Originally posted to Future-tech,
Circuit breaker discrimination (also known as selectivity) is the ability of the local circuit breaker to operate before any upstream circuit breakers under fault conditions. This applies to HV and LV, the illustration below is for a simple single-feed LV system.
With correct discrimination, a single fault on one rack power supply will trip the local MCB in the power distribution unit, but not the distribution circuit breaker or worse still, the main circuit breaker.
In the same way, a fault on the power distribution unit will trip the distribution breaker feeding it, not the main circuit breaker. The intention is to limit the service interruption to the minimum amount for any fault, whether this fault is overload or short-circuit.
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