Imagine a wrench inadvertently left in a starter following service. Touching two terminals, it completes the circuit between them when the panel is energized. What results is a potentially dangerous situation or "fault condition" caused by the low-impedance, phase-to-phase or phase-to-ground connection ... a "short circuit."

Fault current, also called "short-circuit current" (I_SC), describes current flow during a short. It passes through all components in the affected circuit. Fault current is generally very large and, therefore, hazardous. Only the combined impedance of the object responsible for the short, the wire, and the transformer limits its magnitude.

One objective of electrical distribution system design is to minimize the effect of a fault, i.e. its extent and duration, on the uninterrupted part of the system. Coordinating the sizes of circuit breakers and fuses assures that these devices isolate only the affected circuits. Put simply, it prevents a short at an outlet from shutting down power to the entire building!

Calculating the magnitude of short-circuit current is prerequisite to selecting appropriate breakers and fuses. If the distance between transformer and starter is brief, the calculation can be simplified by ignoring the impedance of the interconnecting wiring ... a simplification that errs on the side of safety. We can also assume that the source of the fault has zero impedance, i.e. a "bolted" short. Given these assumptions, the only impedance left to consider is that of the transformer. (Impedance upstream of the transformer is usually negligible.)

Suppose the 1,500-kVA transformer in our example has impedance of 5.75 percent. With this value and the equation below, we can determine how much fault current a short circuit will produce. As you can see, a short would force our wiring to carry more than 30,000 amps when it was designed to handle only 400 amps!

Short-circuit current is often two orders of magnitude greater than normal operating current. Unless a circuit breaker or fuse successfully interrupts the fault, this enormous amperage rapidly heats components to very high temperatures that destroy insulation, melt metal, start fires ... even cause an explosion if arcing occurs. The inherent likelihood of severe equipment and property damage, as well as the risk of personal injury or death, underscores the importance of sufficient electrical distribution system protection.

Continue on to Interrupt Rating, n.