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The principle of Selectivity (Discrimination) is based upon an analysis of several circuit breaker characteristics. These include time-current (tripping) curves, peak-let-through-current (Ipeak) and energy let-through (I²T).
The maximum selectivity level with two nominated breakers in series under short-circuit conditions is often stated by vendors in Technical Reference manuals.
Selectivity can be enhanced beyond the breaking capacity of the downstream breaker provided it is backed up by an appropriately selected upstream breaker, which should not trip (unlatch) under the stated short circuit current.
Selectivity works as the upstream breaker heats up under fault conditions providing impedance in the fault circuit reducing the overall fault severity.