Discrimination (Selectivity)

In a properly coordinated protection system, a fault on a final circuit should trip only the local circuit breaker, not the distribution board main switch or upstream devices. This selective operation — discrimination — prevents unnecessary supply interruption to healthy circuits. Full discrimination means the downstream device always operates first, regardless of fault current magnitude. Partial discrimination means selectivity is maintained up to a specified fault current level, above which both devices may trip. Engineers verify discrimination by comparing time-current curves: the downstream device curve must lie entirely below and to the left of the upstream device curve across all relevant fault current levels. Manufacturers publish selectivity tables showing verified combinations. Achieving discrimination becomes more challenging at high fault levels where circuit breakers operate in their instantaneous region. Solutions include using time-delayed upstream devices, selecting devices with energy-limiting characteristics, or using zone-selective interlocking. Poor discrimination results in widespread blackouts from a single local fault, which is unacceptable in hospitals, data centres, and industrial processes.