Fault protection requires careful selection of the protective devices. Adequate short-circuit interrupting capacity, routine disconnect, and on/off circuit switching are all needed. A switching device should meet these conditions:
The two devices commonly used for the first condition are circuit breakers rated for switching duty and switches. The two devices commonly used to interrupt excessive current are circuit breakers and fuses. Switches can include fuse clips and circuit contacts in a single assembly. Circuit breakers combine switching and overcurrent protection functions.
Overloads, if continued, cause conductor circuit or motor overheating. They may not require immediate disconnect from the system; only before the operating temperatures become too high. The overcurrent protective device used for overloads should have a time-overcurrent characteristic. A short-circuit should be removed from the system immediately; however, a small amount of time may be allowed to permit selective operation of the device closest to the short-circuit location.
Arcing (and other) ground faults may be smaller than the normal full load current. Therefore, a different scheme must be used if the system is to be protected from this type of fault. This ground fault current may cause serious damage at the location of the fault, but still may be too small to trip the overcurrent device. This ground-fault sensing and protection concepts are discussed elsewhere in this manual. They involve auxiliary trip devices operated by external sensors. The NEC (230-95) requires that ground fault protection be provided on certain services rated 1,000A or more, where at least 1,200A of ground fault current might flow. These systems need to be coordinated with the downstream overload protection devices, to prevent shutting down the entire system in the event of a down-stream ground fault.
