Engineering Tutorials

Different layers of OSI model

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• The Physical Layer transforms data into bits that are sent across the physical media.

• The Data Link layer determines access to the network media in terms of frames. Its Media Access Control (MAC) sub layer is responsible for physical addressing.

• The Network Layer routes data through a large network.

• The Transport Layer provides end-to-end, reliable connections, often in terms of segments [Book02].

• The Session Layer allows users to establish connections using intelligently chosen names in packets.

• The Presentation Layer negotiates data exchange formats, also in terms of packets.

• Finally, the Application Layer provides the interface between the user's application and the network through messages.

Data Communication

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OSI Model
• OSI model is just a guideline for protocol design, not the actual protocols

• Not all layers are always used
o Internet uses only five layers
• Some layers may be combined together
o Top three layers are normally combined into one layer .

Why a OSI model?

• Idea was originally to get a message across different networks.

• By layering, each layer performs a separate function. Makes changes and modifications easier. Change of lower layers does not affect higher layers as long as their interfaces are the same.

• Higher layers deal more with end-to-end communications, user services and applications .

• Lowest three layers deal primarily with the details of data transmission in networks.

• Each layer offers certain services to the higher layers, shielding those layers from the details of how the offered services are actually implemented.

The Open Systems Interconnect (OSI), established in 1984 by the ISO (International Standards Organization), divides network functions into seven layers: Physical, Data Link, Network, Transport, Session, Presentation and Application Protocol.

Link Budget

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It ascertains that the RF equipment would cater to the requirements of the network topology and satellite modems in use. The link Budget estimates the ground station and satellite EIRP required. Equivalent isotropically radiated power (EIRP) is the power transmitted from a transmitting object. Satellite ERP can be defined as the sum of output power from the satellite’s amplifier, satellite antenna gain and losses. Calculations of signal levels through the system (from origination earth station to satellite to receiving earth station) to ensure the quality of service should normally be done prior to the establishment of a satellite link. This calculation of the link budget highlights the various aspects. Apart from the known losses due to various cables and inter-connecting devices, it is advisable to keep sufficient link margin for various extraneous noise which may affect the performance. It is also a safeguard to meet eventualities of signal attenuation due to rain/snow. As mentioned earlier a satellite provides two resources, bandwidth and amplification power. In most VSAT networks, the limiting resource in satellite trans-ponder is power rather than bandwidth .

CDMA (Code Division Multiple Access)

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Under this, a central network monitoring system allocates a unique code to each of the VSATs enabling multiple VSATs to transmit simultaneously and share a common frequency band. The data signal is combined with a high bit rate code signal which is independent of the data. Reception at the end of the link is accomplished by mixing the incoming composite data/code signal with a locally generated and correctly synchronized replica of the code . Since this network requires that the central network management system co-ordinates code management and clock synchronization of all remote VSATs, star topology is, by default, the best one. Although this is best applicable for very large networks with low data requirements, there are practical restrictions in the use of spread spectrum. It is employed mainly for interference rejection or for security reasons in military systems .

DAMA (Demand Assigned Multiple Access)

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The network uses a pool of satellite channels, which are available for use by any station in that network. On demand, a pair of available channels is assigned so that a call can be established. Once the call is completed, the channels are returned to the pool for an assignment to another call. Since the satellite resource is used only in pro-portion to the active circuits and their holding times, this is ideally suited for voice traffic and data traffic in batch mode.

DAMA offers point-to-point voice, fax, and data requirements and supports video-conferencing. The ability to use on-board signal processing and multiple spot beams will enable future satellites to reuse the frequencies many times more than today’s’ system. In general, channel allocation may be static or dynamic, with the latter becoming. DE – 5 increasingly popular. DAMA systems allow the number of channels at any time be less than the number of potential users. Satellite connections are established and dropped only when traffic demands them .

PAMA (Pre-Assigned Multiple Access)

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It implies that the VSATs are pre-allocated a designated frequency. Equivalent of the terrestrial leased line solutions, PAMA solutions use the satellite resources constantly. Consequently, there is no call-up delay what makes them most suited for interactive data applications or high traffic volumes. As such, PAMA connects high data traffic sites within an organization .

SCPC (Single Channel Per Carrier) refers to the usage of a single satellite carrier for carrying a single channel of user traffic. The frequency is allocated on a pre-assigned basis in case of SCPC VSAT which is also synonymously known as PAMA VSAT.

FDMA (Frequency Division Multiple Access)

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It is the oldest and still one of the most common methods for channel allocation. In this scheme, the available satellite channel bandwidth is broken into frequency bands for different earth stations. This means that guard bands are needed to provide separation between the bands. Also, the earth stations must be carefully power-controlled to prevent the microwave power spilling into the bands for the other channels. Here, all VSATs share the satellite resource on the frequency domain only. Typically implemented in a mesh or single satellite hop topology, FDMA has the following variants:

TDMA (Time-division multiple access)

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With TDMA networks, numerous remote sites communicate with one central hub – a design that is similar to packet-switched networks. As a leased-line equivalent, SCPC can deliver dedicated bandwidth of up to 2 Mbit/s. Remote sites in a TDMA network compete with one another for access to the central hub, restricting the maximum band-.4 – DE width in most cases to 19.2 kbit/s. Almost all international VSAT services in Asia-Pacific are based on SCPC. Most domestic offerings are based on TDMA, although some domestic operators offer point-to-point SCPC links as well. Here, we will discuss briefly TDMA, pre-assigned or demand-assigned FDMA, CDMA and other accessing techniques featuring merits and demerits of these schemes.

From the above figure the followings are noted:
Switch - routing control between host & modulator/demodulator. Basically on the packets header
Modulator- modulate the outbound carriers (TDM)
Bank of Demodulators- receive inbound carriers, extract data packets.
Radio Frequency Terminal (RFT) - transmits subsystem (up converters, high power amplifier), receive subsystem (low noise amplifier, down converter)
Network Control Center (NCC) - controls and monitors hub and IDUs
Primary Power Subsystem

In a TDMA network, all VSATs share satellite resource on a time-slot basis. Remote VSATs use TDMA channels or in routes for communicating with the hub. There could be several in routes associated with one out route. Several VSATs share one in route hence sharing the bandwidth. Typical in routes operate at 64 or 128 Kbit/s. Generally systems with star topology use a TDMA transmission technique. Most common configuration of TDMA star network use one single high performance hub, many low cost VSAT terminals, centralized management, lower price, optimized use of satellite capacity .

Critical to all TDMA schemes is the function of clock synchronization what is performed by the TDMA hub or master earth station. The VSATs may also access the in route on a fixed assigned TDMA mode, wherein each VSAT is allocated a specific time slot or slots.

Two-way (interactive)

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Following types of networks are two ways

• Multiple Accessing Schemes
• SCPC (Single-Carrier Per Channel)
• TDMA (Time-division multiple access)
• FDMA (Frequency Division Multiple Access)
• PAMA (Pre-Assigned Multiple Access)
• DAMA (Demand Assigned Multiple Access)
• CDMA (Code Division Multiple Access)

Broadcast Network

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Broadcast Networks are used to transmit data, audio and video files to a number of users. The data is broadcast from central site for end user sites. This is a one-way system. The central site broadcasts the data. The remote site only receives data. The channel can provide an uplink speed of 36 to 40 depends on transponder’s BW .

• The main objective of 'Receive only VSAT' services is sharing and spread of information. Thus it is of benefit to society as a whole rather than few users as in the case of normal VSAT.

• The applications of this technology are mainly in areas of information, social services, education and medicine. The broadband multicast capability can provide innovative applications like Tele-Medicine, online newspapers, market rates and Tele-education. It can ensure that these applications are accessible to even the remotest parts of the country.

• This technology is predicted to play a crucial role in facilitating the government efforts to bridge the Digital Divide. Thus the success of Receive Only VSAT is in the favour of the government.

How VSAT Work

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i. The size of a VSAT antenna varies. The feedhorne directs the transmitted power towards the antenna dish or collects the received power from it.

ii. It consists of an array of microwave passive components. Antenna size is used to describe the ability of the antenna to amplify the signal strength.

iii. The Radio Frequency Terminal (RFT) is mounted on the antenna frame and interconnected to the feed-horn (outdoor electronics) includes Low Noise Amplifiers (LNA) and down-converters for amplification and down conversion of the received signal respectively .

iv. LNAs are designed to minimize the noise added to the signal during this first stage of the converter as the noise performance of this stage determines the overall noise performance of the converter unit. The noise temperature is the parameter used to describe the performance of an LNA.

v. Up- converters and High Powered Amplifiers (HPA) are also part of the RFT and are used for up converting and amplifying the signal before transmitting to the feed-horn. The Up/Down converters convert frequencies between intermediate frequency (IF level 70 MHz) and radio frequency.

vi. Extended C band, the down converter receives the signal at 4.500 to 4.800 GHz and the up converter converts it to 6.725 to 7.025 GHz. The HPA ratings for VSATs range between 1 to 40 watts.

vii. The Outdoor Unit (ODU) is connected through a low-loss coaxial cable to the indoor unit (IDU). The typical limit of an (Inter Facility Link) IFL cable is about 500 feet. The IDU consists of modulators that superimpose the user traffic signal on a carrier signal. This is then sent to the RFT for up conversion, amplification and transmission.

VSAT Topology

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Star
The hub station controls and monitors can communicate with a large number of dispersed VSATs. Generally, the Data Terminal Equipment and 3 hub antenna is in the range of 6-13m in diameter. Since all VSATs communicate with the central hub station only, this network is more suitable for centralized data applications .

Mesh
A group of VSATs communicate directly with any other VSAT in the network without going through a central hub. A hub station in a mesh network performs only the monitoring and control functions. These networks are more suitable for telephony applications.

VSAT Services

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i. Interactive real time application
• Point of Sale/retail/Banking (beg. ATM)
• Corporate data
ii. Telephony
• Rural: individual subscribers
• Corporate Telephony
iii. Intranet, Internet and IP infrastructure
• Multimedia delivery (ex. video streaming)
• Interactive distance learning/ training .
iv. Direct-to-home
• Broadband Internet access for consumers and businesses

Specification

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VSAT is a term widely used in the satellite industry to describe an earth station that is installed on the ground to receive communications from a satellite or to communicate with other ground stations by transmitting to and receiving from satellite spacecraft. The ground station may be used only for reception, but is typically capable of both receiving and transmitting. Major components of a VSAT are generally grouped in two categories, ODU (Outdoor Unit) and IDU (Indoor Unit) .

Typical applications for VSAT networks

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o computer communications
o reservation systems
o database enquiries
o billing systems
o file transfers
o electronic mail
o video conferencing
o point of sale transactions
o credit checks and credit card verification
o stock control and management

Steps of VSAT installation

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• Antenna base construction.
• Antenna installation.
• Antenna pointing.
• Cross pole test.
• Equipment configuration.
• BER test.
• Router configuration.
• Data port connects.
• End-to-End data flow test.
• Link test by different way.
• Ready for service

VSAT networks provide

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i. Rapid, reliable satellite transmission of data, voice and video and an ability to allocate resources (bandwidth and amplification power) to different users over the coverage region as needed.

ii. VSAT industry is offering fixed network solutions that can provide a full suite of services at reasonable price. e.g.: a toll quality voice channel via VSAT is available between 3-15 cents/minute today.

iii. Easy to provide point-to-multipoint (broadcast), multipoint-to-point (data collection), point-to-point communications and broadband multimedia services .

iv. VSATs are serviced not only in cases where the land areas are difficult to install, say in the case of remote locations, water areas, and large volumes of air space.

v. An ability to have direct access to users and user premises .

vi. VSAT systems can also provide a variety of services including broadband communication systems satellite-based video, audio, Internet and data distribution networks .

VSAT (Very Small Aperture Terminal)

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VSAT (Very Small Aperture Terminal) is a satellite-based communications service that offers businesses and government agencies flexible and reliable communications solutions, both nationally and internationally, on land and at sea and represents a cost effective solution for users seeking an independent communications network connecting a large number of geographically dispersed sites .

VSAT’s are small, software-based earth stations (generally 0.9 - 4.5 meters), which are used for transmission of data, voice, or video via satellite. It can be operated without additional manpower or technology. VSAT services are delivered using C or KU band GEO satellites .

Type of Satellite Service

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Satellite communication provides services:

i. International Telephony
• using Public Switched Telephone Network (PSTN)
• Intermediate Data Rate (IDR)
• Time Division Multiple Access (TDMA)

ii. Broadcasting
• TV Uplink
• Television Receive Only (TVRO)
• Digital Satellite News Gathering (DSNG)

iii. VSAT ( Very Small Aperture Terminal)
• Personal Earth Station (PES-TDMA)
• Telephony Earth Station (TES-TDMA)
• Domestic IDR/Single Channel Per Carrier (SCPC)
• VSAT Dial away
• VSAT Sky Star Advantage
• VSAT Faraway

For satellite communications the following three types of frequencies are used which are IEEE standards .

C Band : Range between 4 GHz to 8 GHz
Ku Band : Range between 12 GHz to 18 GHz
Ka Band : Range between 20 GHz to 30 GHz
From the above figure the followings are noted:
HPA – High Power Amplifier
LNA - Low Noise Amplifier (Earth station equipment that amplifies the transmit RF signal)
CPE – Customer Premises Equipment (e.g. Telephone, PABX, Ethernet Hub, Host Server etc)

Satellite Communication

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Satellite Communication is a technology of data transmission whether one-way data broadcasting or two-way interactive using radio frequency as a medium.

It consists of-
i. Space Segment or Satellite ( eg. Measat, Intelsat and Inmarsat)

ii. Ground Segment or earth station which includes Antenna, Outdoor Unit, Inter Facility Link, Indoor Unit and Customer Premises Equipment.

satellite communication

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Satellite Communication using VSAT (Very Small Aperture Terminal) since the science fiction on radio transmission through space using geo-synchronous earth satellite, provider has progressed significantly in the field of satellite communications. The early earth stations were large and expensive. The reason for the size and complexity of the early stations was not related to inadequate performance. In fact, the antennas had very high efficiency and the noise temperatures of their receivers were low. However, the satellites at that time had a relatively poor performance providing considerably low RF (radio frequency) power per transponder and a rather high noise temperature for the on-board receivers. Additionally, satellites were then considered suitable only for very long distance communication. Gradually, satellite communications have appeared as regional systems requiring smaller coverage on the earth’s surface enabling higher gain antennas. Subsequently, increase in transponder out-put power, introduction of systems having several spot beams, development of field-effect transistor amplifier for low noise receivers as well as its availability as power amplifier have changed the satellite communication scenario. Once it was possible to envisage an all solid-state transmit and receive earth station even with a rather low power output, low price, large quantity, VSAT-based earth station design could be conceived.

Wavelength

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Wavelength can be calculated as
c = n λ = f λ
So, Wavelength, λ = V/ f

Where,
λ = Carrier wavelength
f = Carrier frequency
c = Velocity of light ( 3 × 108 m/s)

In case of wavelength, when wavelength decreases then the speed of electromagnetic wave propagation decreases and loss increases. Wavelength inversely proportional to rainy season .

Signal-to-noise ratio

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The signal-to-noise ratio which is the ratio of the power in a signal to the power contained in the noise that is present at a particular point in the transmission. Typically, this ratio is measured at a receiver, because it is at this point that an attempt is made to process the signal and recover the data. For convenience, this ratio is often reported in decibels:

SNR dB= 10 log 10signal powernoise power

This expression the amount, in decibels, that the intended signal exceeds the noise level. A high SNR will mean a high-quality signal and low number of required intermediate repeaters .

The signal-to-noise ratio is important in the transmission of digital data because it sets the upper bound on the achievable data rate. Shannon’s result is that the maximum channel capacity, in bits per second, obeys the equation

C= B log2(1+SNR)

Where C is the capacity of the channel in bits per second and B is the bandwidth of the channel in hertz .

Bit Error Rate

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Bit Error Rate. The rate at which errors in transmission occur, normally related closely to the Signal to Noise Ratio (SNR). BER of 10-9, or one bit error for every billion bits, is a typical minimum system requirement .

Waveguide

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A waveguide is a hollow metal tube, either a cylinder or a rectangle in cross section that acts as a pipe for transmission of electromagnetic waves of the microwave range over short distance (1 to 10 cms) . A typical waveguide of rectangular cross section. Since the waveguide is of solid metal, any electromagnetic wave put into its cavity can not escape and travels through the guide to the other end. As a result, waveguide offers a very large bandwidth (2 GHz to 110 GHz) .

The RF power for acceleration of protons inside the accelerating structure, supplied from high-power klystrons, is taken up to the desired ports by means of wave-guide line. The factors of primary importance for a wave-guide system are: power handling capacity, insertion loss, impedance uniformity, band width, physical dimensions/tolerances, economic considerations and self strength. The wave-guides are made from aluminium alloy 6061 plates which are heliarc welded at four corners .

Cable and Connector Loss

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There will always be some loss of signal strength through the cables and connectors used to connect to the antenna.

This loss is directly proportional to the length of the cable and generally inversely proportional to the diameter of the cable .Additional loss occurs for each connector used and must be considered in planning.Cable vendor can provide a chart indicating the loss for various types and lengths of cable. Table A-1 on page A-4 is an example of this kind of chart.

Antenna efficiency

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Consider a dish antenna pointed at an isotropic antenna transmitting some distance away. We know that the isotropic antenna radiates uniformly in all directions, so it is a simple matter of spherical geometry to calculate how much of that power should be arriving at the dish over its whole aperture . Now to measure how much power is being received from the dish (at the electrical connection to the feed) – never greater than is arriving at the aperture. The ratio of power received to power arriving is the aperture efficiency.

The total antenna efficiency accounts for the following losses:

(1) Reflection because of mismatch between the feeding transmission line and the antenna and
(2) Antenna conductor and dielectric losses.

Antenna Gain

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Antenna gain is essential for microwave communication---since it helps both transmitting and receiving, it is doubly valuable.

Antenna gain is an indicator of how well an antenna focuses RF energy in a preferred direction. Antenna gain is expressed in dBi (the ratio of the power radiated by the antenna in a specific direction to the power radiated in that direction by an isotropic antenna fed by the same transmitter). Antenna manufacturers normally specify the antenna gain for each antenna they manufacture .

The relationship between antenna gain and effective area is

G = 4πAe / λ 2 = 4π f 2Ae / c2

Where
G = Antenna Gain
Ae = Effective area
λ = Carrier wavelength
f = Carrier frequency
c = Speed of light ( 3 × 108 m/s)

The hypothetical isotropic antenna is a point source that radiates equally in all directions. Any real antenna will radiate more energy in some directions than in others. Since it cannot create energy, the total power radiated is the same as an isotropic antenna driven from the same transmitter: in some direction it radiates more energy than an isotropic antenna, so in others it must radiate less energy. The gain of an antenna in a given direction is the amount of energy radiated in that direction compared to the energy an isotropic antenna would radiate in the same direction when driven with the same input power. Usually we are only interested in the maximum gain-----the direction in which the antenna is radiating most of the power .

An antenna with a large aperture has more gain than smaller one; just as the captures more energy from a passing radio wave, it also radiates more energy in that direction. Gain may be calculated as

GdBi = 10 log10( η 4π/ λ 2 A)

With reference to an isotropic radiator; η is the efficiency of the antenna.

