Selectivity is the coordination of protective devices so a fault is cleared by the device immediately upstream of it, disconnecting as little as possible. It is achieved by grading current settings, time delays or both, and by energy-based or zone-selective interlocking schemes in low-voltage switchboards. A selectivity study plots the time-current curves of fuses, breakers and relays on log-log axes to prove there are no overlaps.
Lost the whole board because the main went before the 63 A outgoer, there's no selectivity between those two curves.
British engineers say discrimination, IEC documents say selectivity and North Americans say coordination; all three describe the same goal.
Also heard as
- discrimination
- coordination
Related terms
IDMT
Inverse definite minimum time: an overcurrent curve that trips faster as current rises, down to a set minimum.
Protection relay
Device that watches currents and voltages and orders a circuit breaker to trip when it detects a fault.
Nuisance trip
Protective device opening with no real fault present, usually from bad settings, leakage or transients.