Short Circuit & Coordination Study

Many times American Electrical Testing is asked, "What are Short Circuit and Coordination Studies?" and "Why should such studies be performed on electrical distribution systems?" This has been written to give you a better understanding of these types of studies, as well as an understanding of the importance of having such a study performed on a periodic basis. While a Short Circuit Study can be performed independently of a Coordination Study and vice versa, the results of the Short Circuit Study do have a bearing on obtaining meaningful results from the Coordination Study. Now let us examine how each type of study is made and what the study can tell you about your electrical power distribution system.

A Short Circuit Study is made to determine the magnitude of current available at various selected points of the electrical distribution system when a fault occurs. The study is made assuming a three phase fault condition, since this is the type of fault which will produce the maximum current. Maximum current occurs because there is less impedance between the source of power and the point of the fault than there is in any other type of fault (i.e., phase to phase, 2 phases to ground, 1 phase to ground). Impedance between the power source and the point of fault is the deterrent to current flow; the more impedance, the less current flow and vice versa. Current-limiting or impedance sources in a power system are transformers, reactors, cables, switches, breakers, and connections. Current sources are generators and, at the time of faults, capacitors and motors. In the Short Circuit Study, the points at which faults are assumed are located on the source side of the device which will be called upon to interrupt the power system when an actual fault occurs. This is done to determine whether or not the device will be able to interrupt the current available at the time of the fault. Many times the source side of the interrupting device is an electrical bus in a piece of switchgear. When current flows through the bus bars, forces are exerted between the different phases of the bus. When a fault occurs, the current magnitude is greater than normal current flow, and the forces between the bus bars are increased by the square of the current increase. Thus if the fault current is four times normal, the forces between the phases are sixteen times as great. The Short Circuit Study gives us the information necessary to determine if our breakers and fuses are capable of interrupting a fault, as well as the means of determining whether or not the bus sections of the switchgear are supported adequately to withstand the forces generated from the fault currents. In addition, this type of study tells us the maximum current available to operate an electrical protective device (i.e., Molded Case Circuit Breaker, Low Voltage Power Circuit Breaker, Relays, Fuses, etc.).



A Coordination Study is made to determine the proper selection of, and settings for, our electrical protective devices. Ideally in any electrical distribution system, when a fault occurs it is desirable to interrupt and remove only that portion of the electrical system in trouble, leaving the remaining system intact. When this is accomplished a system is said to be coordinated. A coordinated system occurs only with the proper selection of the operating characteristics of its electrical protective devices, starting with the device most remote from the power source. The setting for this remote device is based at its source-side terminals. Therefore, without a Short Circuit Study it is possible to set or select electrical protective devices to operate at a current value higher than the available fault current the protective device will see.

In addition to the Short Circuit and Coordination Studies, a Load Study should be made on an electrical distribution system to determine the adequacy of the system, as well as the proper sizing and setting of electrical protective devices.

In every power system loads are added and removed, transformers are added and replaced and utility companies are adding generation and transmission facilities. As these things take place, the impedance of the overall electrical system changes, as do the protection requirements. With the added capacity on utility systems, our electrical distribution system is subjected to higher values of short circuit currents and greater forces than when the system was designed. Thus, the adequacy or inadequacy can only be determined through a Short Circuit and Coordination Study or when the electrical protective device fails to interrupt a fault and explodes.