Free-Space Path Loss

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A signal degrades as it moves through space. The longer the path, the more loss it experiences. This free-space path loss is a factor in calculating the link viability . Free-space path loss is easily calculated for miles or kilometers using one of the following formulas:

Lp = (96.6 + 20 log10 F) + (20 log10 D)

where
Lp = free-space path loss between antennas (in dB)
F = frequency in GHz
D = path length in miles

or

Lp = (92.4 + 20 log10 F) + (20 log10 D)

where
Lp = free-space path loss between antennas (in dB)
F = frequency in GHz
D = path length in kilometers

Receiver Sensitivity

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The sensitivity of a receiver is its ability to receive quick signal. This sensitivity may be defined in several ways.

First, it may be started in terms of the signal field strength of a signal that will produce a desired demodulated output level under a certain modulation level. The sensitivity is usually started in terms of the voltage developed by the antenna across the receiver antenna terminals in microvolts. This level ranges from a few microvolts to a few hundred microvolts for typical receiver .

Another way of stating the sensitivity is to state the antenna terminal signal voltage required to produce a specified signal- to- noise ratio. In the case of receiver for digital signals, the sensitivity is usually stated as the input signal level required to produce a desired bit- error rate which is related to signal- to- noise ratio.

Receiver sensitivity is typically specified in units of microvolts for a 12 dB SINAD (signal to noise and distortion). The amount of RF power required by the receiver to faithfully represent the actual signal transmitted is arbitrarily set at the 12 dB SINAD, which translates into -114 dBm for a typical receiver.

Calculating a Link Budget or Fade Margin

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The major issues with microwave link engineering are rain fade, multi path and interference. An RF engineer begins a design by doing a link budget analysis. A given radio system has a system gain that depends on the design on the radio and the modulation used. The gains from the antenna at each end are added to this gain. Larger antennas provide higher gain. The free space loss of the radio signal as it travels over the air is then subtracted from the system: the longer the link the higher the loss. These calculations result in a “fade margin” for the link. Anything that affects the radio signal within this margin will be overcome by the radio; if the margin is exceeded, then the link could go down. The next step, then, is to analyze impediments that could potentially affect the radio signal . With good understanding of the potential affects on the signal, the RF engineer can design links with availability and performance equal to or better than a wire line link. A link budget is a rough calculation of all known elements of the link to determine if the signal will have the proper strength when it reaches the other end of the link. To make this calculation, the following information should be available:

Frequency of the link

Free space path loss

Power of the transmitter

Antenna gain

Total length of transmission cable and loss per unit length at the specified frequency

Number of connectors used

Loss of each connector at the specified frequency

Path length

The amount of extra RF power radiated to overcome this phenomenon is referred to as fade margin. The exact amount of fade margin required depends on the desired reliability of the link, but a good rule-of-thumb is 20dB to 30dB.

Fade Margin = SG + AG - LC – LP

Where
SG = System gain (depend on modem)
AG = Antenna gain
LC = Cable loss
LP = Path loss

These parameters are also responsible for gain loss.

System Gain = Radiated Power of radio Equipment - (Receiver Sensitivity)

Earth Bulge

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When planning for paths longer than seven miles, the curvature of the earth might become a factor in path planning and require that the antenna be located higher off the ground . The additional antenna height needed can be calculated using the following formula:

H = D²/8

Where,

H = Height of earth bulge (in feet)

D = Distance between antennas (in miles)

Minimum Antenna Height

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The minimum antenna height at each end of the link for paths longer than seven miles (for smooth terrain without obstructions) is the height of the First Fresnel Zone plus the additional height required to clear the earth bulge . The formula would be:

H = 43.3 √ (D/4F ) + D2/8

Where,
H = Height of the antenna (in feet)
D = Distance between antennas (in miles)
F = Frequency in GHz

Fresnel Zone

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The characteristics of a radio signal cause it to occupy a broad cross-section of space, called the Fresnel Zone, between the antennas. From figure shows the area occupied by the strongest radio signal, called the First Fresnel Zone, which surrounds the direct line between the antennas

Because of the shape of the First Fresnel Zone, what appears to be a clear line-of-sight path may not be. As long as 80 percent of the First Fresnel Zone is clear of obstructions, the link behaves essentially the same as a clear free-space path .

The following formula is used to calculate it:

H = 43.3 √ (D/4F )

Where,

H = Height of the First Fresnel Zone (in feet)

D = Distance between the antennas (in miles)

F = Frequency in GHz

Waves can be deflected by objects in their paths. If a wave from an outer band of the cone (see Radio Signals) is deflected back through the center lobe, it can either strengthen that signal or reduce its strength, depending on how the waves align when they collide. A glancing deflection changes the angle of the wave very little, so it remains generally in phase with the wave at the center lobe Within the signal span, there are zones where deflected signals are generally in phase with the center lobe signal, and there are other zones where deflected signals are generally out of phase with the center lobe signal. We refer to these zones as Fresnel (frnl) zones. The first Fresnel zone surrounds the center lobe where the RF signal is strongest. If more than 40% of the first Fresnel zone is obstructed, the RF line of sight is not sufficiently clear.

In the first Fresnel zone and all the odd numbered Fresnel zones, deflected signals are generally in phase with or the center lobe signal. In the second Fresnel zone, and all even-numbered Fresnel zones, deflected signals are up to 180 out of phase with the center lobe signal . Signals deflected from the second Fresnel zone can cause Inter Symbol Interference (ISI) which can result in great losses of the center lobe signal. To avoid this problem, must place the antenna at a height that is out of range from F2 deflections. (An antenna can be set too high as well as too low.) Where deflection and diffraction from ground-based objects cause interference, even a small relocation of the antenna often produces a substantial improvement.

Tower Height

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H = 43.3 √ (D/4F ) + D²/8

Where,

H = Height of the antenna (in feet)

D = Distance between antennas (in miles)

F = Frequency in GHz

Tower

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When planning antenna placement, it might be necessary to build a free-standing tower for the antenna. Regulations and limitations define the height and location of these towers with respect to airports, runways, and airplane approach paths. These regulations are controlled by the FAA. In some circumstances, the tower installations must be approved by the FAA, registered with the FCC, or both. To ensure compliance, review the current FCC regulations regarding antenna structure .

Best Location of Antenna Placement

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In general, the best location for antenna placement is always the closest to the transmitter. The type of antenna selected also has an effect on placement as discussed earlier. Since the transmitted signal is typically vertically polarized, it is important to orient the receiver antennas in a vertical position. The best place is usually on an adjacent wall, near the vicinity of the transmitter, using the frequency length formula to determine the proper distance between the antennas. When a number of multiple systems are used in a given location it is a common practice to use antenna combiners to eliminate the antenna Farm. Antenna amplifier/combiners are a convenience item that allows a single pair of antennas to feed multiple receivers

Antenna Polarization

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The orientation of the antenna will change the orientation of the signal. The transmitting and receiving antennas should be both polarized either horizontally or vertically. Adjacent antennas on different frequencies can be cross-polarized to help reduce interference between the two, if operating license permits this.

Types of antennas

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Dipole antenna
Other antenna types include the “dipole”, where a section of wire, one-half the wavelength, is positioned either horizontally or vertically in the air to transmit signals. Dipoles emit their signals in more of a two dimensional semi-circular or “doughnut” pattern, the key being both the transmitter and receiver’s antennas must be aligned the same (horizontally or vertically). Dipoles do not require a ground-plane are considered “bi-directional,” in that their signals travel in two opposite directions, depending on how the antenna is oriented .

Yagi antenna
The more focused (uni-directional) type of antenna is called a “Yagi.” A Yagi antenna is basically a standard one-half wavelength antenna, but with additional “elements” placed in front of it to focus the energy for transmission in one direction. The “reflector” and “director” elements are just similar-sized resonators spaced appropriately to increase the strength and narrow the direction of the signal prior to transmission. Again, the key to successfully using Yagi antennas is the correct orientation and alignment of the transmitting/receiving antennas.

Sectoral antenna
The requirement to serve a number of small areas from a single base station has resulted in the development of the multiple beam technique, for which the sectoral antenna is ideal .

Parabolic Reflector antenna
This antenna consists of a parabolic metal surface (dish) with a feed antenna in front. The feed antenna consists of a directive antenna such as a dipole and reflector, log-periodic dipole array or horn antenna. This antenna is capable of producing extremely high gains, usually in the 20 - 30 dBi range

Types of antenna

2009-03-24T06:09:00.000-07:00

Antennas are mainly two types---

Omni directional antenna

Directional antenna

Omni directional Antenna
The omni directional antenna radiates or receives equally well in all directions. It is also called the "non-directional" antenna because it does not favor any particular direction.

Omni antennas usually resemble vertical rods but can come in other shapes as well. Some have horizontal rods of the same length placed at their base to increase their performance/distance. These are called “ground planes”.

The key factor to note is that for receivers all four signals (or signals from any direction, for that matter) are received equally well. For transmitters, the radiated signal has the same strength in all directions. This pattern is useful for broadcasting a signal to all points of the compass (as when calling "CQ"), or when listening for signals from all points.

Directional Antennas

Gain and directivity are intimately related in antennas. The directivity of an antenna is a statement of how the RF energy is focused in one or two directions. Because the amount of RF energy remains the same, but is distributed over less area, the apparent signal strength is higher. This apparent increase in signal strength is the antenna gain. The gain is measured in decibels over either a dipole (dBd) or a theoretical construct called an isotropic radiator (dBi) . The isotropic radiator is a spherical signal source that radiates equally well in all directions. One way to view the omni directional pattern is that it is a slice taken horizontally through the three dimensional sphere

Antenna

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"That part of a transmitting or receiving system which is designed to radiate or to receive electromagnetic waves". An antenna can also be viewed as a transitional structure (transducer) between free-space and a transmission line (such as a coaxial line). An important property of an antenna is the ability to focus and free shape the radiated power in space e.g.: it enhances the power in some wanted directions and suppresses the power in other directions .

Antennas focus the radio signal in a specific direction and in a narrow beam. The increase in the signal power (compared to an omni directional antenna) when it is focused in the desired direction is called gain.

Antennas are tuned to operate on a specific group of frequencies. Other specific attributes such as beam-width and gain are also fixed by the manufacturer. Antennas should be selected and placed according to the site and the application .

In general, the larger the antenna, the higher the gain and the larger the mast required. It is best to use the smallest antenna that will provide sufficient protection from interference and enough signal at the far end of the link to provide good reception even with fading.

Other considerations include antenna beam-width, front-to-side ratios, front-to-back ratios, and cross-polarization rejection. Where interference from other licensees on the same channel or adjacent channels is an issue, narrow beam-widths, high front-to-back and front-to-side ratios, and high cross-polarization rejection are likely to be required. Even when other licensees are not an issue, if using a network deployment using the “cell” approach, all these considerations is still important to reduce interference between own adjacent installations .

Co-Channel and Adjacent Channel Interference

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Co-channel interference results when another RF link is using the same channel frequency. Adjacent-channel interference results when another RF link is using an adjacent channel frequency. In selecting a site, a spectrum analyzer can be used to determine if any strong signals are present at the site and, if they are, to determine how close they are to the desired frequency. The further away from the proposed frequency, the less likely they are to cause a problem. Antenna placement and polarization, as well as the use of high-gain, low-side lobe antennas, are the most effective method of reducing this type of interference.

Control Cable

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When the entire control cable, from the building entrance to the transverter, is encased in steel conduit, no surge arrestors are required. Otherwise, each control cable requires one surge arrestor within two feet of the building entrance, and another surge arrestor within 10 feet of the transverter

Coaxial Cable

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Because the coaxial line carries a DC current to supply power to the transverter, gas-discharge surge arrestors are required. Do not use quarter-wave stub or solid-state type surge arrestors .

When the entire coaxial cable, from the building entrance to the transverter, is encased in steel conduit, no surge arrestors are required. However, local electrical codes require that the conduit be grounded where it enters the building.

When steel conduit is not used to encase the cable, each cable requires one surge arrestor within 2 feet of the building entrance, and another surge arrestor within 10 feet of the transverter .

Radio Links

2009-03-24T05:43:00.000-07:00

Connection of two points by non-visible electromagnetic waves is known as radio link

Types

Extremely Low Frequency(ELF)

Very Low Frequency(VLF)

Low Frequency(LF)

Medium Frequency (MF)

High Frequency (HF)

Very High Frequency (VHF)

Ultra High Frequency (UHF)

Super High Frequency (SHF)

Microwave

2009-03-24T05:38:00.000-07:00

Electromagnetic waves at the frequency range of about 2 to 40 GHz are referred to as microwave . Microwave radio operates in unlicensed bands are 2.4 GHz and 5.7 GHz and are licensed band it could operate like 6GHz, 7 GHz, 8GHz, 10GHz, 11GHz and 13GHz, 15GHz, 18GHz and 23GHz, 38GHz frequency bands. At these frequencies, highly directional beams are possible and microwave is quite suitable for point-to-point transmission. Concentrating all the energy into a small beam using a parabolic antenna (like the familiar satellite TV dish) gives a much higher signal to noise ratio, but the transmitting and receiving antennas must be accurately aligned with each other. It’s a type of unbounded network transmission medium. Microwave is mainly used for satellite communications .

A microwave system includes an antenna, radio, multiplexes, waveguide (hollow metal conductor connecting the RF equipment to the antenna) and feed cables. Based on capacity and radio equipment, antenna size, tower heights and terrain elevation will play a major role in how it will planned and construct the system. These four factors also will dictate system reliability, multi-path fading, fade margin calculations, freshnel zone clearance, interference analysis, system diversity and long-distance specifications.

Co-Channel and Adjacent Channel Interference

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Interference

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An important part of planning for broadband fixed wireless system is the avoidance of interference. Interference can be caused by effects within the system or outside the system. Good planning for frequencies and antennas can overcome most interference challenges.

Control Cable

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Coaxial Cable

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Because the coaxial line carries a DC current to supply power to the transverter, gas-discharge surge arrestors are required. Do not use quarter-wave stub or solid-state type surge arrestors.

When the entire coaxial cable, from the building entrance to the transverter, is encased in steel conduit, no surge arrestors are required. However, local electrical codes require that the conduit be grounded where it enters the building.

When steel conduit is not used to encase the cable, each cable requires one surge arrestor within 2 feet of the building entrance, and another surge arrestor within 10 feet of the transverter .

Lightning Protection

2009-02-20T09:51:00.000-08:00

To provide effective lightning protection, install antennas in locations that are unlikely to receive direct lightning strikes, or install lightning rods to protect antennas from direct strikes . Make sure that cables and equipment are properly grounded to provide low-impedance paths for lightning currents. Install surge suppressors on telephone lines and power lines.

Cisco recommends lightning protection for both coaxial and control cables leading to the wireless transverter . The lightning protection should be placed at points close to where the cable passes through the bulkhead into the building, as well as near the transverter.

Lightning

2009-02-20T09:50:00.000-08:00

The potential for lightning damage to radio equipment should always be considered when planning a wireless link .A variety of lightning protection and grounding devices are available for use on buildings, towers, antennas, cables, and equipment, whether located inside or outside the site that could be damaged by a lightning strike.

Lightning protection requirements are based on the exposure at the site, the cost of link down-time, and local building and electrical codes. If the link is critical, and the site is in an active lightning area, attention to thorough lightning protection and grounding is critical.

Wind

2009-02-20T09:49:00.002-08:00

Any system components mounted outdoors will be subject to the effect of wind. It is important to know the direction and velocity of the wind common to the site. Antennas and their supporting structures must be able to prevent these forces from affecting the antenna or causing damage to the building or tower on which the components are mounted.

Antenna designs react differently to wind forces, depending on the area presented to the wind. This is known as wind loading. Most antenna manufacturers will specify wind loading for each type of antenna manufactured.

Atmospheric Absorption

2009-02-20T09:49:00.001-08:00

A relatively small effect on the link is from oxygen and water vapor. It is usually significant only on longer paths and particular frequencies. Attenuation in the 2 to 14 GHz frequency range is approximately 0.01 dB/mile, which is not significant .

Rain and Fog

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Microwave RF communication is estimated for rain and frequency. Each maps the world into “rain climate regions”. Based on the region, microwave links can be engineered to support any desired availability. The intersection of the radio margin line and the rain zone line is the distance that can be achieved with 99.999% availability . Except in extreme conditions, attenuation (weakening of the signal) due to rain does not require serious consideration for frequencies up to the range of 6 or 8 GHz. When microwave frequencies are at 11 or 12 GHz or above, attenuation due to rain becomes much more of a concern, especially in areas where rainfall is of high density and long duration. If this is the case, shorter paths may be required .

The systems discussed in this guide operate at frequencies below 6 GHz, so rain is not a concern.

In most cases, the effects of fog are considered to be much the same as rain. However, fog can adversely affect the radio link when it is accompanied by atmospheric conditions such as temperature inversion, or very still air accompanied by stratification. Temperature inversion can negate clearances, and still air along with stratification can cause severe refractive or reflective conditions, with unpredictable results. Temperature inversions and stratification can also cause ducting, which may increase the potential for interference between systems that do not normally interfere with each other. Where these conditions exist, Cisco recommends shorter paths and adequate clearances .

Weather

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It is important to research any unusual weather conditions that are common to the site location. These conditions can include excessive amounts of rain or fog, wind velocity, or extreme temperature ranges. If extreme conditions exist that may affect the integrity of the radio link, Cisco recommend that these conditions be taken into consideration early in the planning process.

Radio Links

2009-02-20T09:39:00.002-08:00

Connection of two points by non-visible electromagnetic waves is known as radio link

Fig 2.1 Radio Links

Types

§ Extremely Low Frequency(ELF)

§ Very Low Frequency(VLF)

§ Low Frequency(LF)

§ Medium Frequency (MF)

§ High Frequency (HF)

§ Very High Frequency (VHF)

§ Ultra High Frequency (UHF)

§ Super High Frequency (SHF)

Fig 2.2 Frequencies of radio link

The following sections will help to determine which information is critical to the site and will be an aid in the decision-making process .

Microwave

2009-02-20T09:39:00.001-08:00

Electromagnetic waves at the frequency range of about 2 to 40 GHz are referred to as microwave. Microwave radio operates in unlicensed bands are 2.4 GHz and 5.7 GHz and are licensed band it could operate like 6GHz, 7 GHz, 8GHz, 10GHz, 11GHz and 13GHz, 15GHz, 18GHz and 23GHz, 38GHz frequency bands. At these frequencies, highly directional beams are possible and microwave is quite suitable for point-to-point transmission. Concentrating all the energy into a small beam using a parabolic antenna (like the familiar satellite TV dish) gives a much higher signal to noise ratio, but the transmitting and receiving antennas must be accurately aligned with each other. It’s a type of unbounded network transmission medium. Microwave is mainly used for satellite communications. A microwave system includes an antenna, radio, multiplexes, waveguide (hollow metal conductor connecting the RF equipment to the antenna) and feed cables . Based on capacity and radio equipment, antenna size, tower heights and terrain elevation will play a major role in how it will planned and construct the system. These four factors also will dictate system reliability, multi-path fading, fade margin calculations, freshnel zone clearance, interference analysis, system diversity and long-distance specifications

Microwave

2009-02-20T09:38:00.000-08:00

Electromagnetic waves at the frequency range of about 2 to 40 GHz are referred to as microwave. Microwave radio operates in unlicensed bands are 2.4 GHz and 5.7 GHz and are licensed band it could operate like 6GHz, 7 GHz, 8GHz, 10GHz, 11GHz and 13GHz, 15GHz, 18GHz and 23GHz, 38GHz frequency bands. At these frequencies, highly directional beams are possible and microwave is quite suitable for point-to-point transmission. Concentrating all the energy into a small beam using a parabolic antenna (like the familiar satellite TV dish) gives a much higher signal to noise ratio, but the transmitting and receiving antennas must be accurately aligned with each other. It’s a type of unbounded network transmission medium. Microwave is mainly used for satellite communications. A microwave system includes an antenna, radio, multiplexes, waveguide (hollow metal conductor connecting the RF equipment to the antenna) and feed cables . Based on capacity and radio equipment, antenna size, tower heights and terrain elevation will play a major role in how it will planned and construct the system. These four factors also will dictate system reliability, multi-path fading, fade margin calculations, freshnel zone clearance, interference analysis, system diversity and long-distance specifications

Wireless communication

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The installation of a wireless network requires much the same basic planning as any wired network. The main difference is that the wireless signal requires some additional planning . Compared to landline solutions, RF has significant advantages in speed, ease and cost of deployment. An RF segment between two sides can be installed in a few days. A typical installation consists with indoor cabinet, power and signal cables between the indoor location and the outdoor location, installation of outdoor components such as outdoor units, RF path planning, site preparation, antennas, lightning protection devices, and cabling suitable for outdoor conditions . Usually, also need to investigate the zoning laws as well as Federal Communications Commission (FCC) and Federal Aviation Administration (FAA) regulations. The installation cost is minimal compared to laying cable and the radio equipment cost is comparable to equipment used with landlines .

