Up until now, we have discussed circuit breaker, switch and fuse sizes based upon the normal current used by the load. However, the primary reason for these protective devices is to protect against overcurrents (either overloads or short-circuits). While overloads can be expected to have some relationship to the normal operating currents in a circuit, the short circuit current may have none. The protective device must be capable of successfully interrupting whatever current the system provides (NEC 110-9).
Overcurrent protection can be provided by circuit breakers, fuses, or a combination of these devices. The function of this protective equipment in the operation of a power system is to prevent or limit damage due to faults or overloads, and to minimize their effects on the remainder of the system. This is done by dividing the system into zones separated by protective devices. It is now accepted practice to design the system with sufficient flexibility that any defective part of the system can be disconnected while the remainder of the system can be kept in service.
The most likely fault is a short-circuit caused by insulation failure. Even with the best design possible, materials deteriorate over time. Dust, dirt, and corrosive contaminants accumulate, and normal wear and tear take their toll. Human error and equipment misuse also contribute. And, obviously, a short circuit or accidental ground must be promptly interrupted when it happens. Operating procedures should be changed if overloads occur.
