Selective Tripping of Low-Voltage Air Circuit Breakers

Selective or sequential tripping in low-voltage air circuit breaker application provides for removal from the system of a faulted part only, leaving the rest of the system in operation. Present standard circuit breakers generally are not suitable. Solution of the problem is through direct-acting series tripping means with distinct current-time characteristics in ranges of overcurrent and fault current, with minimum time to complete interruption the objective. This subjects the circuit breaker to the very severe duty of having to carry short-circuit currents for appreciable times, whereas standard circuit breakers have been designed to open instantaneously. Modified standard circuit breakers may be applied to selective tripping, with limitations of application based upon (1) rated interrupting capacity and (2) series trip coil rating. As tests show that modified standard circuit breakers will carry short-circuit current for necessary times, they also may be applied with relay schemes requiring delayed tripping. In systems using cascade or backup arrangements of circuit breakers, little is gained by selective tripping. It is recommended that for a particular installation choice be made between selective tripping and cascade arrangement, according to relative importance of advantages of each.