Multiplexing

2009-02-20T09:15:00.000-08:00

The technique of transmitting more than one information signal through a single channel is called multiplexing. There are two types of multiplexing

(a) Frequency Division Multiplexing (FDM) and

(b) Time Division Multiplexing (TDM)

When the information signal is digital and the carrier signal is Sinusoidal, modulation is called shift keying [Book03]. According to the change of the parameter of the carrier (sinusoidal) by the digital signal we have,

(a) Amplitude Shift Keying (ASK)

(b) Frequency Shift Keying (FSK) and

In addition, a combination of ASK and PSK is employed at high bit rates. This method is called Quadrature Amplitude Modulation (QAM) .

Types of digital modulation

2009-02-20T09:13:00.000-08:00

In digital modulation the carrier signal is a train of pulses. Pulses have amplitude, width and position. We have

(a) Pulse Amplitude Modulation (PAM)

(b) Pulse Width Modulation (PWM)

Pulse Position Modulation (PPM)

Types of Analog Modulation

2009-02-20T09:08:00.000-08:00

The basic idea here is to superimpose the message signal in analog form on a carrier which is a sinusoid of the form

A cos (ω_ct + )

There are three quantities that can be varied in proportion to the modulating signal: the amplitude, the phase, and frequency. The first scheme is called Amplitude Modulation and the second two are called Angle Modulation schemes.

Fig.1.2 Types of Analog Modulation

There are two types of analog modulation.

1. Amplitude Modulation

2. Angle Modulation.

Angle modulation are also have two different types.

1. Frequency Modulation(FM)

2. Phase Modulation(PM)

In amplitude modulation, the message signal will be present in the amplitude of the transmitted signal.

Analog modulation is non linear modulation and requires high bandwidth and also have good performance in the presence of noise .

In frequency modulation, the message signal will be present in the instantaneous frequency.

In phase modulation, the message signal will be present in the phase.

In quadrature amplitude modulation, the message will be present in both the amplitude and the phase .

Modulation

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The information signal is placed on the carrier. The information signal introduces certain change in the parameter of the carrier. In the case of analog sinusoidal carrier, it is possible to change the amplitude or frequency or phase or two of them or all of them by the information signal.

The point to modulation is to take a message bearing signal and superimpose it upon a carrier signal for transmission. For ease of transmission carrier signals are generally high frequency for several reasons [Book03]:

For easy (low loss, low dispersion) propagation as electromagnetic waves.
So that they may be simultaneously transmitted without interference from other signals.
So as to enable the construction of small antennas (a fraction, usually a quarter of the wavelength).
So as to be able to multiplex that is to combine multiple signals for transmission at the same time.

Modulation is the process of putting an information signal on a carrier signal for some technical advantages.

Demodulation is the opposite of modulation. That is separation of the information signal from the received modulation signal. For data transmission demodulation is the process of recovering the data signal from the received asked sinusoidal signal.

Modulation is the process by which some characteristic of a carrier signal is varied in accordance with a modulating signal. Many ways exist to modulate a message signal m(t) to produce a modulated (transmitted) signal x(t). For amplitude, frequency, and phase modulation, modulated signals can be expressed in the same form as

s(t) = A (t) cos(2 ƒ_c t+ (t))

where A(t) is a real-valued amplitude function (a.k.a. the envelope), ¦_cis the carrier frequency, and θ(t) is the real-valued phase function.

Communication systems are often organized according to the following structure

Fig 1.1 Communication systems

Modulation

2009-02-12T10:31:00.004-08:00

Very rarely base band transmission is used for long distance transmission. A technique called modulation is used for the purpose. In modulation there is a carrier signal. The carrier signal may be an analog sinusoidal signal of a fixed frequency or a train of pulses of certain frequency .

The information signal is placed on the carrier. The information signal introduces certain change in the parameter of the carrier. In the case of analog sinusoidal carrier, it is possible to change the amplitude or frequency or phase or two of them or all of them by the information signal.

The point to modulation is to take a message bearing signal and superimpose it upon a carrier signal for transmission. For ease of transmission carrier signals are generally high frequency for several reasons:

For easy (low loss, low dispersion) propagation as electromagnetic waves.
So that they may be simultaneously transmitted without interference from other signals.
So as to enable the construction of small antennas (a fraction, usually a quarter of the wavelength).
So as to be able to multiplex that is to combine multiple signals for transmission at the same time.
Modulation is the process of putting an information signal on a carrier signal for some technical advantages.

Demodulation is the opposite of modulation. That is separation of the information signal from the received modulation signal. For data transmission demodulation is the process of recovering the data signal from the received asked sinusoidal signal .

Modulation is the process by which some characteristic of a carrier signal is varied in accordance with a modulating signal. Many ways exist to modulate a message signal m(t) to produce a modulated (transmitted) signal x(t). For amplitude, frequency, and phase modulation, modulated signals can be expressed in the same form as

s(t) = A (t) cos(2 piƒc t+ θ(t))

where A(t) is a real-valued amplitude function (a.k.a. the envelope), ¦c is the carrier frequency, and θ(t) is the real-valued phase function.

Communication systems are often organized according to the following structure

Best way for search in Google

2009-02-09T22:08:00.004-08:00

What you’ll learn in this class
•How to use internet search and internet resources for schoolworkand home use
•Lecture, demonstration and hands-on practice
•Develop effective search query formation
•Learn how to organize your search
•What to do when you’re stuck
•Learn how to evaluate the trustworthiness of web sites

Search Basics
•Some basic terms:
–Web site: a collection of web pages on a web server
–Browser: the software you use to view web pages
such as:Internet Explorer (IE), Firefox, Safari, Opera
–Query: the terms you send to Google to ask your question
–URL:the string that refers to the web page
–Plugin:an extra piece of code that plugs into the browse
such as:Quicktime, Flash, SVG, ….

To get started
•I need to tell you a few things: .

Miniature Circuit Breakers

2009-01-31T03:29:00.004-08:00

Miniature Circuit Breakers

As previously stated, a miniature circuit breaker is a device that switches and/or protects the lowest common distributed voltage in an electrical system. It is designed to protect conductors and insulation from damage due to Overload (or Overcurrent) and Short Circuit.
For most people, the easiest way to visualize this application picture is to think in terms of a loadcenter in a home (Figure 3). The circuit breakers inside the loadcenter are miniature circuit breakers.
Think about the electrical utility and where the electricity is generated. The residential loadcenter is certainly at the end of the distribution system. It is here that the voltages are the lowest of the distributed voltages in the electric utility's system.

End of the Line Distribution System (Loadcenter)

Miniature circuit breakers are not just for residential applications only. They are used in residential, commercial and industrial applications.
In an industrial or commercial application, miniature circuit breakers can be found in loadcenters, lighting Panelboards and individual mountings.
Applications
Miniature circuit breakers fall into two categories. These are:
• Residential-Residential miniature breakers are only of the Plug-In type. These are designed for residential loadcenters, commercial units, and light industrial applications. They typically range from 10 to 125 amps, with an interrupting rating of 10 or 22 KAIC.
• Industrial-These breakers are designed for three types of mounting applications: plug-in, Bolt-On, and Cable-In/Cable-Out. (We will look at mounting methods shortly.)
Industrial miniature breakers are designed to protect small branch circuits in commercial or industrial electrical distribution systems. They are applied in loadcenters, lighting panelboards or individual mounting applications. They typically range from 6 to 125 amps, with an interrupting ratings as high as 65 KAIC.
Some potential customers are original equipment manufacturers (OEMs) involved in industrial control panels and electrical machinery, such as machine tool equipment, material handling and packaging systems. In addition, look for involvement with printing machines, food-processing systems, uninterruptable power supplies (UPS) and HVAC (heating, ventilation and air conditioning).
Pictured here is a typical residential loadcenter. Each miniature circuit breaker in the loadcenter protects a branch circuit. Two branch circuits are shown, each providing power to common residential loads.

Each miniature breaker is rated to handle a specific load. For example, a circuit breaker protecting a branch used with kitchen appliances has a higher rating than a circuit breaker protecting a branch with an overhead lighting fixture on it.

Introduction

2009-01-31T03:26:00.004-08:00

We will discuss two types of products in this module. These are the Miniature Circuit Breaker and the Supplementary Protector.
We group these products together because they perform the same function. They both switch and protect the lowest common distribution voltage in an electrical system.
Some other similarities include:
• Both have molded case enclosures
• Both are used in low voltage (under 600 volts) systems
• Both devices are small: Typically 1" wide
The big difference between the two is that the supplementary protector is not UL 489 (Underwriters Laboratory) approved. For this reason, it cannot be used as a Branch Circuit Overcurrent Protective Device, or in the place of the branch circuit protector. A miniature circuit protector protects the whole branch circuit, but the protector is only used to protect a particular device.
Figure shows the difference between using just a miniature circuit breaker (shown on the left) and using a supplementary protector.
Figure Branch Circuit Overcurrent Protection (on left) Vs.
Supplementary Protector (on right)

Therefore, this module will concentrate on miniature circuit breakers. Supplementary protectors will be discussed only briefly

Welcome to Module 9

2009-01-31T03:21:00.004-08:00

Welcome to Module 9, which is about miniature circuit breakers and supplementary protectors.

Typical Miniature Molded Case Circuit Breakers

Like the other modules in this series, this one presents small, manageable sections of new material followed by a series of questions about that material. Study the material carefully, and then answer the questions without referring back to what you've just read.
You are the best judge of how well you grasp the material. Review the material as often as you think necessary. The most important thing is establishing a solid foundation to build on as you move from topic to topic and module to module.

Mounting and Enclosures

2009-01-27T02:54:00.004-08:00

Generally, molded case circuit breakers can be mounted in any position. Mounting them up, down, horizontal or vertical does not affect the tripping or interrupting characteristics of the breaker. However, mounting them in a vertical position with the ON position as anything other than up, is in violation of National Electric Code.
In some cases because of the physical arrangement of a panelboard or switchboard, it is necessary to reverse feed a circuit breaker. The circuit breaker must be tested and listed accordingly for this type of application. Only breakers that have fixed trips can be used, and they usually have sealed covers. They often do not have "Line" and "Load" marked on the cover, so the power source can be connected to either the line or the load terminations.
In addition to being mounted in motor control centers, switchboards and panelboards, molded case circuit breakers are mounted individually in separate enclosures. The National Electric Code and local electric codes determine the proper selection of an enclosure type for a particular application. The National Electrical Manufacturers Association (NEMA) and the International Electro-technical Commission (IEC) have set standards for the protection of devices in various environmental situations. Enclosure types are rated to withstand water, dust, oil, and other environmental conditions. NEMA assigns Type classifications to enclosures. When an enclosure is rated a particular type, it means it is made of the specified materials and has passed specific tests. IEC also has tests and standards that enclosures must conform to. They assign an IP classification.

Mounting Circuit Breakers

The most common types of enclosures are:
NEMA Type 1 (Conforms to IP40) - These enclosures are designed for indoor applications. They are suitable for installations where unusual conditions do not exist, but where a measure of protection from accidental contact is required. They are commonly used in commercial buildings and apartment buildings. They are often referred to as general purpose enclosures.
NEMA Type 3R (Conforms to IP52) - These enclosures are designed for outdoor use where falling rain, sleet or external ice might form. They have a gasket on the cover to keep out water. Some versions have a top hinged front cover which must be opened to gain access to the circuit breaker handle. Other versions have an external side operated handle mechanism. These enclosures are often referred to as raintight enclosures.
NEMA Type 4 (Conforms to IP65) - These enclosures are designed for use either indoors or outdoors. They provide protection from splashing water, wind blown dust or rain. They even protect the circuit breaker from hose directed water. They are well suited for application in dairies, breweries, paper mills, food processing plants and other process industries. These enclosures are often referred to as watertight enclosures.
NEMA Type 4X (Conforms to IP65) - These are much the same as the Type 4 except that they are made of gasketed, stainless steel. In some designs, they are made of a nonmetallic material. They provide better resistance to corrosion than the Type 4. Industries that have a high amount of corrosive liquids, require a high measure of hose-down cleaning, or are in a salt-water environment use these enclosures. They are often referred to as corrosion-proof enclosures.
NEMA Type 12 (Conforms to IP62) - These enclosures are designed for indoor use in dirty and dusty applications. They are constructed of sheet metal and provide protection from dripping liquids (non-corrosive), falling dirt and dust. A special NEMA 12K version provides Knockouts for conduit. These enclosures are often referred to as dust-tight enclosures.
There are other ratings of enclosure types, but these are the most commonly used.

The selection of molded case circuit breakers is generally determined in two ways:
• For protection of non-motor circuits.
• For protection of motor circuits.
We will discuss protection of non-motor circuits first.
Protecting Non-Motor Circuits
Non-motor circuit applications usually center around cable protection and require molded case circuit breakers with both overload and short circuit capabilities. They should be able to distinguish between harmless and destructive conditions, and function appropriately for its application. It is very important that the MCCB selected be adequately rated and equipped for all the electrical and physical conditions that are likely to exist when the system is energized.
The standard selection factors for molded case circuit breakers include:
• Voltage Rating
• Frequency
• Continuous Current Rating
• Interrupting Rating
• Number of Poles
• Fixed or Interchangeable Trip Unit
• Trip Unit Functions
• Accessories
Protecting Motor Circuits
The selection of an HMCP is based on the full load current of the motor it is to protect. Data shown in the National Electric Code (tables 430-148 and 430-150) list the full load currents of induction motors running at speeds normal for belted motors and with normal torque characteristics. However, actual motor nameplate ratings should be used for selecting the motor running overload protection.
Other considerations in the selection of HMCPs include:
• The ambient temperature outside the enclosure should not exceed 40°C (104°F).
• Infrequent starting, stopping and reversing of the motor.
• Motor accelerating time of 10 seconds or less.
• Locked rotor rating is a maximum of six times the motor FLA rating.

Accessories and Modifications

2009-01-27T02:34:00.004-08:00

Handle Operating Devices
Handle operating devices provide indirect electrical or manual operation of the circuit breaker handle.Electrical operators provide complete remote control of a molded case circuit breaker by means of a pushbutton or similar pilot device. When energized from a remote location, the operator mechanism moves the circuit breaker handle to either the ON or OFF position. Shunt trips and undervoltage release mechanisms can only be used to trip the breaker. Electrical operators deliver a positive switching action. In case of a power failure, means are provided for manual operation. They come in a variety of designs and, depending on the circuit breaker type, are mounted in different ways.

The newer designs are front mounted on the breaker cover and fit within the trim line of the circuit breaker. For smaller frame breakers, a solenoid is used. On larger frame sizes, a motor is used to provide the increased operational force required to move the breaker handle.

Front Mounted Electrical Operator (Solenoid Type)

Some designs are side mounted and use an extended arm to move the circuit breaker handle. These older motor driven operators are not usually suitable for generator synchronizing because of the time it takes for operation.

Side Mounted Electrical Operator (Motor Type)

Contemporary electrical operator designs, whether solenoid or motor driven, are capable of performing a closing operation in five cycles or less. This makes them very suitable for generator synchronizing applications.

Handle mechanisms allow for the manual operation of the circuit breaker toggle handle. These mechanical devices allow personnel to open and close the breaker without opening the enclosure in which the breaker is mounted. They come in a variety of designs for different applications and enclosure types.

The flex shaft type handle mechanism is an extra heavy-duty mechanism designed for mounting in flange-type enclosures. An operating handle, flexible shaft and mechanism are required for standard application.

Flex Shaft Handle Mechanism

Handle mechanisms are often designed to indicate the status of the circuit breaker because the breaker itself is often not visible when the handle operates. Some designs allow for protection against tampering, or to tag and lockout during maintenance. The unit shown here will accept up to three padlock shackles.

Safety Handle Mechanism

Rotary handle mechanisms mechanically transfer the rotating operation to the in-line toggle operation of the circuit breaker handle. Depending on their design, they may allow mounting an auxiliary switch on the handle for undervoltage release. Some devices are available with red handles and yellow background labels to meet local codes.

Rotary Handle Mechanisms

Lock and Interlock Devices
Lock and interlock devices are used to deter undesired circuit breaker operation and establish interlocked control systems. They do not interfere with the trip-free operation of the molded case circuit breaker.

The nonlockable handle block secures the breaker handle in either the ON or OFF position to prevent accidental operation.

Nonlockable Handle Block

The padlockable hasp mounts on the circuit breaker cover and will allow multiple padlocks to secure the handle in either the ON or OFF position.

Padlockable Hasp

The cylinder lock internally blocks the breaker trip bar in the tripped position. This prevents the breaker from being switched ON. However, use of a cylinder lock may reduce the interrupting rating of the breaker.

Cylinder Lock

A key interlock is used to externally lock a circuit breaker handle in the OFF position. An extended deadbolt blocks movement of the breaker handle. The key is only removable in the locked position.

Key Interlock

To prevent adjacent breakers from being switched to the ON position, a sliding bar interlock can be used. When the sliding bar handle is moved from one side to the other, a bar extends to alternately block the breaker handle.
Sliding Bar Interlock

Along the same lines, the walking beam interlock provides mechanical interlocking between breakers of the same pole configuration. It mounts on a bracket behind and between the breakers.

Walking Beam Interlock

Miscellaneous Devices
Certain molded case circuit breakers can have their interrupting rating increased to 200,000A symmetrical at up to 600V. The device used for this is called a current limiting attachment. It is bolted to the load end of the circuit breaker so that it will interrupt normal fault current up to 50 or more times the breaker's continuous rating. The limiter will only trip when a very high fault is encountered. As the limiter trips, the breaker is also tripped magnetically, preventing a single-phasing condition.
A spring loaded indicator is on each pole of the limiter to identify the faulted phase. When a current limiter trips, it is an indication of a serious circuit problem that must be corrected before restoring service. To ensure the proper limiter is used with a compatible breaker, they are always of a noninterchangeable design.

Current Limiting Attachment

Another device designed to protect personnel and equipment is the low level earth leakage (ground fault) protector. This device usually trips the circuit breaker through a shunt trip or undervoltage release mechanism when it detects a low level ground fault.

Earth Leakage Protection

Accessories and Modifications

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When a comparison is made between a fusible switch and a molded case circuit breaker, it is easy to see the application flexibility MCCBs provide. This is even more apparent when you look at the array of accessories and modifications available. We are going to review in general what is available with a brief explanation of their purpose and applications. We will not cover specific accessories for specific lines of breakers in this module.

