Dry Type Specialty Transformers

00:17 / Posted by tech data / comments (4)

A variety of specialty dry type transformers exist. Typically not ventilated, this type of transformer usually has a small rating, and is capable of moving excessive heat away from the core and coils naturally, without the need for ventilation openings or other heat dissipation means. In most designs, this is accomplished by surrounding the core and coils with special material mixtures which absorb the heat and provide a solid seal. This type of transformer is ideal for hazardous locations, and is usually referred to as an Encapsulated Transformer.

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Standards and Codes

22:57 / Posted by tech data / comments (0)

Standards and Codes

The electrical industry is guided by a set of Standards and Codes for designing, manufacturing and supplying electrical equipment, and because we are part of a global economy, both domestic and international considerations must be included. There is no room for compromise when performance, quality, and safety are involved. Exacting standards and codes are established to provide a set of guidelines relative to the design, testing and manufacture of all types of electrical equipment.

Standards
A number of standards are established through a consensus process within a particular industry. Once the consensus is achieved, the standards are published by an independent standards organization, such as ANSI (American National Standards Institute).
Some standards are not required, but it may be impossible to sell a particular piece of equipment in a certain area of the world unless the relevant standard is met.

Today, many standards from different countries stipulate similar guidelines. A piece of equipment meeting one set of standards might very well meet another set with minor changes. In most instances, however, meeting the requirements of any set of standards must be proven and certified through testing.

Testing is frequently carried out at independent testing laboratories. For example, you are probably already familiar with UL (Underwriters Laboratories, Inc.). A great deal of equipment is designed, built and tested in accordance with UL Standards. Appliances in your house display the UL approval. You may not know exactly what UL requires of that appliance, but the UL approval gives you confidence that it will function safely, if used correctly.


Codes
Electrical codes are sets of rules established by governing bodies which state:
• Type of equipment to be used in a given situation
• Appropriate use
• Installation procedures, including how and where it should be installed

Codes usually carry mandatory compliance, and can apply nationally or to a more limited area, such as a single local municipality. In any case, such codes can be used to facilitate the successful installation of equipment, or stop it dead in its tracks. Codes are powerful, and there must be a keen awareness of the various codes and their applications.

One of the best known set of codes is the NEC (National Electrical Code) , which works in conjunction with UL requirements. These codes are applicable throughout the United States, and regulate all electrical equipment used in power distribution systems, from the source to private residences, and even to the configuration of the circuits within homes.

As you learn about different types of electrical equipment, you will become very aware of the standards and codes that are most relevant to that particular type of equipment. For now, just be aware of their existence and importance.

Here is a list of the most common standards and codes (but it is far from all-inclusive):

• ANSI (American National Standards Institute)
• BSI (British Standards Association)
• CE Mark (Certified European Mark)
• CEC (Canadian Electric Code)
• CSA (Canadian Standards Association)
• IEC (International Electrotechnical Commission)
• IEEE (Institute of Electrical and Electronic Engineers)
• ISO (International Standards Organization)
• NEC (National Electrical Code)
• NEMA (National Electrical Manufacturers Association)
• UL (Underwriters Laboratories, Inc.)

Nameplates and Labels
A piece of electrical equipment usually has a nameplate affixed to it that provides valuable information about the equipment ratings and the conditions under which it can operate. In addition, electrical assemblies have nameplates to identify applicable standards and to determine appropriate applicable equipment. Make it a habit to look at nameplates and the information they provide.

Nameplates may be found in the following places:

• Attached to the equipment itself
• Attached to the packing material during shipment
• In literature relating to the equipment
Labels can serve a wide variety of purposes:
• Provide handling and installation instructions
• Provide operational information
• Act as a safety feature by issuing warnings and/or cautions concerning specific dangers or

problems
One of the most important uses of labels is as a safety feature. The best way to eliminate hazards is to design foolproof equipment. Because individuals have, from time to time, defeated the best efforts of designers to provide fail-safe designs, it is necessary to warn people about potential hazards.

Safety labels will contain a single enlarged word, such as DANGER, WARNING, CAUTION, or NOTICE. The word used depends on the classification of the potential hazard.

Typical Look of Warning and Caution Safety Labels

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In the Work Place

22:52 / Posted by tech data / comments (0)

If a building has 65,000 amperes of fault current available at the service entrance, every circuit protection device must also be rated at 65,000 amperes. Below is the one-line of what this would look like. A system such as this provides excellent equipment protection and is highly reliable.

Service continuity is a little less than a selectively coordinated system, but the initial cost is also less. When compared to a series-combination system, the fully rated system provides the same level of service continuity with a higher initial cost.

