All components and wiring in an electrical distribution system offer some degree of resistance. Under normal conditions, the heat produced when current flows against this resistance readily dissipates to the surroundings. However, the enormous current generated during a short circuit produces damaging heat at a much faster rate than can be safely dispersed. Interrupt the current and you stop adding heat to the system.

As Figure 2 suggests, time is a critical determinant of the amount of heat (energy) added. An electrical short that lasts three cycles, for example, adds six times the energy of one lasting just one-half cycle. It's in this sense that all circuit breakers and fuses "limit" current.

Figure 2 also shows the effect of a current-limiting device. To be truly current-limiting, the interrupting device must open the circuit within one-quarter cycle (1/240 second), i.e. before the fault current peaks.

Remember our chiller-motor scenario? If there's no starter available with a short-circuit withstand rating greater than 31,400 amps, compliance with NEC Section 110-10 requires that we either:

• Add a current limiting device, i.e. usually a fuse, but sometimes a circuit breaker and fuse in series, that restricts the fault current to a value less than the starter's short-circuit withstand rating. Or ...

• Redesign the electrical distribution system to reduce the fault current. Choosing this approach warrants a more detailed fault-current analysis.

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