We are going to divide our review into the following categories:
• Operational Devices
• Termination Devices
• Handle Operating Devices
• Lock and Interlock Devices
• Miscellaneous Devices

Operational Devices
A shunt trip provides remote controlled tripping of a molded case circuit breaker. A solenoid coil is remotely energized using a pilot device, such as a pushbutton. That moves the plunger to activate the trip bar of the breaker. At the same time, a cutoff switch operates which disconnects power to the solenoid so the coil doesn't burn out. Often, pigtail leads are supplied for connecting the shunt trip to either an AC or DC control power source

In the workplace
One application for a shunt trip is for use on a welding machine. Normal thermal overload protection is not practical because of the high, frequent cycling of the machine. Often, the machine is equipped with a Thermistor to provide overload protection. The Normally Open (NO) contacts of the thermistor close when it reaches a preset temperature.

Shunt Trip Application Example

The thermistor is connected to a shunt trip on a magnetic only circuit breaker. The breaker provides short circuit protection, the thermistor provides the overload protection.
An undervoltage release mechanism trips the breaker whenever the voltage falls below a predetermined level. These undervoltage release mechanisms (UVRs) come in two different styles:
The handle reset UVR (standard on current breakers) consists of a continuous rated solenoid with a plunger and tripping lever. The UVR mechanism is reset by a tripping lever when normal voltage has been restored, and the circuit breaker handle is moved to the reset OFF position. With no voltage applied to the UVR, the circuit breaker contacts will not touch when a closing operation is attempted.

Handle Reset UVR

An automatic reset UVR (standard replacement type breakers) has a tripping lever extension for resetting during the tripping action cycle. It works like the manual reset UVR except there is no plunger to be reset. When the breaker trips, it resets the UVR mechanism.
It is important to point out, that undervoltage release mechanisms are not designed to be used as circuit interlocks.

Due to long lengths of cables from power supplies in underground mining, low voltage conditions are common. To protect personnel and equipment, the circuit breaker trips. It cannot be energized until the power has been restored to at least 85% of the coil rating on the undervoltage release mechanism.

Providing circuit breaker main contact status, an auxiliary switch is mounted in the breaker. In this diagram, the contacts are shown as "A" and "B". An "A" contact is open when the breaker is open or tripped. A "B" contact is closed when the breaker is open or tripped. The contacts are rated 120V for pilot duty.

Auxiliary Switch Contacts

If you wanted to give a visual indication that a circuit is energized, you could mount an indicating light on the panel. Using an auxiliary switch with an "A" contact would allow the light to be illuminated whenever the breaker is closed. When the breaker trips, the light goes off, letting you know the breaker has tripped or been opened.
Auxiliary switches can be used for circuit interlocking purposes. The NEC requires that motor control circuits be disconnected from all sources of supply when the disconnect means is in the open position. For starters with common control, one circuit breaker would disconnect the voltage for both the power circuit and the control circuit. If the control circuit has a separate power supply, the circuit breaker would not disconnect that supply source. You could use separate disconnect means, or simply use an auxiliary switch in the breaker. When the breaker is open, the auxiliary opens the control circuit, disconnecting it from its supply source.

Auxiliary Contacts Disconnect Separate Control Power Source

Alarm switches differ from auxiliary switches in that they function only when the breaker trips automatically. The normally open contact of the switch closes when the breaker trips due to a short circuit, an overload condition, or when operated by a shunt trip. An indicating light, buzzer or bell can be placed in the circuit to provide indication that the circuit has tripped. When the breaker is reset, the alarm switch is reset. A manual opening of the circuit breaker does not affect the alarm switch contact.

Termination Devices
Line and load terminals provide the means for connecting the circuit breaker to the power source and the load. They are sized for various continuous current capabilities and wire types. Depending on the breaker, it may be supplied factory equipped with both line and load lugs, load terminals only, or line and load terminals for field installation.
The terminal body is usually made of aluminum to accommodate aluminum or copper wire or cable. Terminals with copper or steel bodies are also available. Terminals are available that support single or multiple conductors.

Typical Termination for Use with 3/0-250 MCM Conductors

A keeper nut slides onto the line or load conductor and acts as a threaded adapter to accept a ring terminal or other bolt-on connector.

Typical Keeper Nut

A plug nut is used where screw connected ring terminals are preferred to connect cables to the breaker conductors. They are press-fit into the opening in the breaker terminal conductor.

Plug Nut with Ring Terminal

Endcap kits are used to connect bus bar or similar electrical connections.

Endcap Kit

Rear connected studs are available in different sizes to accommodate specific fixed mounted breaker applications and ratings. The breakers are front removable from switchboards and other equipment by unscrewing the nut that holds the stud to the breaker. Studs must be assembled in accordance with UL required clearances.

Rear Connected Studs

As with the rear connected studs, plug-in adapters simplify installation and front removal of the breaker from applications such as switchboards. Tulip-type connectors and threaded studs or flat bus bars are built into the molded support block and connect to the main bus.

Plug-in Adapters

To connect circuit breaker terminals to the panelboard bus, panelboard connecting straps can be used. Because depth and bus spacing vary depending on type of panel and manufacturer, the panelboard builder should be consulted to determine the correct strap.

Panelboard Connecting Straps

So far, these termination devices have been a means for connection of the circuit breaker to the power source and load. The next few items offer terminal isolation and protection.
In motor control center applications, because of confined spaces, line side conductors are often custom fitted. Terminal end covers fit together with the circuit breaker case to form terminal compartments. This allows isolation of the discharged ionizing gasses that form during the tripping operation.

Terminal End Covers

To provide protection against accidental contact with live line side terminations, terminal shields fasten over the front terminal access openings.

Terminal Shield

Interphase barriers are high dielectric insulating plates that fit between the terminals to provide additional electrical clearance between circuit breaker poles.

Interphase Barriers

Terminal covers provide the required electrical clearance between circuit breaker poles when extended terminals are used.

Terminal Covers

Motor Circuit Protectors

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Because special considerations need to be taken when using circuit breakers with motors, we will dedicate this section on their particular characteristics and applications.

Most faults on a motor circuit are caused by a breakdown of the insulation within the motor windings. The initial fault current is usually low when compared to the overall system capacity. However, because it causes an Arcing condition, it could cascade and short out more and more of the motor windings. If the fault is allowed to continue, serious motor and starter damages occur, increasing repair costs. Although fusible switches and thermal magnetic breakers can provide motor branch circuit protection, the level of protection is not as effective against this type of fault.
For this reason, the motor circuit protector was developed. A motor circuit protector (MCP) operates on a magnetic only principle. It has a specially designed current sensing coil in each of its three poles to provide sensitive low level protection. It can clear a fault faster than a fusible device. It does not, however, provide overload protection for the motor. As a result, a contactor with an overload relay or motor starter must be used in conjunction with the motor circuit protector. (See Module 19 for information on contactors, overload protection and starters.)

Fuse vs. Circuit Breaker
This chart shows the typical fault Clearing Time (Fuse) of a dual element fuse on Low Level Faults. It required from 12 to 84 cycles to clear the fault. This test was based on a 3 hp motor with an FLA of approximately 4.2A. The short circuit was 150A, or 35 times the full load current. The fusible device had a 5A dual element fuse, sized at 120% the FLA. The NEC allows fuses to be sized up to 175% of motor full load current.

Fault Clearing Time of Dual Element Fuse

When an MCP rated at 7A was used, it was able to clear the fault in less than one cycle. It was set to trip at 51A, or approximately 12 times the full load current. The MCP also did not create a single-phasing condition, whereas the dual element fuse did. The fuse cleared the fault in phase B in only 12 cycles, but did not clear it in phases A and C until 22 cycles had elapsed.

Fault Clearing Time of MCP

In a single-phasing condition, the current to a motor in the remaining phases increases significantly above normal. This can lead to severe equipment damage. Because the motor circuit protector cleared the fault in all three phases in less than one cycle, the motor failure did not result in starting an electrical fire.

Components
The components of motor circuit protectors are very similar to molded case circuit breakers. They are:
• Molded Case
• Operating Mechanism
• Arc Extinguishers
• Contacts
• Trip Mechanism
• External Adjusting Mechanism
Motor Circuit Protector Components

How It Operates
Motor circuit protectors disconnect the motor load from an electrical supply under three conditions. They are:

• When the handle is switched to OFF.
• When an automatic trip operation occurs.
• When a manual trip is initiated with a push-to-trip button.

As with the molded case circuit breakers, the operating mechanism is a spring-loaded toggle that provides quick-make, quick-break and trip-free operation.
Its design provides an increased air gap between the stationary and moveable contacts when in the tripped position. This air gap results in greater arc extinguishing during contact opening and provides higher interrupt ratings.

The magnetic trip unit operates when a fault current exceeds the magnetic pickup setting. It consists of an electromagnetic coil and plunger assembly. Certain HMCPs also have a transient inrush trip suppression device. This allows the startup of energy efficient motors without nuisance tripping the sensitive short circuit protection of the current sensing coil.
A tuned spring introduces a time delay of approximately 8 ms into the trip sequence under normal conditions. It allows the HMCP to ignore the initial high inrush current during the first half-cycle of start-up. A true fault current would supply a magnetic force to override the spring action and provide instantaneous tripping of the device.

Transient Inrush Suppressor

Larger HMCPs (600A) and above may use electronic trip units to provide an adjustable three-phase instantaneous trip setting. At start-up, an electronic time delay acts as the inrush trip suppressor. However, any current in excess of the predetermined setting level, such as with a short circuit, would override the time delay and trip the HCMP.
A trip setting adjustment allows for precise motor protection. Press in on a cam and turn the arrow until it is aligned with the required trip setting shown on the nameplate.
However, in keeping with NEC requirements, they cannot be set at more than 1300% of the motor full load current rating.

Trip Setting Adjustment

Applications
Motor circuit protectors can be used in combination starter units within a motor control center. They allow for protection against both low and high level fault currents without requiring current limiters. They can also be applied in standalone combination starters.
When properly sized, they can provide short circuit protection for resistance welding devices. The normal high welding currents can flow, but the HMCP trips instantaneously if a short circuit develops.

HMCPs can be used in panelboards. You can have both distribution branch circuit protection and protection of the motor circuits within the same enclosure.

Ratings and Environment

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When selecting the proper circuit breaker for an application, the ratings and environment need to be considered.

Ratings
The voltage rating of a circuit breaker is determined by the maximum voltage that can be applied across the terminals, the type of distribution system and how the breaker is being applied in the system.

The voltage system of 480Y/277V is the most common found in commercial and institutional buildings. It has a solidly grounded neutral. This system is also very prevalent in industrial plants and some high-rise residential buildings.
When a breaker is applied in a panelboard, it is important that it have the lowest possible voltage rating that will do the job and meet the specifications. It can save the customer a lot of money if the breaker is chosen.
A 2-pole, 480/277V breaker can be used on this system because it is a three-phase, 4-wire, grounded system. The maximum line to ground voltage is 277 volts across one pole of the breaker.
That is not the case in a three-phase, 3-wire Delta system.

Here, a fault condition could occur that would allow the breaker to see the full 480V across one pole. UL requires that each pole of the breaker be rated to interrupt this full 480V.

The continuous current rating of a molded case circuit breaker is the amount of current it is designed to carry in open air. The breaker has a specific ampere rating and is Ambient Compensated. Most manufacturers calibrate their breakers for a 40°C (107°F) ambient. The National Electric Code (NEC) allows a breaker to be applied to a maximum of 80% of the breaker's continuous current rating. Some manufacturers offer breakers that can be used at 100% if they are specifically designed and tested for such use. They must also specify the minimum size enclosure, ventilation needs and conductor size for the application.

The Interrupt Rating of a molded case circuit breaker is the amount of fault current it can safely interrupt without damaging itself. The interrupt rating must be equal to or greater than the amount of fault current available at the point in the system where the breaker is applied. The interrupt rating always decreases as the voltage increases. The interrupt rating is one of the most critical factors in the breaker selection process.

Most molded case circuit breakers retain the same tripping characteristics whether they are applied to a 50 Hz or 60 Hz system. On higher frequency systems, the breaker may need to be specially calibrated or derated. A molded case circuit breaker that has a thermal magnetic trip unit might not have the same thermal or magnetic performance at a higher frequency than 60 Hz. MCCBs with electronic trip units require special derating factors and cables or Bus at higher frequencies.

The number of poles of a molded case circuit breaker is determined by the type of distribution system in which it is applied. Except in certain special applications, each hot conductor is considered a pole. For single-phase applications with a grounded neutral, a single-pole breaker can be used. Two-pole and three-pole breakers are used in three-phase systems.

Environment
Thermal magnetic breakers can be affected by large differences in the ambient temperature. At ambient temperatures below 40°C, the breaker carries more current than its continuous current rating. The mechanical operation of the breaker could be affected if the temperature is significantly below the 40°C standard. The breakers will carry less current than their continuous rating if the temperature is above 40°C, and could cause Nuisance Tripping. It could also cause unacceptable temperature conditions at the terminals of the breaker.
Electronic trip circuit breakers often have a wider temperature range (-20° - 55°C) and so are less susceptible to ambient temperature fluctuations. At very low temperatures, the mechanical parts of the trip unit could require special lubrication. At very high temperatures, the electronic circuitry components could be damaged. Some MCCBs with electronic trip units have special self-protection circuitry to trip, should the internal temperature rise to an unsafe level.
An atmosphere that has a high moisture content or the presence of corrosive elements should be avoided. Electrical equipment should be mounted in clean, dry environments. When moist conditions cannot be avoided, special fungus treatments may be needed. Although the glass-polyester molded cases may not support the growth of fungus, terminals and other parts may. If changes in temperature create condensation, space heaters in the enclosures may be required.
Because the air is thinner at high altitude, it reduces the cooling and dielectric characteristics from those of denser air found at lower altitudes. Circuit breakers must be derated for voltage, current and interrupting ratings at altitudes above 6000 feet.

Special shock resistant breakers must be used for installations subject to high mechanical shock. Special installed anti-shock devices hold the trip bar latched under shock conditions, but don't inhibit the proper functioning of the breaker for short circuits or overload conditions.
A large steel manufacturer was experiencing a lot of nuisance tripping of circuit breakers located near the smelting side of their factory. Upon investigation, it was determined the circuit breakers had thermal magnetic trip units. The ambient temperature at that end of the factory was often around 110°F (43°C), particularly in the summer. The thermal magnetic breakers were only rated for 104°F (40°C).

The solution? They changed to breakers with electronic trip units that had a wider temperature range (up to 131°F/55°C) which easily handled the ambient heat, even on the hottest summer day.

Circuit Breaker Components

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Although there are many types of molded case circuit breakers manufactured, all are made up of five main components. These are:
• Molded Case or Frame
• Operating Mechanism
• Arc Extinguishers
• Contacts
• Trip Units

Frame
The function of the frame is to provide an insulated housing to mount all of the circuit breaker components. The frame is often of a glass-polyester material or thermoset composite resin that combines ruggedness and high dielectric strength in a compact design. The frame is also known as a molded case.

A frame designation is assigned for each different type and size of molded case. This designation is used to describe the breaker's characteristics such as maximum voltage and current ratings. However, each manufacturer has their own identification system to account for the differences between breaker characteristics.

Operating Mechanism
The operating mechanism is the means to open and close the contacts. The speed with which the contacts open or close is independent of how fast the handle is moved. This is known as Quick-Make, Quick-Break. The breaker cannot be prevented from tripping by holding the handle in the ON position. This is known as Trip-Free. The handle position indicates the status of the contacts - closed, open or tripped. When the contacts are in the tripped position, the handle is in a midway position.

To restore service after the breaker trips, the handle must be moved first to the OFF position from its center tripped position. Then the handle must be moved to the ON position. When breakers are mounted in a group, as in a panelboard, the distinct handle position clearly indicates the faulted circuit. Some breaker designs also incorporate a push-to-trip mechanism. This allows a manual means to trip the breaker and test the mechanism.

Arc Extinguisher
Whenever a circuit breaker interrupts current flow, an arc is created. The function of the arc extinguisher is to confine and divide that arc, thereby extinguishing it. Each arc extinguisher is made up of a stack of steel plates held together by two insulator plates. When an interruption occurs and the contacts separate, the current flow through the ionized region of the contacts induces a magnetic field around the arc and the arc extinguisher. (Module 5, "Fundamentals of Circuit Breakers," covers this topic in detail.)

The lines of magnetic flux created around the arc and its force drives the arc into the steel plates. The gas goes through Deionization and the arc divides, allowing it to cool.

Standard molded case circuit breakers use a linear current flow through the contacts. Under short circuit conditions, a small blow-apart force is created, which helps open the contacts. The majority of the opening action comes from mechanical energy stored in the trip mechanism itself. This is because the current in both contacts are going in the same direction and attract each other.

Newer design breakers use a reverse loop of current flowing in essentially opposite paths. This creates a repulsion action and results in a greater blow-apart force. This force assists with rapid arc extinguishing by causing the contact to open faster. The force is directly proportional to the size of the Fault Current. The greater the fault, the greater the force, and the faster the contacts open.

Trip Unit
The trip unit is the brains of the circuit breaker. The function of the trip unit is to trip the operating mechanism in the event of a short circuit or a prolonged overload of current. Traditional molded case circuit breakers use electromechanical (thermal magnetic) trip units (Module 5 covers this in more detail). Protection is provided by combining a temperature sensitive device with a current sensitive electromagnetic device, both of which act mechanically on the trip mechanism. Electronic trip units are now available and they can provide much more sophisticated protection and monitoring.

Most molded case circuit breakers utilize one or more different trip elements to provide circuit protection for different applications. These trip elements protect against thermal overloads, short circuits and arcing ground faults.

Conventional MCCBs are available with either a fixed or interchangeable electromechanical trip unit. If a new trip rating is required for a Fixed Trip breaker, the entire breaker must be replaced. With an Interchangeable Trip Unit, as its name implies, only the trip unit needs to be changed, up to the maximum current rating of the breaker frame. Interchangeable trip units are also called Rating Plugs. Some breakers offer interchangeability between electromechanical and electronic trip units within the same frame.
To provide short circuit protection, electromechanical trip circuit breakers have adjustable magnetic elements.

To provide overload protection, electromechanical trip circuit breakers contain thermal trip elements. Breakers using the combination of magnetic elements and thermal elements are often called thermal magnetic breakers.

Increasingly, molded case circuit breakers with conventional thermal magnetic trip units are being replaced by breakers with electronic trip units. These units provide increased accuracy and repeatability. Some units have built-in ground fault protection, removing the need for separate ground fault relays and Shunt Trips. Some units can also provide system monitoring, data gathering and communication to energy management systems.
In general, electronic trip systems are composed of three components:
• A current transformer (sensor) is used on each phase to monitor the current. It also reduces the current to the proper level for input to a printed circuit board.
• Electronic circuitry (printed circuit board) that interprets the input and makes a decision based on predetermined values. A decision to trip results in sending an output to the next component.
• A low power flux-transfer internal shunt trip that trips the breaker. This is typically a mechanical, spring loaded device held in place by a permanent magnet.

Welcome to Module 8

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Welcome to Module 8, covering Molded Case Circuit Breakers. In previous modules, you have learned about the fundamentals of circuit breakers (Module 5), medium voltage power circuit breakers (Module 6), and low voltage power circuit breakers (Module 7). In this module, we will specifically cover molded case circuit breakers (MCCBs), where they are used, their components and accessories.
This module is intended to be a continuation of the study of circuit breakers. You should have a good understanding of the concepts discussed in Module 5. Because various terms and phrases will be used in this module without providing further explanation, you might wish to review Module 5 before proceeding.

