When an electrical distribution system is designed and constructed, a fault-current coordination study should be conducted, and circuit protective devices should be sized and set according to the results of the study. In time, however, the electrical system configurations are often changed due to the changing needs of the end users.
If the coordination and capability of the electrical equipment are not reviewed at the time of the changes, faults could result in unnecessary tripping of a main breaker or, even worse, an explosion of equipment that was thought to be in good condition.
When system conditions change, the results that were obtained in the original fault-current coordination study may no longer apply to the current system. Unnecessary tripping, known as lack of selectivity, could be caused by poor coordination.
An equipment explosion could result from the interrupting capability of the circuit breaker being exceeded. Both indicate a clear need for an updated fault-current coordination study.
Utility systems delivering higher fault currents
The demand for electricity, particularly in the industrial and commercial environment, has been steadily increasing. Consequently, utility systems have grown much larger and have become capable of delivering much higher fault-currents at service points than in the past.
Therefore, protective devices that were properly applied at the time they were installed may have become inadequate after system changes, and the protective system may no longer be coordinated. When available fault current increases to the point at which it exceeds protective device interrupting and withstand ratings, violent failure is possible, regardless of how well the devices are maintained.
Protection in an electrical distribution system
System and equipment protective devices are a form of insurance. This insurance pays nothing as long as there is no fault or other emergency.
When a fault occurs, however, properly applied protective devices reduce the extent and duration of the interruption, thereby reducing the exposure to personal injury and property damage. If, however, the protective system does not match system needs, just as an insurance policy should keep up with inflation, it is no help at all. It is the responsibility of the system operator to ensure proper system protection and coordination.
Protective equipment set to sense and remove short circuits
In medium-voltage systems, the protective equipment for feeder conductors is often set to sense and remove short circuits, but not necessarily to provide overload protection of circuits. Device settings sometimes are purposely chosen low enough to sense and provide a degree of overload protection.
Operators should be aware of this so that a protective device that is set lower than necessary for coordination does not cause a false tripping action during system switching procedures. System protection coordination is an important consideration in switching systems with loop feeds and alternate sources. To avoid false tripping action, operators should be aware of the settings and any probable temporary overloads or circulating currents during switching.
SAFETY AND POWER SYSTEM MANAGEMENT BASIC INFORMATION AND TUTORIALS
A well designed and constructed power system will not provide a safe and reliable operation unless it is properly managed. Any electrical power distribution system, from the smallest system to the largest and most complex system, needs to be managed. As systems become larger in size and complexity, the problems of system management increase, thereby requiring more time and attention from the system-operating personnel.
Good design, proper installation, quality assurance, and sound operating and maintenance programs provide the basic foundation for the safe and reliable operation of industrial electric power systems. A plant engineer who is faced with the task of improving the plant's electric power system performance, however, will likely find that programs to reduce human error are more cost-effective than system modifications or additional preventive maintenance. In fact, given good design and a sound maintenance program, the inherent system reliability can only be achieved by the reduction of operating error.
The operation of an electric power system should also address the problem of human errors. The following examples should be considered:
Following a severe thunderstorm, a plant shift supervisor made a walk-through inspection of the plant's primary distribution switchgear. Upon seeing a red light for each circuit breaker, he immediately tripped each circuit breaker in order to obtain a green-light indication. Because he incorrectly thought that the red light meant "open," he shut down the entire plant.
One of a plant's two steam boilers was down for annual inspection and maintenance. An electrician who was assigned to make a modification to the boiler control circuit erroneously began working on the operating boiler control circuit and shut down the operating boiler.
An investigation of a 15 kV outdoor bus duct fault revealed that production personnel routinely turned off outside lighting at the beginning of the day shift by switching off circuit breakers in a 120 V distribution panel. The bus duct heater circuit was incorrectly identified, and was being switched off with the lighting circuits.
It is a natural tendency to blame equipment for failures, rather than human error. The bus duct fault in the last example could have been classified as an equipment failure; however, the prime cause was improper operation (human error) of the bus duct heaters.
Most plant electrical outages that clearly are not due to equipment failure, lightning, or utility disturbances can be prevented by making an objective investigation of the potential for outages and by following these guidelines:
a) Document the system and identify the equipment.
b) Plan switching operations in detail.
c) Secure equipment from unintentional operation.
d) Clearly define operating responsibility and adhere to it rigidly. System operation can and should be managed.
Effective managers of a power system will consider load distribution, system integrity, power factor, system protection coordination, and operating economics. Each of these areas is discussed in this chapter, thus showing how all of these considerations relate to each other. No area of industrial and commercial power system management is independent of the other.
