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.
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
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.
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
Labels:
Circuit Breaker
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
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
Labels:
Circuit Breaker
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
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.
Labels:
Circuit Breaker
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.
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.
Labels:
Circuit Breaker
• 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.
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].
Labels:
Circuit Breaker
• 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].
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
Labels:
Circuit Breaker
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
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.
Labels:
Circuit Breaker
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.
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