Typical Collection of Industrial and Miniature Molded Case Circuit Breakers

Like the other modules in this series, this one presents small, manageable sections of new material followed by a series of questions about that material. Study the material carefully then answer the questions without referring back to what you've just read. You are the best judge of how well you grasp the material. Review the material as often as you think necessary. The most important thing is establishing a solid foundation to build on as you move from topic to topic and module to module.

Ratings and Performance

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The rigid frame housing also provides for the rigid mounting of circuit breaker components which:
• Improves the mechanism's life by eliminating frame deflection
• Provides for the consistent operation of mechanism
• Eliminates the need for mechanism adjustments

Some of the most useful information about a low voltage power circuit breaker can be found in its ratings table. Ratings tables were briefly covered during the Circuit Breaker Selection discussion. The ratings published in these tables are backed up by testing as outlined in applicable standards. These ratings tell quite a story. They indicate how a particular circuit breaker will perform during a given set of application circumstances. In a similar regard, the ratings indicate what circuit breaker should be selected for a specific application.
Although ratings tables from different manufacturers often look similar and even reflect many of the same ratings, it is not a good idea to assume that all low voltage power circuit breakers have the same ratings. Get into the habit of reviewing different tables and comparing the published ratings. There is no room for unexpected surprises in circuit breaker selection.

To demonstrate the value of making these comparisons, let's once again look at a partial ANSI ratings table, contrasting the ratings for one type low voltage power circuit breaker (Type XYZ) with those of the Magnum DS low voltage power circuit breaker (Figure 44). Once you have taken the time to look the table over a bit, several comparison examples will be discussed. Keep in mind that this table only presents excerpts and is not intended to show the ratings of all available circuit breakers for an entire line. It can be assumed that both circuit breakers used in the examples meet all applicable ANSI standards for low voltage power circuit breakers.
The comparisons to be discussed are identified in the table by a circled letter, for example.
A - Notice first that the ratings given for the Magnum DS power circuit breaker apply for all system voltages. Ratings for other power circuit breakers can vary by voltage. Uniform ratings across all application voltages for the 3200 ampere frame circuit breakers and below simplifies the selection process. Interrupting ratings higher than 100,000 amperes are available with Magnum DS frame sizes above 3200 amperes for certain application voltages. Therefore, the uniformity is not totally maintained on the larger frame circuit breakers.
B - The short time rating (withstand) for Magnum DS once again applies across the entire range of application voltages, while the other is only applicable to 480 volts. In addition, notice that Magnum DS has higher short time current ratings than does the other power circuit breaker. A broader range and higher short time ratings allow Magnum DS to be applied in systems with higher available short circuit currents while maintaining full selectivity.
C - An interrupting rating of 100,000 amperes is available with 3200 ampere frame and below Magnum DS circuit breakers at all application voltages. This rating is not at all available with the other power circuit breaker. Notice further that the short time rating is 85,000 amperes (3200 amperes and below). What does this accomplish?
1. These ratings mean that this particular Magnum DS rating has a hefty short time (withstand) capability and can withstand a short circuit of 85,000 amperes for the full 30 cycles required by ANSI.
2. It also means that this Magnum DS rating can be applied on a system that could experience a short circuit as high as 100,000 amperes. The circuit breaker would be selective up to 85,000 amperes, and would open instantaneously from 85000 to 100,000 amperes.
D - One more area on a ratings table to be alert to is with and without instantaneous trip. It is possible for some circuit breakers to have higher interrupting ratings if they can trip instantaneously. In those instances, the interrupting rating might be lower if the circuit breaker is required to provide short time protection. This is not the case, however, with Magnum DS at interrupting ratings of 85,000 amperes and below.
As a general rule, the ratings for most ANSI rated low voltage power circuit breakers are similar where system requirements are less stringent. When the application requirements become more stringent and/or special, the capabilities of different power circuit breakers begin to differ. Look at ratings tables closely. They have a story to tell.
Extended Interrupting Ratings: Interrupting ratings can be extended up to 200,000 amperes by providing a combination of a low voltage power circuit breaker connected in series with current limiting fuses (current limiters). This combination is provided on systems where the overload and switching functions of the circuit breaker are required, and available fault currents could exceed the interrupting rating of the circuit breaker by itself. For smaller frame circuit breakers, the current limiters usually are integrally mounted in the circuit breaker (Figure 45). For larger frame circuit breakers, the current limiters are usually mounted in a separate drawout truck and positioned adjacent to the circuit breaker.

Altitude and Ratings: Low voltage power circuit breakers are applicable at their full voltage and current ratings up to a maximum altitude of 6600 feet (2000 meters) above sea level. When a circuit breaker is installed at higher altitudes, the ratings are subject to correction factors in accordance with ANSI Standards. Fortunately, you are not often faced with this situation. Even if you are, it probably has already been taken into account by the specifier. It is good, however, to be aware of such application exceptions.
Current Waveforms and Ratings: The subject of current waveforms and their effect on circuit breaker ratings will not be discussed in great detail in this module. Once again this consideration is usually made by the specifier and taken into account when a particular circuit breaker is specified. You should be somewhat familiar with the concepts. If for no other reason, it could help to explain why a certain circuit breaker rating selection was made for a particular application. It will also be good background information later in this module when applications are discussed. You noticed in the ratings table just discussed that interrupting ratings and short time ratings were expressed in symmetrical amperes. For most discussions and selections involving low voltage power circuit breakers, this will be the case. Without getting into too much detail, it is important to know that there are two common ways to rate protective devices in amperes. These ratings are symmetrical and asymmetrical. The symmetrical and asymmetrical ampere ratings can be quite different for the same device. To clarify the significance of the two terms, let's briefly discuss each individually.
The graphic shown of a Symmetrical AC Current represents a fault current flowing in a circuit (Figure 46). The fault current has a sine wave shape and is symmetrical with respect to the horizontal axis. That is, the current rises and falls equally above and below the horizontal axis, so the shapes are symmetrical.
The graphic shown of an Asymmetrical Current becoming Symmetrical represents an offset fault current wave (Figure 47). It rises above the horizontal axis considerably more than it goes below for the first few cycles. This wave is said to be offset from or asymmetrical with respect to the horizontal axis. This condition occurs in circuits containing reactance which are short-circuited at some time other than when the current is passing through the zero point on the cycle. It occurs in all 3-phase circuits in one or more phases. When this happens, a DC current is superimposed on top of the AC current causing asymmetry. The DC component actually decays to zero within a short time after the fault occurs. The final decay of the DC component signifies a change from asymmetrical to symmetrical. How fast this actually happens depends upon the quantity relationship of reactance and resistance in the circuit, the X/R ratio. The more resistance in the circuit, the faster the DC component decays, or the larger the X/R ratio, the longer the decay time.

Asymmetrical Current Becoming Symmetrical as DC Components Decay

Continuing this discussion of current waveforms, let's take a look at some background information. Low voltage power circuit breakers typically part their Contacts after several cycles of fault current, assuming there is no intentional time delay. In short, contact parting only begins once time delays are expired. The circuit breaker can be called upon to interrupt more than the symmetrical value of fault current, as calculated from the impedance of the circuit. This is because of the presence of the DC component just discussed.
We will not discuss here why the degree of asymmetry can be different, just know that the degree can vary. What is of real importance is the rate at which the DC component decays and the change to symmetrical takes place. This rate, and hence the current value, can be related directly back to the ratio of circuit reactance to circuit resistance or the X/R ratio.
For this reason, the X/R ratio is a significant and specified factor for standards testing. It is also an important ratio to know when selecting circuit breakers for application on a system. As long as the X/R ratio for the system does not exceed the tested X/R for the circuit breakers, you are home free.
If the system X/R ratio exceeds the tested X/R for the circuit breakers, the circuit breakers would have to have their published interrupting capacity and short delay current capability de-rated. A circuit breaker with a higher interrupting capacity and higher short delay current rating might be needed to accommodated the de-rating factor.
As previously mentioned, these types of system determinations are normally made well ahead of time by the specifier or consultant, which is when the equivalent system short circuit rating is normally specified.
For your information, the ANSI tested X/R ratio for power circuit breakers is 6.6. This 6.6 ratio was not always the case. As a matter of fact, power circuit breakers were not always rated in symmetrical terms. That is history and we will not discuss why a change was made from asymmetrical to symmetrical.

At some point, the industry made the change and determined that an X/R ratio of 6.6 for power circuit breakers was typical, and would be a good base to work with for testing and application. At least all the manufacturers were working with the same standardized starting point. Now, when the calculated short circuit current X/R ratio for a particular system is higher than the standard 6.6 X/R ratio for low voltage power circuit breakers, a table can be consulted to determine what de-rating factor should be applied to the circuit breaker's interrupting rating to insure proper circuit breaker sizing and selection.

To give you an idea of what these de-rating factors look like, refer to the partial table of low voltage power circuit breaker de-rating factors (Figure 48). Keep in mind that these considerations and decisions must be made for all types of circuit breakers, not just low voltage power circuit breakers. Before this discussion is concluded, let's take a look a one simple example.

Example: The total available fault current from all sources that this low voltage power circuit breaker could see was calculated to be 48,000 amperes symmetrical. For this example, we are only considering what circuit breaker could be used to deal instantaneously with a potential fault of this magnitude. From some selection chart you might select a circuit breaker with a 50,000 ampere interrupting capability.

In this example that would not be a good selection because it has also been determined that the system X/R ratio is 9.94. You can see from the de-rating table that a 9.94 ratio equates to a de-rating factor of 0.937. This factor used on the circuit breaker's interrupting capability of 50,000 amperes reduces it to 46,850 amperes, not sufficient to deal with the potential fault current of 48,000 amperes [50,000 x 0.937 = 46,850].

Obviously, a circuit breaker with a higher interrupting capability must be selected for this application. If a circuit breaker with an interrupting capability of 65,000 amperes is selected, you can see from the calculation that it would do the job [65,000 x 0.937 = 60,905].

Construction Method

2009-01-25T03:53:00.004-08:00

You learned that low voltage power circuit breakers are essentially an assembly of parts on a metal frame or in an encased housing of insulating material. Because the makeup of both approaches was adequately discussed, the details will not be repeated. One point, however, that should be repeated centers around what applicable standards are required relative to the construction method. The frame construction used must hold all the circuit breaker parts in place and be capable of withstanding the tremendous physical forces and severe heating effects a power circuit breaker could be subjected to while performing its function. Standards do not specify the exact nature of the construction or the construction material. Those decisions are left to the circuit breaker manufacturer.

Obviously, the construction method and materials used must result in a strong, rigid design. For many years, the preferred approach was the open type metal-frame which had a number of pieces welded and/or bolted together. With the significant technological strides made in the areas of insulating material and molding processes, versatile rigid frame housings of high strength engineered thermoset composite resins have become available. Not only does the rigid frame housing type low voltage power circuit breaker meet the stringent requirements of ANSI, it exceeds them in a number of instances.

The Magnum DS circuit breaker uses a 3-piece construction:

A 2-piece engineered thermoset composite resin case completely encloses the current paths and arc chambers.
The operating mechanism sits on the front of the case and is electrically isolated from the current contact structures. It is in turn covered by an insulating front cover.

The rigid frame construction results in a more compact, lighter weight low voltage power circuit breaker. Previous circuit breaker designs with many parts that were once produced and attached individually to the frame can now be molded as an integral part of the rigid frame. The overall strength and rigidity of the engineered thermoset composite resin design can even contribute to higher performance capabilities by the power circuit breaker. This was the result with Magnum DS which has higher short time ratings (withstand) than previously available power circuit breakers, along with higher interrupting capabilities. In the way of review, remember that the short time rating consists of the following two components:

• Short delay current component (kA)
• Short delay time component (cycles)
In addition to the improved performance characteristics just mentioned, the rigid frame housing type low voltage power circuit breaker has individual arc chambers that (Figure 43):
• Insulate and isolate Arcing from other poles, the rest of the circuit breaker, and personnel
• Provide support for the current path pole assembly

Standards and Testing

2009-01-25T03:46:00.004-08:00

The standards that are applicable to low voltage power circuit breakers and the testing involved to prove compliance by a specific low voltage power circuit breaker design are in Module 5 and previously in this module. You learned from those discussions that standards and testing go to the heart of the matter. This is true from three very important standpoints:
1. This is the industry's determination as to whether or not a particular circuit breaker design is capable of meeting a wide range of published operational and physical requirements.
2. The proven and stated compliance to specific standards tells potential users that the equipment from the manufacturers under consideration all meet certain basic standards, which makes the user's evaluation process much simpler. Once this determination is made, a particular manufacturer can still gain an evaluated advantage by offering additional unique features and/or an operational design approach preferred by the user.
3. It is a solid way of defining specific types of circuit breakers within a larger general grouping. For example: The larger general grouping is "Low Voltage Circuit Breakers." Specific types within the Low Voltage Circuit Breaker grouping would be "Low Voltage Power, Insulated Case, Molded Case and Miniature."
As you can see, when a specific type circuit breaker is specified, such as a low voltage power circuit breaker, the specifier already knows what the base expectations are from each manufacturer.
You will recall, from both Module 5 and previous sections, a map of the world showing the standards most influential in different parts of the world (Figure 37). It bears revisiting the map again to emphasize the importance, in today's global economy, of having flexible designs capable of complying with all major standards around the world. In this module the emphasis will be primarily on ANSI and IEC Standards. You should never lose sight of the fact, however, that there are a number of other standards that can play a critical role in determining what equipment is acceptable for application in a given area of the world. Even local and/or individual city codes and requirements may have to be considered.

