A protection relay takes the secondary outputs of current and voltage transformers and energises a breaker trip coil when its measured quantities cross set thresholds for a set time. Types range from electromechanical induction-disc and static relays to numerical relays that pack overcurrent, distance, differential, frequency and breaker-failure functions into one unit with event and disturbance records. Settings are coordinated across the network so the relay nearest a fault acts first.
Relay flagged A-phase instantaneous on feeder 4, pull the disturbance record off the protection relay before anyone resets it.
Also heard as
- protective relay
- relay
Related terms
IDMT
Inverse definite minimum time: an overcurrent curve that trips faster as current rises, down to a set minimum.
Distance protection
Line protection that measures impedance to a fault and trips fastest for faults close to the relay.
Differential protection
Scheme that compares current entering and leaving a protected zone and trips when the two disagree.
86 relay
Lockout relay that latches after a serious trip and holds breakers open until someone resets it by hand.