Circuit breakers and fuses are examples of equipment that is intended to interrupt current at fault levels. These are defensive devices since their job is to defend against an overcurrent condition by eliminating the fault or overload.

Since the 1978 NEC^®, circuit breakers and fuses have been required to have an interrupting rating sufficient for the current available at the line terminals of the equipment (the available fault current).

Lighting contactors, timers, relays and molded case switches are examples of passive equipment that requires additional protection from an overcurrent device in order to defend against a fault condition that can occur somewhere in the circuit controlled by the passive device. These devices do not interrupt current at fault levels but do interrupt current when called upon to do so. An example would be a time clock that is set to allow current to flow to an exterior luminaire during the night and then interrupt the current when the sun rises.

Since the 1956 NEC^®, equipment that interrupts current at other than fault levels has been required to be rated for the voltage and the current that must be interrupted. An example would be selecting a 277-volt, 30 ampere rated time clock to interrupt 30 amperes of 277-volt parking lot lighting.

See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).