In previous modules, references other than ANSI and IEC were made with respect to standards and testing, such as UL, IEEE. There is a strong relationship between ANSI, UL and IEEE. As a matter of fact, you will notice in manufacturer's publications for low voltage power circuit breakers and even low voltage metal enclosed switchgear references made to all. The following two samples are typical statements you might encounter when reading publications for both the power circuit breaker and the metal enclosed switchgear:
Typical Low Voltage Power Circuit Breaker Statement: "Type XYZ low voltage power circuit breakers are UL listed, and built and tested to applicable NEMA, ANSI, IEEE and UL standards (ANSI C37.50, C37.13, UL 1066)."
Typical Low Voltage Metal Enclosed Switchgear Statement: "Type XYZ low voltage metal enclosed switchgear conforms to NEMA SG3, NEMA SG5, ANSI C37.20.1, ANSI C37.51 and UL1558."
It may seem to you like a confusing web at this point. Once the relationship is understood, it will be clear as to why these references are made. There will be no detailed discussion of the standards relating to low voltage metal enclosed in this module, only those relevant to the power circuit breaker. Keep in mind, however, it works the same way. The standards state different requirements for the different pieces of equipment, but the intent is the same - an uncompromised piece of equipment with proven performance capabilities.
For the purpose of this section, let's identify the key players as a minimum and elaborate on a couple. This should not be considered as a substitute for the standards themselves. For a full explanation of any standard, consult the standard itself for details and proper conformance instructions.
IEEE (Institute of Electrical and Electronic Engineers)
• IEEE is an objective technical organization made up of manufacturers, users, and other general interest parties.
• IEEE defines technical definitions, technical requirements, temperature limits, altitude correction, insulation limits, and service conditions. For electrical equipment, including switchgear, it supplies the test requirements for the low voltage power circuit breaker construction and test standards, namely ANSI C37.13 and ANSI C37.50.
NEMA (National Electrical Manufacturers Association)
• NEMA is an electrical equipment manufacturer only organization, such as Cutler-Hammer, General Electric, and Square D. NEMA defines preferred ratings, related requirements, and application recommendations.
• NEMA Standards normally cover additional information about a product of specific interest to the manufacturing community, which the American National Standards Committee does not include in its scope. NEMA votes on the suitability of standards for ANSI designation and adopts, by reference, the appropriate American National Standards.
• The applicable low voltage power circuit breaker NEMA Standard is SG-3, and it adopts ANSI C37.16 in its entirety.
UL (Underwriters Laboratories Inc.)
• UL is an independent, non profit, third party testing and certification company headquartered in Northbrook, Illinois. It functions to develop standards and to insure that equipment meets relevant published standards.
• UL also adopts otherwise recognized standards, and, in some instances, develops their own independent certification tests. In the case of low voltage power circuit breakers, the UL Standard is UL1066, which was previously mentioned. UL1066, entitled "Standard for Low Voltage AC and DC Power Circuit Breakers Used in Enclosures," calls for testing to demonstrate compliance with ANSI/IEEE C37.13 without change. A UL Label is affixed to the circuit breaker to indicate successful compliance.
CSA (Canadian Standards Association)
• The Canadian Standards Association is in the category of a major international standard. Its design and testing requirements are essentially the same as required by UL. In fact, harmonization programs between UL and CSA are ongoing to close the gap and/or eliminate differences. The Canadian Standards Association standard most associated with low voltage power circuit breakers is CSA 22.2-31 for Switchgear Assemblies.
ANSI (American National Standards Institute)
You were briefly introduced to ANSI. Now let's take the time to get to know ANSI much better because ANSI is the key to low voltage power circuit breakers. It is the recognized North American Authority on equipment standards.
ANSI's Purpose - ANSI is a nonprofit, privately-funded membership organization that coordinates the development of U.S. voluntary national standards, called American National Standards. It is also the U.S. member body to the non-treaty international standards bodies, such as the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC). ANSI serves both the private and public sectors' need for voluntary standardization.
ANSI's History - The institute was founded in 1918. It was prompted by the need for an umbrella organization to coordinate the activities of the U.S. voluntary standards system and eliminate conflict and/or duplication in the development process. The institute serves a diverse membership of over 1300 companies, 250 professional, technical, trade, labor and consumer organizations, and some 30 government agencies.
A simple yet very typical example of why ANSI came into existence can be related to the low voltage power circuit breaker. In the early days of low voltage power circuit breaker development, manufacturers and users were building and applying equipment with little thought given to uniform performance or design standardization. The C37 standard was developed and implemented to establish minimum performance standards for the circuit breaker and its physical design features.
The standard was meant to address even the smallest detail. A close button, for example, might not say close on it or it varied in color from one manufacturer to the next. These inconsistencies in design made products confusing for use by customers. This might seem to be one trivial point, but you can imagine how big the problem would be when compounded with every aspect of a low voltage power circuit breaker.
ANSI's Functions - ANSI functions to:
• Coordinate the self-regulating, due process consensus voluntary standards system
• Administer the development of standards and approve them as American National Standards
• Provide the means for the U.S. to influence development of international and regional standards
• Disseminate timely and important information on national, international and regional standards activities to U.S. industry
These standards are intended to provide guidance, direction, and requirements. Compliance to these standards does not, nor is it meant to limit manufacturers in construction, materials, or the technology used.
Specifically relating to power circuit breakers, ANSI standards are written by either the IEEE Switchgear Committee or NEMA. The electrical standards written by both of these organizations are reviewed and clarified by the Accredited Standards Committee (ASC) for power switchgear and power circuit breakers. The ASC standards group is entitled C37.
ANSI Defined Standards for Low Voltage Power Circuit Breakers - Although there are a multitude of ANSI standards relating to many different types of equipment, only those standards relating to low voltage power circuit breakers are outlined here. The intent is just to make you aware of just how many are applicable to just one category of electrical equipment. You will notice that each standard is followed by a specific year. As additions or changes are made to a standard, the year is altered to indicate the latest version. Obviously, staying on top of the latest version is an ongoing process. You should also note that each standard is given a broad word definition.
1. ANSI/IEEE C37.13-1990, "Low Voltage AC Power Circuit Breakers Used in Enclosures"
2. ANSI C37.16-1997, "Preferred Ratings Related Requirements and Application Recommendations for Low Voltage Power Circuit Breakers and AC Power Circuit Protectors"
3. ANSI C37.17-1997, "Trip Devices for AC and General Purpose DC Low Voltage Power Circuit Breakers"
4. ANSI C37.50-1989, "Test Procedures for Low Voltage AC Power Circuit Breakers Used in Enclosures"
5. IEEE Standard C37.100-1992, "IEEE Standard Definitions for Power Switchgear"
6. IEEE C37.20.1-1993, "Standard for Metal-Enclosed Low Voltage Power Circuit Breaker Switchgear"
7. ANSI C37.51-1989, "Standard for Switchgear - Metal-Enclosed Low Voltage AC Power Circuit Breaker Switchgear Assemblies - Conformance Test Procedures"
8. NEMA SG-3-1981, "Low Voltage Power Circuit Breakers"
9. UL1066-1993, "Standard for Low Voltage AC and DC Power Circuit Breakers Used in Enclosures"
This lengthy list gives you some indication why it is a matter of practicality when a manufacturer states that a piece of equipment is built and tested to all applicable NEMA, ANSI, IEEE and UL standards. It was also mentioned that a great deal of referencing to other standards takes place within the body of a specific standard. Successful testing and compliance with respect to one standard often includes automatic compliance with other standards. It is worth repeating one of the examples given.
Example: ANSI C37.13 details the physical attributes, such as Stored Energy, that a low voltage AC power circuit breaker must have to comply. At the same time, ANSI C37.50 references C37.13 and details how the described circuit breaker should be tested. The key here is that successful testing in keeping with ANSI C37.50 brings with it compliance to C37.13. There is no need to mention C37.13, when it is stated that the circuit breaker complies with C37.50.
IEC (International Electrotechnical Commission)
IEC presides over the standardization of equipment for a number of other parts of the world. In view of today's global markets, there is a significant amount of interaction between the organizations just discussed and IEC.
IEC 947-2 is a multi-part international testing standard covering a variety of devices, including circuit breakers of all types. It is entitled "Low Voltage Switchgear and Controlgear."
As far as IEC is concerned, every device tested to IEC 947-2 must be subjected to several test sequences in order to be approved. Because IEC 947-2 covers both low voltage power circuit breakers and low voltage molded case circuit breakers, the exact test sequences performed are not necessarily the same. They depend on the category of the device.
Category A Device - In general, this is a device without a short time Withstand Rating, such as a molded case circuit breaker.
Category B Device - This is a device with a short time withstand rating, such as a power circuit breaker and certain molded case circuit breakers. Typically, these devices are referred to as Air Circuit Breakers or just ACBs.
IEC 947-2 was developed with assistance from members of the U.S. National Committee. Still, a number of significant differences exist between IEC 947-2 and applicable ANSI standards. In particular, the various ratings of a circuit breaker can differ when tested to each standard. Therefore, any product comparisons made between products tested to these different standards (domestic versus international) should only be made with a thorough understanding of the differences.
Standards Conclusion
This might seem to be a monumental amount of information about standards. It is only the tip of the iceberg. This is not to imply that you must be an expert on standards to deal with power circuit breakers. You can, however, begin to appreciate just how much effort, investment, and plain hard work goes into being able to legitimately print in a document a statement such as:
"Magnum DS Low Voltage Power Circuit Breakers are UL Listed and built and tested to all applicable ANSI Standards." Keep in mind that all these standards establish minimum requirements. There is nothing prohibiting a manufacturer from exceeding standards by offering additional features and/or using newer and improved operational techniques for more efficient and higher levels of performance. Magnum DS does just that in a number of areas.
The majority of the remaining discussions in this module will be presented as they relate to applicable ANSI Standards. Remember, however, that other standards do exist in other parts of the world. They must be complied with to participate in the international segment (Figure 38).
Figure 38. Circuit Breaker Identification
Low Voltage Power Circuit Breaker C37.50 Testing
Testing of a low voltage power circuit breaker in keeping with required ANSI Standards provides the first glimpse at what makes a low voltage power circuit breaker unique. Remember, low voltage power circuit breakers are applied at or below their nameplate ratings. That nameplate rating is a result of having successfully completed a series of rigorous tests. This is referred to as a 100% rating.
Although every detail of the testing will not be covered here, you will have an appreciation for just how demanding these ANSI defined tests are for low voltage power circuit breakers. The tests will be described as four test sequences. It should be pointed out here that all tests are performed using a Drawout circuit breaker in its enclosure for each frame size.
The first three test sequences are similar in many ways to the tests performed on other types of low voltage circuit breakers, such as a molded case circuit breaker. It is the fourth test sequence performed on a low voltage power circuit breaker that differentiates the power circuit breaker from other types of circuit breakers.
Note that all the following test sequences, except for Test Sequence 4 in which the circuit breaker has no Trip Unit, are preceded and followed by a calibration test and dielectric check.
Test Sequence 1 - This test sequence consists of a temperature rise test, an overload switching test, and then a short circuit test. The circuit breaker is equipped with an instantaneous trip.
1. The circuit breaker is loaded to 100% of the maximum rating of the frame size (in normal enclosure) until the temperature is constant. The standard lists the maximum permissible temperature rises at various parts of the circuit breaker.
2. The circuit breaker is then subjected to number of opening operations on over-load switching.
3. The circuit breaker is then given a 3-phase short circuit test at its maximum voltage rating, which is 635 volts for this 600 volt rated circuit breaker. The short circuit current in this case can be no less than the 600 volt interrupting capacity of the circuit breaker being tested. The three maximum voltages used during testing and typically listed on the nameplate are listed below along with their corresponding application voltages:
Maximum Voltage Application Voltage
635 volts 600 volts
508 volts 480 volts
254 volts 240 volts
4. The short circuit test consists of initiating current through the closed circuit breaker, causing it to trip. After 15 seconds, the circuit breaker is re-closed on the fault, and then allowed to trip open to clear the fault. This is known as an O-CO (open-close open) test.
5. The short circuit test is followed by a calibration check and a dielectric test.
Test Sequence 2 - This test sequence consists of a series of short circuit tests on a circuit breaker equipped with selective tripping (no instantaneous).
1. Once again, all short circuit tests are preceded by a dielectric test and a calibration test. After the interruptions, the circuit breaker is given another dielectric test and the calibration is again checked.
2. Each short circuit test is an O-CO test meaning that the circuit breaker interrupts the full fault current twice.
3. After the 3-phase short circuit tests are completed at different prescribed voltages, single phase tests are performed. A new circuit breaker may be used for each test, or each test may be done on different poles of the same circuit breaker.
4. One single-phase test is done at each of the same three maximum voltage ratings used for the three-phase tests (635, 508 and 254 volts) at the appropriate Interrupting Rating for that voltage.
Test Sequence 3 - This test sequence includes tests of mechanical and electrical endurance.
1. A circuit breaker is calibrated, given a dielectric test, and subjected to a large number of operations. Some of the operations are at no load and some at full load.
2. The required number of operations varies by circuit breaker frame size.
3. After the endurance test, the same circuit breaker is given a full 3-phase O-CO short circuit test at 635 volts and a dielectric withstand test.
The required number of operations for the endurance test just described is about the same for larger frame power circuit breakers and somewhat higher for smaller frame power circuit breakers compared to molded case circuit breaker endurance tests. For the sake of comparison, refer to the two endurance ratings tables, one for power circuit breakers and one for molded case circuit breakers .

Test Sequence 4 - This test sequence includes a short time current (withstand) test. Molded case and insulated case circuit breakers are not usually subjected to this type of test and, therefore, have no full 30 cycle Short Time Rating. This is one of the key differences.
1. For this test, the circuit breaker does not have a trip unit or the trip unit is disconnected. During the short-circuit testing, the circuit breaker can be tripped instantaneously by a shunt trip.
2. The circuit breaker is closed and then energized at its full short time current rating. The short time rating is usually equal to the 600 volt short circuit current rating.
3. The current is left on for 30 cycles (1/2 second), then off for 15 seconds, then back on for another 30 cycles. The circuit breaker remains closed during this test sequence.
4. After the short time current (withstand) tests, the same circuit breaker is given a 3-phase short circuit test sequence at full short circuit current rating and 635 volts. The circuit breaker is opened as quickly as possible by the shunt trip, which is energized at the same instant the power is applied. The intent is to force the circuit breaker to open during the worst case conditions (full current asymmetry). This is where the short time rating for the circuit breaker comes from.
5. After the final test, the circuit breaker calibration is checked and given a dielectric withstand test.
During Test Sequence 4, the circuit breaker is subjected to tremendous physical forces from the magnetic fields and to severe heating effects from the current. Think about it. Both the magnetic forces and the heating vary with the square of the current (Figure 41). For example, a 4000 ampere frame low voltage power circuit breaker at 85,000 amperes is subjected to forces and heating more than 450 times normal for each 30 cycle test. It is quite awesome.

Advanced Low Voltage Power Circuit Breakers

2009-01-25T03:44:00.004-08:00

The Magnum DS Family of low voltage power circuit breakers is not an extension of any other low voltage design (Figure 36). It is at the forefront of technology and development. For this reason, it is an excellent design to discuss when certain specific examples are required in this module. Keep in mind, however, all low voltage power circuit breakers do not necessarily offer as many features or use the same advanced technology as Magnum DS. Even though this might be the case, it does not mean that another design does not qualify as a low voltage power circuit breaker, or that there are not other capable low voltage power circuit breakers.

Magnum DS is a low voltage power circuit breaker. It is built and tested to all applicable ANSI Standards for low voltage AC power circuit breakers and Underwriter's Laboratories Listed. Because of its flexible design, an International Electrotechnical Commission rated version of Magnum DS is also available to address international requirements. This IEC version is called Magnum. Everything that is expected of an ANSI rated low voltage power circuit breaker is delivered by Magnum DS, and then some. If you think this sounds a bit biased, it is. Cutler-Hammer is justifiably proud of Magnum DS, and as you learn more and more detailed information about low voltage power circuit breakers, you will most certainly agree.
You will recall a discussion of the areas that might set a low voltage power circuit breaker apart from other types of low voltage circuit breakers.
Namely:
• Method used to make and break circuits
• Ratings
• Construction/Maintainability
• Integrally Mounted Trip Units
• Operating Mechanisms
• Testing

You were initially introduced to the primary factors that make the low voltage power circuit breaker unique. Other factors and methods that were rather common with low voltage power circuit breakers but not necessarily unique were also discussed. You will revisit a number of the areas just mentioned. Each topic discussed, however, will be presented in more detail with special emphasis placed on those factors that set the low voltage power circuit breaker apart from other types of low voltage circuit breakers, such as molded case and insulated case circuit breakers. The general topics to be discussed are:

• Standards and Testing
• Construction Methods
• Ratings and Performance
• Operational Techniques
• Integral Trip Unit
• Applications
• Low Voltage Power Circuit Breaker Summary

The last section reiterates many of the facts learned with special attention given to the unique factors associated with low voltage power circuit breakers. This summary can serve as a review and a future quick reference.

Testing

2009-01-25T03:40:00.004-08:00

The testing required and the standards that must be met by a low voltage power circuit breaker depend on the area of the world where the circuit breaker is applied. To play a major international role, low voltage power circuit breakers should be able to meet the requirements of ANSI, UL and IEC .

As you continue through this module, you will become well aware that the required testing is the key factor in defining the type of circuit breaker. In a very general and simplistic way, low voltage power circuit breakers undergo a sequence of four rigorous tests.

1. The first sequence consists of a temperature rise, an overload, and a short-circuit test.
2. The second sequence is a series of short-circuit tests.
3. The third sequence is an endurance test.
4. The fourth is a momentary rating test.

Molded case circuit breakers, for example, are subjected to tests similar to numbers 1, 2 and 3. The fourth test sequence, momentary rating test, is specific to power circuit breakers and to some IEC molded case circuit breakers.
Specific testing details will not be covered in this module. It should be pointed out, however, that the momentary rating test just mentioned (test sequence 4) subjects the circuit breaker to tremendous physical forces and severe heating effects. Very simply speaking, the circuit breaker is subjected to its full short time current rating for two (2) time periods up to 30 cycles each. The short time rating indicates what magnitude of current the circuit breaker can stand with its contacts closed for a short period without being damaged. The circuit breaker's short time rating is often equal to its 600 volt interrupting capacity. A low voltage power circuit breaker must be strong enough to survive this test and function properly after completion.

Helping the Customer
Selection of the proper low voltage power circuit breaker for a specific application is not a difficult process. There are some important questions, however, you must be able to answer. Fortunately, the most difficult part of the job has already been done by other qualified individuals when they determined the requirements of the system.
This includes determining things like:
• Circuit Breaker type required
• Application voltage
• Maximum fault current system could see
• Continuous current for the system and each branch
• System frequency
• Types of trip unit capabilities
• Programmable functions
• Accessory needs
• Mounting configuration
• Special requirements

Your job is to make sure these types of questions are answered. The more familiar you are with what a particular circuit breaker line has to offer, the easier the task. Let's start by looking at what circuit breaker manufacturers do to help.
Manufacturers normally provide a great deal of assistance in the way of printed material, computer accessible information and direct contact. This does not mean, however, you should not put forth that extra effort to know personally what is available. Learn to use all the information provided.
Most selection factors fall into one of two categories:

• Standard selection factors
• Special selection factors

Standard Selection Factors

Standard selection factors normally are associated with the circuit breaker's ratings/standards, operation method, accessory items, and how the breaker will be mounted. The most common points to consider will be discussed.
1. Standards - Applicable standards were discussed in this module and earlier modules. You should be told or it will appear in a written specification what standards the circuit breakers must meet. Newer low voltage power circuit breakers meet a wide array of standards which will make them acceptable in most parts of the world. In addition, make sure you are aware of any special local requirements and/or standards.
2. Ratings - This is a critical part of the selection process. You should already know what is required. Now you must determine what specific circuit breakers will meet the rating requirements. Manufacturers normally provide easy to read tables outlining the ratings of every circuit breaker frame. Keep in mind there could be more than one table. This is especially true for newer circuit breaker designs that meet both ANSI and IEC requirements. A manufacturer might choose to present it as one combined table or two tables. If you know what is required, you will be able to make a selection from the tables under normal circumstances.

ANSI Example
Let's take a look at a typical type ANSI table for a low voltage power circuit breaker and see what it has to offer (Figure 35). The table used in this example will not cover every circuit breaker rating for a particular design.
Enough of the table is presented to give you a good working knowledge on how to proceed. Each area of the table that is discussed is identified by a circled letter to simplify the discussion. One last important point should be made before beginning. Always read footnote references carefully. They provide important information and could be critical to the proper selection.

A: The "Breaker Type" is usually the name given to the circuit breaker by the manufacturer along with some general information about the ratings of that specific circuit breaker type. In the example ratings table shown, XYZ-508 is the first circuit breaker listed. The XYZ is the circuit breaker's name. The first number "5" gives you a general idea what the interrupting rating is of the XYZ-508 circuit breaker at a voltage of 480 volts. This is a common presentation method because the widest used application voltage domestically is 480 volts. The last two numbers, "08" in this case, usually tell you the maximum continuous current rating of the circuit breaker. XYZ-508 can, therefore, carry 800 amperes continuously, and interrupt 50,000 amperes at 480 volts.
B: This column outlines specifically the maximum continuous current the circuit breaker will carry. Notice that circuit breaker type XYZ-616 in the example table will carry a maximum continuous current of 1600 amperes. Notice also that the last two numbers of the circuit breaker type XYZ-616 ("16") give you the same information, with 16 meaning 1600. Take the time to make this same comparison with circuit breaker type XYZ-632.
C: Notice that the rest of this table is devoted to the interrupting capabilities in amperes of the different circuit breaker types at different application voltages. Also notice that the application voltages listed are:
• 208-240 volts
• 480 volts
• 600 volts
The nominal voltage range for the ANSI market is 208 to 600 Volts AC at a frequency of 50 or 60 hertz. Get comfortable with seeing these voltages when talking about ANSI rated low voltage power circuit breakers.

D: You will notice that these two columns are labeled differently. The first column entitled "With Instantaneous Trip" outlines the interrupting capabilities of each circuit breaker frame at the different application voltages. These ratings are applicable when the circuit breaker's trip unit provides instantaneous protection. In other words, the circuit breaker can be applied to safely handle faults of the magnitudes shown.

You will also notice in the column entitled "Without Instantaneous Trip" that some of the interrupting ratings are somewhat lower than the left column under 208-240 volts. These ratings are the magnitudes that the circuit breaker can tolerate safely for a short delay period of time (30 cycles) before opening at the short delay current ratings shown. This might sound like a contradiction. It really is not for a number of reasons. Consider the following points.

1. You will recall from material presented earlier that a low voltage power circuit breaker's short time rating is normally the same as its interrupting rating. The key word here is normally. The partial ratings table being considered here already indicates that there are some very limited times when a low voltage power circuit breaker could have a higher interrupting rating if it has instantaneous protection versus just short time protection and no instantaneous. This was probably the result of a conscious decision to meet some very specific application requirement for a particular customer or industry, knowing the fact that a circuit breaker had to have instantaneous to be applied at these somewhat higher ratings.

2. The fact still remains that low voltage power circuit breakers must be and are only applied in keeping with their nameplate rating. This, in almost all cases, shows the interrupting rating and the short time rating to be the same. When electrical systems are being considered, fault calculations are done to determine the maximum fault current a system can experience. Low voltage power circuit breakers are then selected with ratings that are able to deal successfully and safely with the worst case fault scenario calculated. In other words, if a low voltage power circuit breaker with an adequate short delay current rating is applied, it will stay closed for the appropriate short time no matter what. This is true because it will not see (experience) more that it was designed to safely handle. End of that part of the story.

3. On the other hand, a low voltage power circuit breaker, which is already in the open position, will trip open instantaneously if an attempt is made to close the breaker on an existing fault. This safety feature prevents damage that could result from closing on a fault. Today, this feature is normally accomplished through circuitry which is part of the trip unit. This self protecting circuitry is often called a discriminator circuit or may be called a making current release in newer designs like Magnum DS. Its purpose has nothing to do with a circuit breaker that is already closed and functioning.

For now, how this feature is technically accomplished will not be discussed. Just be aware that such a feature exists with low voltage power circuit breakers. Future training material specific to a particular low voltage power circuit breaker design will discuss just how it is accomplished.
Remember:

• Low voltage power circuit breakers are applied at their nameplate ratings.
• Low voltage power circuit breakers are sized and selected for application to handle the maximum fault that could be encountered where they are applied.
• Low voltage power circuit breakers are provided with a means to trip (open) instantaneously if they are closed in on an existing fault.

E: Let's just briefly take a look at the footnote. It tells you that these ratings are also the short time ratings of the circuit breaker. The material in D was discussed as though we already knew these were short time ratings, and we did. Suppose we did not know that fact and failed to read the footnote. We would not be as informed as we should be for the proper circuit breaker selection. It could be like making the selection blindfolded. Be sure to read the footnotes.

IEC Example

IEC Example - An IEC example similar to the one just presented will not be offered here. Ratings tables and their appearance as to how data is presented can change from country to country and even manufacturer to manufacturer. The information presented, however, is usually similar. You should be aware of some of the noticeable differences in the presented data, and start now to become familiar with IEC rated breakers. For now, consider the following to get started:
• The voltage range for the international market is 380 through 690 Volts AC at a frequency of 50 or 60 hertz.
• The general continuous current range for low voltage power circuit breakers is 800 through 6300 amperes.
• The voltage and current abbreviations and names are different, such as:
Ue - application voltage, such as 380 or 690 volts.
In - rated current such as 800 or 6300 amperes.
Ics - rated service short circuit breaking capacity.
Icu - rated ultimate short-circuit breaking capacity.
Icw - rated short time withstand current (similar to the ANSI short time rating and the circuit breaker is expected to function properly again after having dealt with the Icw).

