Prospective short-circuit current

The prospective short-circuit current (PSCC), available fault current, or short-circuit making current is the highest electric current which can exist in a particular electrical system under short-circuit conditions. It is determined by the voltage and impedance of the supply system. It is of the order of a few thousand amperes for a standard domestic mains electrical installation, but may be as low as a few milliamperes in a separated extra-low voltage (SELV) system or as high as hundreds of thousands of amps in large industrial power systems. Protective devices such as circuit breakers and fuses must be selected with an interrupting rating that exceeds the prospective short-circuit current, if they are to safely protect the circuit from a fault. When a large electric current is interrupted an arc forms, and if the breaking capacity of a fuse or circuit breaker is exceeded, it will not extinguish the arc. Current will continue, resulting in damage to equipment, fire, or explosion. In designing domestic power installations, the short-circuit current available on the electrical outlets should not be too high or too low. The effect of too high short-circuit current is discussed in the previous section. The short-circuit current should be around 20 times the rating of the circuit to ensure the branch circuit protection clears a fault quickly. Quick disconnecting is needed, because during a line-to-ground short circuit the grounding pin potential on the power outlet can rise relative to the local earth (concrete floor, water pipe etc.) to a dangerous voltage, which needs to be shut down quickly for safety. If the short-circuit current is lower than this figure, special precautions need to be taken to make sure that the system is safe; those usually include using a residual-current device (a.k.a. ground fault interrupter) for extra protection. The short-circuit current available on the electrical outlets is often tested when inspecting new electrical installations to make sure that the short-circuit current is within reasonable limits.