Good design, proper installation, quality assurance, and sound operating and maintenance programs provide the basic foundation for the safe and reliable operation of industrial electric power systems. A plant engineer who is faced with the task of improving the plant's electric power system performance, however, will likely find that programs to reduce human error are more cost-effective than system modifications or additional preventive maintenance. In fact, given good design and a sound maintenance program, the inherent system reliability can only be achieved by the reduction of operating error.
The operation of an electric power system should also address the problem of human errors. The following examples should be considered:
Following a severe thunderstorm, a plant shift supervisor made a walk-through inspection of the plant's primary distribution switchgear. Upon seeing a red light for each circuit breaker, he immediately tripped each circuit breaker in order to obtain a green-light indication. Because he incorrectly thought that the red light meant "open," he shut down the entire plant.
One of a plant's two steam boilers was down for annual inspection and maintenance. An electrician who was assigned to make a modification to the boiler control circuit erroneously began working on the operating boiler control circuit and shut down the operating boiler.
An investigation of a 15 kV outdoor bus duct fault revealed that production personnel routinely turned off outside lighting at the beginning of the day shift by switching off circuit breakers in a 120 V distribution panel. The bus duct heater circuit was incorrectly identified, and was being switched off with the lighting circuits.
It is a natural tendency to blame equipment for failures, rather than human error. The bus duct fault in the last example could have been classified as an equipment failure; however, the prime cause was improper operation (human error) of the bus duct heaters.
Most plant electrical outages that clearly are not due to equipment failure, lightning, or utility disturbances can be prevented by making an objective investigation of the potential for outages and by following these guidelines:
a) Document the system and identify the equipment.
b) Plan switching operations in detail.
c) Secure equipment from unintentional operation.
d) Clearly define operating responsibility and adhere to it rigidly. System operation can and should be managed.
Effective managers of a power system will consider load distribution, system integrity, power factor, system protection coordination, and operating economics. Each of these areas is discussed in this chapter, thus showing how all of these considerations relate to each other. No area of industrial and commercial power system management is independent of the other.
WIRE SIZES AMERICAN WIRE GAGE (AWG) FORMER BROWNE & SHARPE
Wire Sizes
In the United States, it is common practice to indicate wire sizes by gage numbers. The source of these numbers for electrical wire is the American Wire Gage (AWG) (otherwise known as the Brown & Sharpe Gage).
A small wire is designated by a large number and a large wire by a small number as shown in below.
The diameter of a No. 0000 wire is 0.4600 inch or 460 mils; the diameter of a No. 36 wire is 0.0050 inch or 5 mils. There are 38 other sizes between these two extremes. For example, a No. 8 wire is 0. 1285 inch (128.5 mils) in diameter and a No. 1 wire is 0.2576 (257.6 mils) in diameter.
It has proved convenient to discuss the cross-section area of a wire in circular mils. A circular mil (cm) is the area of a circle having a diameter of 0.001 inch or 1 mil. Because it is a circular area unit of measure, it is necessary only to square the number of mils given in the diameter of a wire to find the number of circular mils in a circle of that diameter.
Thus, a conductor with a 1-mil diameter would have a 1-circular-mil (cm) cross-section area; a 3-mil diameter wire would have a 9-cm area; and a 40-mil-diameter wire, a 1600-cm area.
For conductors larger than 0000 (4/0) in size, the wire sizes are expressed in circular mils; for example, 350,000 cm, 500,000 cm, and so on. (Sometimes these are expressed as 350 mcm, 500 mcm, etc.)
In the United States, it is common practice to indicate wire sizes by gage numbers. The source of these numbers for electrical wire is the American Wire Gage (AWG) (otherwise known as the Brown & Sharpe Gage).
A small wire is designated by a large number and a large wire by a small number as shown in below.
The diameter of a No. 0000 wire is 0.4600 inch or 460 mils; the diameter of a No. 36 wire is 0.0050 inch or 5 mils. There are 38 other sizes between these two extremes. For example, a No. 8 wire is 0. 1285 inch (128.5 mils) in diameter and a No. 1 wire is 0.2576 (257.6 mils) in diameter.
It has proved convenient to discuss the cross-section area of a wire in circular mils. A circular mil (cm) is the area of a circle having a diameter of 0.001 inch or 1 mil. Because it is a circular area unit of measure, it is necessary only to square the number of mils given in the diameter of a wire to find the number of circular mils in a circle of that diameter.
Thus, a conductor with a 1-mil diameter would have a 1-circular-mil (cm) cross-section area; a 3-mil diameter wire would have a 9-cm area; and a 40-mil-diameter wire, a 1600-cm area.
For conductors larger than 0000 (4/0) in size, the wire sizes are expressed in circular mils; for example, 350,000 cm, 500,000 cm, and so on. (Sometimes these are expressed as 350 mcm, 500 mcm, etc.)
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