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Short Circuit
The short circuit study is fundamental. Fault levels vary from system to system and location to location. This study calculates the maximum available short circuit current at various points throughout the system. Calculated values are then used to evaluate the application of protective devices, and to develop circuit breaker trip settings (see Protective Device Coordination).
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.).