ANSI and IEC Example
Let's make a quick comparison from a presentation standpoint. Keep in mind, the important things are:
1. Will the circuit breaker being considered do the job?
2. Will the circuit breaker being considered meet the standards in effect where the circuit breaker is to be used?
It is not possible to simply take a product designed and tested to one standard (ANSI or IEC) and certify it to the other standard. A manufacturer must undertake a concerted design effort to satisfy both standards individually.

This is by far not an all inclusive example. It is only intended to draw some simple ANSI and IEC comparisons between some of the most common selection points that must be considered when selecting low voltage power circuit breakers. You can see that although not exactly the same, it is primarily a matter of familiarization.

3. Operation Method - As discussed earlier, low voltage power circuit breakers are either manually or electrically operated. You must always specify the method of operation. At some point, you will need to know the secondary control voltage being used for an electrically operated circuit breaker. Even if the circuit breakers are manually operated, it is a good idea to find out the secondary control voltage. The control voltage is necessary for the final selection of a number of items, not just electrically operated circuit breakers.
4. Accessory Items - Many of the common accessory items associated with low voltage power circuit breakers were discussed earlier. You have to be alert for these items in a specification or ask the customer if any are required. A determination can then be made if a compatible accessory is available to meet the need. Make a list of the required accessories and the specific requirements that are appropriate for them, such as control voltage, number and types of contacts and overall function.
5. Mounting Method - You will need to know whether the breakers will be fixed mounted or drawout. Always check to see if there are any special requirements for either configuration.
Special Selection Factors
There may not be special conditions to consider, although this should be determined as soon as possible. You may be able to deal with certain special conditions and others might call for assistance from the manufacturer. Do not hesitate to ask for help. Some conditions or requirements to look for that might not be considered standard are:

• High or low ambient temperatures
• Moist or corrosive atmospheres
• Altitude
• High shock conditions
• Unusual circuit breaker mounting conditions

Governing Standards

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You will recall from Module 5 that circuit breakers are designed, built and tested in accordance with one or more specific sets of standards. In this module, you will be introduced to the standards specific to low voltage power circuit breakers. The intent here is not to present and study the different applicable standards word for word. That type of undertaking would be a course unto itself. Our goal is to understand a little about low voltage power circuit breaker related standards, where they were, and where they are today.

You will hear many people repeat specific standards designations. Many of those same people do not have an intimate knowledge of what the standards actually say, nor are we saying they should. The actual product selection based on standards compliance should be left to the experts. It is helpful, however, to know what specific standards your products comply with and what general topic a specific standard addresses.

Keep in mind that a standard exists for almost everything. There are specific standards for circuit breakers and others for the structural assembly. Compliance with these exacting standards ensures customers of the very best possible product selection with a high degree of comfort. There is no room for compromise when performance, quality and safety are involved.
A number of years ago, low voltage power circuit breakers and most other types of equipment were designed and built primarily with only domestic standards in mind. This approach also was taken by foreign suppliers. A manufacturer would offer a circuit breaker designed, tested and manufactured in keeping with applicable standards for that part of the world or even particular country. Trying to play a significant role in other world markets was, at best, extremely difficult. If manufacturers today expect to be global participants, they must offer products that comply with the standards applicable to a variety of different markets around the world. This will require that you become familiar with both domestic and international nomenclature, ratings, procedures and governing standards. The task is greater, but so is the reward.
Some of the lines separating different types of low voltage circuit breakers were at times blurred in the past. Low voltage metal-frame power circuit breakers were built and tested to certain ANSI and UL specifications, while some low voltage encased circuit breakers were tested to UL specifications specific to molded case circuit breakers. The newest low voltage power circuit breakers today, like Magnum DS, are tested to specific low voltage power circuit breaker standards, like ANSI. They are also tested to standards that cover a much broader product scope, like IEC. The primary testing standards associated with low voltage power circuit breakers today are:

ANSI
The American National Standards Institute's ANSI C37.50 is a specific North American testing standard entitled "Low Voltage AC Power Circuit Breakers Used In Enclosures." This standard specifies rigorous tests for product performance. There are additional C37 standards which govern power circuit breaker and trip unit construction, such as C37.13 and C37.17 respectively.

UL
Underwriter's Laboratories Incorporated's UL1066, for the most part, calls for testing to demonstrate compliance with ANSI C37.50 just mentioned. A UL Label is affixed to the breaker to indicate successful compliance.

IEC
The International Electrotechnical Commission IEC 947-2 is a more general international testing standard covering a variety of devices, including circuit breakers of all types, and is entitled "Low Voltage Switchgear and Controlgear."

Closing Comments on Standards
Before concluding this section, it might help to minimize confusion if you remember that there is often a great deal of referencing to other standards that takes place within a specific standard. Successful testing with respect to one standard often includes automatic compliance with other standards.

Example 1 - ANSI C37.13 details the physical attributes, such as stored energy, that a low voltage AC power circuit breaker must have, while ANSI C37.50 references C37.13 and details how the described breaker should be tested. The key here is that successful testing in keeping with ANSI C37.50 brings with it compliance to C37.13.
Example 2 - In a similar fashion, IEC 947-2 references IEC 947-1 (General Rules). Compliance with IEC 947-2, therefore, brings with it IEC 947-1 compliance.

Mounting Methods

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As briefly discussed earlier, low voltage power circuit breakers are usually available in the two following mounting configurations:

• Fixed
• Drawout

Total usage of low voltage power circuit breakers today is dominated by the drawout configuration because it provides for easier maintenance and continuity of service. Most circuit breaker manufacturers, however, offer both types.

Fixed Circuit Breaker
Fixed low voltage power circuit breakers usually have fixed primary conductor stabs protruding from the rear of the circuit breaker. The circuit breaker is bolted in position within its assembly compartment, and the rear conductor stabs are bolted to primary bus connections (Figure 30). Secondary connections are also made manually. Power must be turned off to the assembly to connect a fixed circuit breaker into the system or to remove it from the system.

Drawout Circuit Breaker
Drawout low voltage power circuit breakers have a levering device to move the circuit breaker from one compartment position to the next. Usually part of the levering mechanism is on the circuit breaker with a corresponding part is in its compartment. Working together, they provide the mechanical means required to move the circuit breaker. Drawout circuit breakers are designed to be removed and connected without cutting power to the entire assembly under load conditions because the circuit breaker, by design, automatically opens before racking can take place. This means that power to the assembly does not have to be turned off when a circuit breaker is removed from or inserted into the assembly, thus ensuring continuity of service.
Drawout circuit breaker compartments are provided with extension rails which, when not in use, are stored inside the compartment (Figure 31). The extension rails provide a means by which a drawout circuit breaker can be easily removed from its compartment for inspection, maintenance or movement to another area.

Primary electrical connections between the circuit breaker and the primary bus are automatically made or broken as the circuit breaker is moved into or out of the "Connected" position within the circuit breaker compartment. Primary connectors mounted to the back of the circuit breaker slide onto the primary bus connectors. These primary connectors, often called finger clusters or disconnect contacts, are frequently composed of a number of spring loaded fingers (contacts) . The number of fingers (contacts) used is dictated by the amount of current they will carry. Fingers (contacts) are made of an excellent conducting material or material combination, such as silver plated copper.

Secondary electrical connections are usually automatically made or broken as the circuit breaker is moved into and out of its compartment. As the circuit breaker is moved into the "Test" position from the "Disconnect" position, the secondary connections are made providing the required secondary power for testing or operating the breaker, but no primary power. The secondaries remain connected as the breaker moves into the "Connected" position. When the circuit breaker is moved out of the "Connected" position, the secondaries remain connected and stay connected until the circuit breaker is moved farther out of its compartment past the "Test" position. The graphics of the four positions presented earlier in the module demonstrate the movement and connections.

Arc Extinguishers

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In Module 5, a number of ways or combinations of ways to extinguish an arc was discussed. Low voltage power circuit breakers use some type of Arc Extinguishers (arc chutes or arc chambers) mounted above and around the main contacts to extinguish arcs in air (Figures 21 and 22). This leads to the name low voltage power air circuit breakers.

Arc chutes, in some form, have been used to extinguish arcs for more than a half century. The primary purpose of an arc chute is to extinguish an arc each time a circuit breaker interrupts a current. This is accomplished by confining, dividing and cooling the arc. This accomplished, the arc is not able to sustain itself through current zero.

Not all arc extinguishers are created equal and, therefore, some are more efficient than others. Efficiency is very important because the amount of contact damage caused by arcing is directly related to how fast or efficiently arcs are extinguished. More efficiency leads to longer contact life.

During the arcing process, ionized gases are generated and normally vented, in some fashion, harmlessly away from the circuit breaker, breaker compartment, and any operator who might be in close proximity to the equipment. It is also known that the high pressure created by these gases, if controlled properly, can be put to good use during interruption.
To this end, the molded case low voltage power circuit breaker design, for example, utilizes this gas pressure to help with the interruption process while minimizing gas leakage back into the circuit breaker itself. This improvement is accomplished through the use of seals in the arc chamber and a close fit of pieces and parts. This can only be accomplished with molded frame designs. Obviously, the design and process is a bit more involved than just described. For now, the most important thing to remember is that the original arc extinguisher concept is still used today, but great strides have been taken to improve upon the original concept with significant improvements in overall efficiency.
Operating Mechanism
You learned in Module 5 that some type of a mechanism is provided with all circuit breakers for opening and closing. Low voltage power circuit breakers are no exception. A low voltage power circuit breaker operating mechanism is composed of a number of different parts, assemblies and accessories, all dedicated to ensuring that the circuit breaker opens and closes consistently.
The mechanism is a two-step spring charged stored energy type providing three basic functions:

• A means to charge the closing springs
• A means to close/open the circuit breaker using the stored energy of the closing and opening springs
• A means to perform an Open-Close-Open duty cycle
Two varieties of the mechanism exist:
• Manual
• Electrical (Motor Operated)

The manually operated circuit breaker has its closing springs charged manually through the use of some type of charging handle. The circuit breaker is closed using a manual close button which is a mechanical process. As the circuit breaker closes, a set of smaller opening springs are charged. The circuit breaker is opened using a manual trip (open) button, which is a mechanical process.

Safety interlocks, accessory items and trip units can also cause the circuit breaker to trip through mechanical means. Most manually operated power circuit breakers can be equipped with an optional device to electrically release the spring's stored energy, thus closing the circuit breaker.

Previously, it was not practical or even possible to convert manually operated low voltage power circuit breakers to electrically operated circuit breakers in the field. This is no longer impossible with newer low voltage power circuit breaker designs. Such designs permit manually operated circuit breakers to be converted to electrically operated circuit breakers by field installing UL Listed electrical operators.

An electrically operated circuit breaker can be operated every way a manually operated circuit breaker can be operated. In addition, a small electric motor is normally used to automatically charge the closing springs, and an electrical means to close or trip (open) the circuit breaker is provided.

Integral Trip Unit
For a circuit breaker to do its job, a means must be provided enabling the circuit breaker to perform automatically or in response to other commands. In short, the circuit breaker is a rather dumb device without a brain (intelligence source). This source of intelligence is the trip unit.
As required by ANSI Standards, low voltage power circuit breakers must be provided with an Integrally Mounted Trip Unit. This means that the trip unit must be inside of, or part of, the circuit breaker. Prior to the advent of the first solid state trip unit, electromagnetic type tripping devices, commonly called dual-overcurrent magnetic trips, were used with all low voltage power circuit breakers. In recent times, this type of tripping device on low voltage power circuit breakers has disappeared from the scene. For this reason, only microprocessor-based trip units will be discussed in this module.

A typical microprocessor-based trip unit used with low voltage power circuit breakers offers the following capabilities:
• Programming
• Monitoring
• Diagnostic
• Communication
• Testing

The capabilities of a particular trip unit depends on the trip unit design itself and system requirements. Some trip units can only offer basic features, while others can offer basic features or, if required by the system, additional sophisticated and highly advanced features.
The operating response of a trip unit is graphically represented by time-current characteristic curves. These curves show how and when a particular trip unit will act for given values of time and current. A characteristic curve is represented by a band created by a minimum and maximum value of time or current.

The programmable or adjustable features of a trip unit permit movement of its characteristic curve or parts of the curve . This movement can be done in both a horizontal and vertical direction. Some trip units even allow the shape of the curve to be changed.

Most trip units offer protection combinations of:

• (L) Long delay protection - protection against overloads and short circuits
• (S) Short delay protection - protection against short circuits
• (I) Instantaneous protection - protection against short circuits
• (G) Ground fault protection - protection against ground faults

A trip unit offering all four of these protection at one time is commonly called an LSIG Trip Unit. Other combinations are also available, such as LI, LS, LSI, LIG and LSG.
The long, short and ground functions would have programmable values of current and time. Obviously, instantaneous has no associated time because the trip is instantaneous (Figure 26). Trip units have these different programmable features programmed so they coordinate with one another and with the requirements of the system being protected to provide the closest possible system coordination and protection against all eventualities. This coordination discipline is where you start hearing phrases like curve shaping and close coordination. No attempt will be made in this module to get into the details of this discipline. It is quite specialized and best left to individuals with the proper training.

More advanced trip units are able to monitor and display currents, energy, power, power quality and power factor. They also may be able to diagnose problems and provide advance warnings of potential problems, such as harmonics. Two way communications for remote monitoring and control is also available. This affords the user a cost effective way to monitor and control expansive, multi-location facilities with a wide array of protective equipment and operational machinery.

Trip and no trip tests can usually be performed on the trip unit and circuit breaker utilizing integral testing capabilities or separate test kits. Normally, the tests can be performed with the circuit breaker in service and full protection provided during the testing. This type of testing is secondary testing. Primary testing involves specialty testing equipment and a testing expertise, and is not discussed in this module.

Accessory Items
Accessories used with low voltage power circuit breakers are usually added to the circuit breaker to provide additional features, such as status indication and remote operation. It is possible, however, that some accessories for some circuit breaker designs might be mounted remotely from the circuit breaker. These devices might be totally mechanical, totally electrical or a combination. The intent here is to briefly discuss the function of commonly used accessory items, although all low voltage power circuit breakers do not necessarily offer all of the devices being discussed, nor is this list all inclusive.
• Electrical Operator - This is an assembly of devices including a small spring charging motor that when added to a manually operated circuit breaker converts it to an electrically operated circuit breaker. This allows for remote operation (open/close) of the circuit breaker. The ability to field install this device is more common with newer low voltage power circuit breakers. Power circuit breakers normally use to be either manual or electrical by design, and could not be easily converted.
• Operations Counter - An operations counter is a counting device, usually linked in some fashion to the operating mechanism. It is used to count the open and close operations of the circuit breaker, and serves as a maintenance aid.
• Auxiliary Switch - An auxiliary switch consists of "normally open" (NO) and "normally closed" (NC) contacts (Figure 27). The contacts on some switches are convertible from NO to NC and vice versa. The contacts are frequently referred to as "a" or "b" contacts. The "a" being open when the circuit breaker is open and the "b" closed when the circuit breaker is open. In short, these auxiliary contacts change "state" when the circuit breaker main contacts change "state." An auxiliary switch is normally mounted on the circuit breaker. Contacts from these switches are frequently used for electrical operation of a circuit breaker, remote signaling, and electrical interlocking.

• Undervoltage Release (UVR) - An undervoltage release, normally a circuit breaker mounted electromechanical device, trips the circuit breaker when the voltage falls below a predetermined level.
• Shunt Trip (ST) - A shunt trip is an electromechanical device which is standard on most electrically operated power circuit breakers. When added to a manually operated circuit breaker, it provides for remote controlled electrical tripping.
• Spring Release (SR) - The spring release device is standard on most electrically operated power circuit breakers. When added to a manually operated circuit breaker, it permits the circuit breaker to be closed electrically from a remote location.
• Bell Alarm (OTS) - The bell alarm, frequently called an overcurrent trip switch (OTS) on a power circuit breaker, is normally circuit breaker mounted. Its function is to provide a signal to indicate that the circuit breaker has tripped open automatically (trip unit command). It will not operate if the circuit breaker is tripped open by other means, such as the use of a manual trip button, an electrical control switch, or the operation of an undervoltage release device.
• Locking Devices - Low voltage power circuit breakers normally have a wide array of mechanical locking devices to prevent unauthorized circuit breaker operation.

• Mechanical Interlocks - These devices provide a way to mechanically interlock two circuit breakers. A typical use for such a device is to prevent one circuit breaker from closing while another circuit breaker is already closed.
• Capacitor Trip Device - A capacitor trip device is normally mounted externally from the circuit breaker. It uses a small storage capacitor for AC control of the circuit breaker to ensure reliable tripping power during fault conditions.
• Lifting Device - Because some low voltage power circuit breakers can be sizable and heavy, a variety of devices is usually available to lift and move the circuit breaker once it is out of its compartment (Figure 29).
Rail Mounted Lifting Device Being Used to Lift a Magnum DS Power Circuit Breaker

• Truck Operated Cell Switch (TOC) - A TOC switch is usually mounted in the circuit breaker compartment and is activated by movement of a drawout circuit breaker into and out of the "Connected" position. As the circuit breaker moves, the contacts are activated providing a means for remote indication of the circuit breaker's position.

Design and Functional Considerations

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In Module 5, you learned that all circuit breakers have a number of design and functional characteristics in common:
• Compliance with Specific Standards
• Set of Open/Close Contacts
• Means to Open and Close the Contacts
• Means to Extinguish an Arc
• Means to Respond to Overcurrents/Commands
• Method for Enclosing Circuit Breaker Components
• Method For Mounting Circuit Breaker
Specific methods used for mounting and using low voltage power circuit breakers will be covered in the next section. In this section, the concentration will be on how low voltage power circuit breaker operate to accomplish their tasks and what accessory items are available to enhance their capabilities.
Basic low voltage power circuit breakers are generally composed of:
• Frame or Chassis
• Primary Contacts
• Arc Extinguishers
• Operating Mechanism
• Integral Trip Unit
• Accessory Items
Let's take a look at each.
Frame or Chassis
You will recall from Module 5 that all circuit breakers utilize some method to hold all the parts that make up a circuit breaker, usually called the frame or chassis. A low voltage power circuit breaker chassis today will be one of two types (Figure 14 and 15):
• Open Type Metal-Frame (Older Designs)
• Molded Frame of Engineered Thermoset Composite Resins (Newer Designs)

The open type metal-frame has a number of pieces welded and/or bolted together on which the different circuit breaker components are assembled. Components have a tendency to be larger, heavier, and may need to be adjusted.

in the workplace
The new Magnum DS power circuit breaker utilizes a rigid frame molded from engineered thermoset composite resins.

Molding improves the structural rigidity of the frame, allowing for higher interrupting and short time ratings.
Many individual circuit breaker parts are molded as integral assemblies. This improves the design by making it smaller and stronger with fewer individual parts, unlike the metal-frame approach.

Primary Contacts

Primary open/close contacts in a low voltage power circuit breaker provide a means for isolating or connecting a part of a circuit from or with the rest of the system. The design of the primary contacts is one of the most critical design considerations relative to the efficiency and overall effectiveness of any low voltage power circuit breaker. These contacts are used to carry or break the main continuous load current associated with the system in which the circuit breaker is applied. Each phase has an associated primary contact. A three-phase low voltage power circuit breaker, for example, would have three sets of primary contacts. Keep in mind that primary contacts come in a wide variety of designs and appearances. All designs do not use the same number of parts nor are all designs equally efficient. However, all designs act to provide the same general service.
Low voltage power circuit breaker primary contacts usually have separate arcing and main current carrying parts. This does not mean that they are necessarily separate pieces. They could both be part of the same component piece, although the arcing and main contacts act as individual pieces and perform distinctly different functions.

