![]() When a fault occurs in a particular zone, the device dedicated for its protection will sense the current and isolate the fault from the remaining system. They only operate on faults that lie within their “zone of protection”. Each protective device is dedicated to a particular zone. Selective coordination is accomplished by adjusting and rearranging the time current curves of protective devices such that their settings or curves have minimum or no overlapping. In case, the breaker C.B-5, due to any reason, does not clear the fault, then C.B-2 clears it after some delay and if, due to any reason, C.B-2 is not able to clear fault, then C.B-1 issues a trip (which could be the worst case scenario). ![]() In this case, C.B-5 should be able to clear the fault in the least possible time and no other breaker (in this case C.B-2 and C.B-1) should trip during this time. ![]() The above figure shows a fault that occurs below circuit breaker 5 (C.B-5). In order to understand how protective devices are coordinated, let us take an example: “ Localization of an over-current condition to restrict outages to the circuit or equipment affected, accomplished by the choice of over-current protective devices and their ratings or settings.” So, you won't have the power outage if there is a fault somewhere downstream.Īccording to NEC article 100, Selective coordination is defined as: The fuse or a circuit breaker closest to the fault opens without opening the fuse or the circuit breaker that feeds it (from the upstream side). Complete selectivity means that the protective devices will minimize the effect of a short circuit or other undesirable event on the power system. Selective Coordination is defined as a method of adjusting the opening times of overcurrent protection devices so that the fuses or breakers nearest to the faults open first. The magnetic part of the breaker senses high overcurrent or short circuit and issues a trip signal. Instantaneous Trip: There is no intentional delay in tripping.As the current increases, heating goes on and overcurrent clearing time decreases. Bimetallic strip in the breaker heats up on high current causing the contacts to break up after a delay. Delay Trip: This trip is due to overcurrent sense by thermal part of the breaker. ![]() Thermal magnetic breakers have slightly different characteristic graphs than electronic (solid state) breakers as they have only two settings:
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