Selectively Coordinated Systems:
As with the fully rated system, all circuit breakers are fully rated to interrupt the maximum fault current available at their point of application. The selectively coordinated system maximizes service continuity because only the breaker nearest the fault operates to isolate the faulted circuit.

Each upstream breaker in the power distribution system incorporates short time delay tripping. The upstream breaker must be capable of withstanding the thermal and magnetic stresses delivered by the fault current for the time period required by the breaker nearest the fault to trip.
The selectivity of the system can be based, up to a point, on:

• Magnitude of the fault current providing current selectivity.
• Fault withstand time providing time selectivity.
• Both current and time providing complete selectivity.

The selectively coordinated system is the most costly of the three basic systems. However, it provides the best overall protection of equipment and maximum continuity of service.

Series Rated System: The series rated system states that the main upstream circuit protection device must have an interrupting rating equal to or greater than the available fault current of the system, but downstream devices connected in series can be rated at lower values. Under fault conditions, both the main device and the downstream device would open to clear the fault. Series rated breaker combinations must be tested in series in order to be UL Listed.

In the Work Place

For example, a building with 42,000 amperes of available fault current might have the breaker at the service entrance rated at 42,000 amperes and the additional downstream breakers rated at 18,000 amperes.

Series rated systems are intended for use on systems where the branch circuits are primarily lighting and other resistive loads. If the branch circuits supply motor loads, fully rated or selectively coordinated systems should be used. The major advantage to this system is it allows you to use lower-cost branch breakers. However, because both breakers trip on a major fault, service continuity may not be as high as the other systems.

The three basic systems offer protection of electrical conductors and equipment with equal  effectiveness. The initial cost and continuity of service are the varying factors. The decision of which protection system to use should be based on the application variables and needs.

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System Protection

22:48 / Posted by tech data / comments (0)

The primary goal of all electrical power distribution systems is to provide power to electrical equipment with the utmost safety. System protection is designed to add the remaining goals of equipment/conductor protection and service continuity at the most reasonable cost.

Protective equipment, such as molded case circuit breakers, during Overcurrent conditions, must quickly isolate the affected section of the power system to maintain service to other sections. They must also minimize equipment damage and limit the extent or duration of outages. We will first discuss what overcurrent conditions are and then talk about system coordination.

Overcurrent Conditions

Overcurrent: This is a current that is higher than the amount of current a conductor or piece of equipment can carry safely. An overcurrent condition left unchecked can cause insulation and/or equipment damage as a result of excessive temperature and/or dynamic stresses. The cable insulation is the most vulnerable to overcurrent conditions. The conductor itself may be able to withstand extremely high heat, but the insulation around the conductor cannot.

There are three types of overcurrent conditions:
• Overloads
• Short circuits
• Ground faults

Overloads:
Overloads are the result of placing excessive loads on a circuit, beyond the level the circuit was designed to handle safely. Insulation deterioration in electrical conductors is most often the result of such overload conditions. When an overload condition exists, a temperature buildup occurs between the insulation and the conductor.

How many times have you had to go to the loadcenter in your house and reset a circuit breaker? An overload condition is created, heat builds up, and the circuit breaker opens to protect the cable and, ultimately, the house.

Short Circuits: Short Circuits, frequently called Faults, are usually caused by abnormally high currents that flow when insulation on a conductor fails. When the insulation that protects one phase from another or one phase from ground breaks down, short circuit currents can be expected to flow. The short circuit condition must be eliminated quickly to protect against damage to the system.

A simple water analogy can be used to compare the current that normally flows in a circuit to a short circuit current.


A large dam is built and feeds a controlled amount of water into a small river. Downstream, a small town is built along the river's banks. The amount of water permitted to enter the river safely is independent of the amount of water behind the dam. Should the dam break and suddenly release the water behind it, the town could be severely damaged or even washed away. This sudden rush of water is like the flow of current in a circuit under fault conditions. The amount of damage done depends on the amount of water stored behind the dam or the amount of current available to feed the fault.

Ground Faults: A Ground Fault is a particular type of short circuit. It is a short circuit between one of the phases and ground. It is probably the most common low level fault experienced, especially on lower voltage circuits.

Ground fault currents are often not large in magnitude and can go undetected for a period of time. This type of fault might occur in the electrical outlets located in the bathroom or in other areas where water could be present.

System Coordination
Because of the overcurrent conditions that can occur in distribution systems, thought has to be put into properly coordinating that system. There are three types of system coordinations: Fully Rated, Selectively Coordinated and Series Rated.

Fully Rated Systems: In a fully rated system, all circuit breakers are rated to operate independently. They all have an Interrupting Rating adequate for the maximum Fault Current available at their point of application. All of the breakers are equipped with long time delay and instantaneous overcurrent trip elements.

The fully rated method selects circuit protection devices with ratings equal to or greater than the available fault current.

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