In Module 5, the discharge of electric current crossing a gap between two contacts was discussed . This phenomenon, on a small scale, can occasionally be observed when pulling a plug from a wall socket.

Arcing also occurs when opening and closing low voltage power circuit breakers under load, except to a much larger degree. The primary contact design challenge is to ensure that the arcing is dealt with first to protect the surface of the main contacts from arc damage. For this reason, primary contacts are mechanically designed such that on closing of the circuit breaker, the arcing contacts touch (make) before the main contacts. Also on opening of the circuit breaker, the main contacts part (break) before the arcing contacts. This construction ensures that arcing takes place on the heat resistant arcing contacts. Usually, primary contacts are replaceable on low voltage power circuit breakers, which can be needed in time if the operating duty of the breaker is severe enough.
A primary contact assembly is composed of:

• fixed (stationary) part
• moving part

A rigid insulating piece through a pushing or pulling motion is used to operate the moving part of the primary contact assembly.
The fixed and moving main and arcing portions of the assembly can be in any number of configurations, some more efficient than others (Figures 18 and 19). Usually the designs for a particular type circuit breaker are the same. The only variable is the number of parts used to handle the amount of current available. Larger circuit breaker frames require more and/or larger arcing and main contact pieces.

Keep in mind that the design goal is to efficiently handle arcing through the heat resistant arcing contacts so that the main contacts are protected from unnecessary damage. This approach permits the main contacts to be made from low resistance materials, such as silver or silver alloys to minimize the heat developed during normal operation.

Finally, it was pointed out in Module 5 that some newer low voltage power circuit breaker designs take full advantage of certain natural facts of physics to assist with the opening process. You will recall that the concept centers around magnetic fields established in conductors when current is flowing in the conductors.
The low voltage power circuit breaker design takes full advantage of this electromagnetic force to assist with opening and keeping the circuit breaker closed. In certain configurations, the force and also the insulator are used to help hold the contacts closed temporarily during a fault condition, which is where a power circuit breaker's short time rating comes from. Circuit breaker designs taking advantage of this concept can be smaller and lighter and still maintain the higher withstand (short-time) capabilities associated with low voltage power circuit breakers. When it is time for the contacts to open, this same force can be used in the opposite direction to speed the opening action.

Think about the concept of electromagnetic assistance with opening and closing of contacts in the following fashion . A door could be viewed as the movable contact. Our super-hero can be considered the rigid insulator used to push closed or pull open the door (contact). Assistance from the wind (electromagnetic force) in the proper direction would help our super-hero open or keep the door closed.

Control Voltage

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Control Voltage
The Control Voltage (or secondary voltage), is usually secondary with respect to the voltage rating of the circuit in which the circuit breaker is applied. Control voltage is used to operate secondary devices. The voltage used to run the motor that charges a circuit breaker's springs automatically is an example.

Drawout
A drawout circuit breaker refers to a circuit breaker that can be moved within a compartment from one defined position to another without manually disconnecting any connections or turning off the line side power. This is usually accomplished through the use of a mechanical levering device, sometimes in combination with the manual assistance of an operator. This is called racking the circuit breaker into or out of a position. The circuit breaker is first opened, and then automatic main disconnect devices on a drawout circuit breaker allow for the circuit breaker to connect or disconnect from the bus. These automatic main disconnect devices are often referred to as Finger Clusters. The phrase finger cluster comes from the fact that many designs utilize a number of conductive pieces (fingers) assembled into one cluster. The four typical defined positions are:
• Connected
• Test
• Disconnect
• Remove (Withdrawn)
In the Connected position, the circuit breaker is into its compartment as far as it will go with both primary and secondary electrical connections made. The circuit breaker is now ready for normal operation .

In the Test position, the circuit breaker is farther out of its compartment with the primary electrical connections no longer made (Figure 9). Secondary electrical connections are still made in this position to provide the secondary power required to test the circuit breaker's operation, including the trip unit.

In the Disconnect position, the circuit breaker is even farther out of its compartment with the main Contacts open (Figure 10). Neither the primary nor secondary electrical connections are made. This is a typical compartment storage position for a circuit breaker not in use.

In the Remove (or Withdrawn) position, the circuit breaker is out of the compartment on extension rails with the main contacts open and the closing springs discharged (Figure 11). There are neither primary nor secondary electrical connections. This is the typical last position for a circuit breaker to be in before it is physically removed from its rails to another location.

Behind Door Drawout
This is related to the specific drawout breaker design (Figure 12). Behind the door drawout means that the breaker compartment door usually must be opened to Lever (or "rack") the breaker from one position to another as just discussed under "Drawout."
The breaker normally has a Faceplate Shield (or "deadfront shield") to protect the operator from dangerous voltages while the door is open. This type of design usually permits the breaker to be in any of three positions (Disconnect, Test, Connected) with the door closed. This design does not permit an individual to know the status of the circuit breaker or its trip unit without opening the compartment door.

Through Door Drawout
This is also a drawout related circuit breaker design (Figure 13). Through the door drawout permits the operator to lever the circuit breaker from the "Connected" position to the "Test" position to the "Disconnect" position and vice versa without opening the compartment door. The door has a hole in it to accommodate protrusion through the door of some small portion of the circuit breaker as it reaches a position well to the front of the compartment. The operator is also protected by a deadfront shield, usually a combination of the door and the faceplate of the circuit breaker. The benefits associated with this design are that a full view of the circuit breaker front is given along with access to the racking (drawout) device without opening the compartment door.

Continuous Current Rating
The Continuous Current Rating of a circuit breaker is the maximum current rating the breaker is designed to carry on a continuous basis and remain within the applicable guidelines for the breaker. It is also referred to as the "Frame Rating" or the "Frame Size."
100% Rated ANSI specifies that low voltage power circuit breakers are to be rated for continuous operation at 100% of their current ratings in their compartment. To meet these requirements, they are tested for operation within a specific enclosure and, therefore, do not need to be de-rated.

Interrupting Rating
The interrupting rating is the maximum short-circuit current that the circuit breaker can safely interrupt. ANSI prescribes its minimum preferred ratings for power circuit breakers to meet.

Short Time Rating
The short time rating of a low voltage power circuit breaker is the maximum value of current the circuit breaker is designed to handle safely for a short period of time (30 cycles or 0.5 seconds) in the closed position, without damage to the circuit breaker. This test is repeated twice for a total of one (1) second. The short time rating is usually equal to the 600 volt interrupting capacity. This attribute is one of the main features that differentiates a power circuit breaker from other types of circuit breakers and allows for system selectivity. The short time rating was also discussed earlier in this module.

Trip Free
When a circuit breaker is in a Trip Free condition, it cannot, by design, be closed. Even when intentional efforts are made to close the circuit breaker and it is in a trip free condition, the main contacts will not touch and the circuit breaker will automatically return to the tripped position. This is an important safety feature specific to power circuit breakers.

Current Sensor
Sensor, as used with respect to a circuit breaker, is a common term for a current transformer which steps current down to useful levels for a specific purpose, such as providing an input to a trip unit (circuit breaker's intelligence package).

Principles of Operation and Terminology

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A low voltage power circuit breaker can be applied on any system within the interrupting rating of the circuit breaker. Low voltage power circuit breakers are ideally suited for applications where there is a requirement for the circuit breakers to be selective when faced with short-circuit conditions. In addition to our earlier discussion of selectivity, we could also say that "selective" means that the circuit breaker is capable of remaining closed for a certain period of time with a short-circuit present to allow the problem to be cleared up by a downstream device before the power circuit breakers open and the larger system is shut down (short time delay rating capacity). This is the area where short time delay ratings from 0 to 30 cycles play a key role. Obviously, it is assumed that the circuit breaker is applied properly and will not face short-circuit conditions beyond its capabilities. If it does see a condition beyond its short time rating, it will open instantaneously.

Time will be taken here to introduce several additional principles and common terms associated with low voltage power circuit breakers and their application. This material will be especially helpful from a practical standpoint. These are the types of terms and topics encountered on the job when working with low voltage power circuit breakers and their assemblies. Principles and terms presented here are certainly not all inclusive. Even after this module is completed and you return to your work location, new terms will surface that should be part of your low voltage power circuit breaker vocabulary. The intent here is to provide a solid background on which to build.

Stored Energy
Stored energy was briefly touched on earlier in this module and in Module 5. Because this is a phrase frequently heard with respect to circuit breakers, it deserves some elaboration. All low voltage power circuit breakers, whether manually or electrically operated, utilize two-step stored energy mechanisms. Stored energy mechanisms are needed to overcome inherent forces opposed to the closing process. They also make it possible to close the circuit breaker very quickly, 5 cycles or less time.

Stored energy is energy held in waiting, ready to open or close the low voltage power circuit breaker in five cycles or less. Designs are such that the energy required to open a low voltage power circuit breaker is always available.

On manually operated circuit breakers, closing springs are charged by hand. For electrically operated circuit breakers, springs are normally charged by a small electric motor, although they can also be charged manually if power is not available.

Bus
Bus refers to a conductor or conductors, usually made of copper or aluminum bars. Bus bars carry current and serve as a common connection for two or more circuits

What is a Low Voltage Power Circuit Breaker

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Like much other terminology in the industry, the designation low voltage power circuit breaker can be confusing at times. For now, let's just say that the set or sets of standards a circuit breaker complies with determines whether or not the circuit breaker can be classified as a low voltage power circuit breaker. Applicable standards will be discussed later in this module.
As you might imagine by now, there is a wide variety of low voltage power circuit breakers available in the market today. We will not concentrate on what circuit breakers are called. Instead, we will look at characteristics, features and governing standards. Then, no matter who the manufacturer or what a circuit breaker is called, you will be better prepared to discuss the subject.

Low voltage power circuit breakers are considered rugged, long-lived, flexible and, to varying degrees, field-maintainable. Let's briefly look as some of the areas that might set a low voltage power circuit breaker apart from other types of low voltage circuit breakers, such as:

• Method used to make and break circuits
• Ratings
• Construction/Maintainability
• Integral Trip Units
• Operating Mechanisms
• Testing

Method Used to Make or Break Circuits
Because they make and break power circuits in air using Arc Chutes, as opposed to Vacuum, SF6 or oil, they are considered Air Circuit Breakers.

Ratings
Low voltage power circuit breaker interrupting ratings and frame size designations can vary to some degree from one manufacturer to another or from one part of the world to another. One thing that is common to most power circuit breakers is the fact that they are rated for continuous operation at 100% of their current rating in their enclosure. What you see on the nameplate is what you get. There is no derating necessary when enclosed, if they are applied as specified by the manufacturer. This is not the case with all types of low voltage circuit breakers when applied in an enclosure. Low voltage power circuit breakers also have a short time rating in addition to an interrupting rating making them naturally suited for selectivity and coordination with downstream devices. Downstream devices are devices, such as other circuit breakers, that are farther into the electrical system.

You will recall from an earlier discussion, and it is worth mentioning again, that the short time rating is composed of two components - short delay current and short delay time, which are adjustable (programmable). As far as selectivity is concerned, let's say it is the response to a set of circuit or system conditions, usually in terms of current, in a certain time frame. It is really the ability to withstand a certain level of current (kA) for a given time period (cycles) while a downstream device selectively takes care of the problem by interrupting. This is also known as discrimination. The degree of selectivity is usually limited by the sophistication of the trip unit and the physical ability of the circuit breaker to withstand the potentially large thermal and mechanical stresses created by a fault current.

Construction/Maintainability
Low voltage power circuit breakers are essentially an assembly of parts on a metal frame or in an encased housing of insulating material. It is important to know that no set of standards dictates the type of frame construction for low voltage power circuit breakers. That decision is left in the hands of the manufacturer. You could look at it like the frame and body of a car holding all the other parts, like the motor, wheels, bumpers, seats and radio. This type of circuit breaker, to varying degrees, has the ability to be maintained in the field.

In addition, it is available in either a Fixed or Drawout configuration, with drawout being the most commonly used type.
Trip Units
Trip Units today used on low voltage power circuit breakers are almost universally of the solid state, microprocessor-based design. Years ago this same type circuit breaker used only electromechanical type trip units. Because this type of trip unit used with a low voltage power circuit breaker is almost non-existent, it is only mentioned briefly in this module. It is important to note that ANSI Standards require that the trip units on low voltage power circuit breakers be integrally mounted.

Operating Mechanisms
Low voltage power circuit breakers operate through two-step stored-energy spring mechanisms. The springs used to close the circuit breaker contacts, called closing springs, can be manually or electrically charged. The springs used to open the circuit breaker, called opening springs, are usually charged automatically when the breaker is closed.
Because of the increased closing forces required and the closing speed, low voltage power circuit breakers use two-step, stored energy mechanisms. That is, the closing springs are charged and remain charged with the breaker open until a "close" button or some other type of release is activated to close the breaker. As mentioned in Module 5, the low voltage power circuit breaker is required by ANSI Standards to provide an open-close-open duty cycle. This dictates the need for a two-step stored energy mechanism.

In the workplace

Low voltage power circuit breakers are most commonly applied in switchgear assemblies like the one shown here.

Frequently, low voltage power circuit breakers are used to control (and protect against overloads and short-circuits on) fans, pumps and lighting panels.
An assembly such as this one could be used to serve the HVAC needs of a manufacturing facility.
Because they are built to withstand such intense service conditions, low voltage power circuit breakers are ideal for industrial applications such as this.

Introduction

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There are both low voltage DC power circuit breakers and low voltage AC power circuit breakers. The interruption of direct current is distinctly different from the interruption of alternating current, and generally more difficult at comparable voltages and currents. Large quantities of low voltage AC power circuit breakers are used throughout industry in comparison to very small numbers of DC devices. For this reason and the fact that this is an introduction to low voltage power circuit breakers, only AC designs will be covered. Keep in mind, however, low voltage DC power circuit breakers do exist and are used in a number of specialty applications, such as rapid transit.

Circuit Breakers are often classified by certain modifying words, such as low voltage power. Low voltage AC power is considered to be for applications at 1000 volts AC and below. For comparison reasons then, medium voltage AC power is considered to be for application above 1000 volts AC. In general, however, low voltage power circuit breakers are viewed as 600 volt circuit breakers applied at a number of different voltage levels, such as 240 or 480 volts.
Sound confusing? Let's try to clear it up a bit by taking a brief look at why a low voltage power circuit breaker might be used along with some background information.
Why use a low voltage power circuit breaker over another type of low voltage circuit breaker? Most often the determination is made by the specific application. Let's consider a number of the more prominent reasons why a low voltage power circuit breaker is ideally suited for certain applications. Keep these reasons in mind as you proceed through this module. You will learn about the features and requirements that support and further explain the following reasons for applying low voltage power circuit breakers:

• Continuity of Service - Continuity of service allows for the maximum up time and minimum down time of equipment. A low voltage power circuit breaker has a significant Short Time Rating (also: "withstand rating"). This means that the low voltage power circuit breaker has the strength to withstand the stresses of a fault for up to 1/2 second or 30 cycles, instead of opening immediately. This ability to delay opening allows for a circuit breaker nearest the fault to clear the fault. This helps to prevent facility outages or a wide shutdown of facility equipment.
• Maintainability - A low voltage power circuit breaker is designed to be maintained in the field. This extends the useful service life of the circuit breaker. Especially for heavy, repetitive duty applications, maintenance of the circuit breaker is quite an important feature. Low voltage power circuit breakers allow for the inspection and replacement of parts on site.
• Safety - Low voltage power circuit breakers are tested as drawout devices in an enclosure. As such, four distinct circuit breaker positions relative to its enclosure are provided for maximum operator safety. The four drawout circuit breaker positions allow for the following uniquely different functions:

Connected Position: The circuit breaker is fully connected and functional.

Test Position: The circuit breaker's primary connections are disconnected. Secondary connections are not disconnected and testing can be safely performed because the circuit breaker is not energized. This is not possible with a circuit breaker that is permanently mounted.
Disconnect Position: Neither the primary nor secondary electrical connections of the circuit breaker are made. This position is often used as a storage position for the circuit breaker within its enclosure.

Withdrawn Position: In this position, the circuit breaker has no electrical connections. It is far enough out of its enclosure, usually on some type of integral extension rails, to permit inspection and maintenance without turning the power off to an entire assembly of equipment.
Reliability - Low voltage power circuit breakers are tested for and must be able to meet high electrical and mechanical endurance ratings. Electrical endurance is the number of operations at rated continuous current and maximum system voltage. Mechanical endurance is the number of operations with no voltage applied.

Remote Operation and Reclosing - Low voltage power circuit breakers are designed for operation remotely. They have two-step Stored Energy mechanisms which permit circuit breakers to rapidly reclose after a fault. The two-step stored energy mechanism makes multiple charge-close operations possible, such as the operating sequence: charge-close-recharge-open-close-open.

Custom has led to using phrases such as low voltage power circuit breaker, low voltage metal-frame circuit breaker, low voltage air circuit breaker, and 600 volt power circuit breaker. Although these circuit breakers fall into the classification of 1000 volts and below, real world applications are usually 600 volts and below, thus the 600 volt reference. In general, such a device must be built and tested in accordance with a very specific set of standards, such as ANSI Standards. A low voltage power circuit breaker is a device with both an Interrupting Rating and a short time rating, where the short time rating is composed of two components:

• Short Delay Current (expressed in kA)
• Short Delay Time (expressed in cycles)

This is the primary differentiating feature between a power circuit breaker and a molded case circuit breaker. The importance of this difference will be discussed a number of times later in this module.

For many years, low voltage power circuit breakers were essentially an assembly of parts on a welded metal frame, thus the phrase metal-frame circuit breaker. Distinguishing one low voltage circuit breaker from another at that point was rather simple. If it was a metal-frame circuit breaker, it was probably a power circuit breaker. If the circuit breaker parts were enclosed by an insulating material, it was called a molded case circuit breaker.

Certain hybrid low voltage circuit breakers were later developed and quite successful in certain markets. These circuit breakers had their parts encased in an insulating material, not unlike a molded case circuit breaker. From a performance standpoint, however, they performed more like a power circuit breaker. They had several of the same physical attributes as the power circuit breaker, but were never able to achieve the short time ratings of a power circuit breaker or pass all the power circuit breaker test standards.

This type of circuit breaker, although not tested to all the same standards as a power circuit breaker, found its application niche to be similar to traditional power circuit breakers. This design became known as a low voltage insulated case circuit breaker .
At that point, the line between frame material to identify the type of circuit breaker became blurred. All this said, the differentiating feature still remains the device's ability to meet power circuit breaker test standards, not the frame's type of construction.

Module 7

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Welcome to Module 7, which is about low voltage power circuit breakers.

Figure 1. Molded Case Version of Low Voltage Power Circuit Breakers

You will find the Advanced Low Voltage Power Circuit Breakers to be an advanced discussion of the low voltage power circuit breaker. It is intended to be a continuation of the discussion of low voltage power circuit breakers and Module 5, "Fundamentals of Circuit Breakers." It also addresses many of the same topics in a more detailed fashion. Because the Cutler-Hammer Magnum DS Low Voltage Power Circuit Breaker is the latest and most advanced product in the industry, it will be the low voltage power circuit breaker used for most examples.

Like the other modules in this series, this one presents small, manageable sections of new material followed by a series of questions about that material. Study the material carefully then answer the questions without referring back to what you've just read. You are the best judge of how well you grasp the material. Review the material as often as you think necessary. The most important thing is establishing a solid foundation to build on as you move from topic to topic and module to module.