A protective relay is a device that detects the fault and initiates the operation of the circuit breaker to ioslate the defective element from the rest of the system.
The relays detect the abnormal conditions in the electrical circuits by constantly measuring the electrical quantities which are different under normal and fault conditions. The electrical quantities which may change under fault conditions are voltage, current, frequency and phase angle. through the changes in one or more of these quantities, the faults signals their presence, type and location to the protective relays. Having detected the fault, the relays operates to close the trip circuit of the breaker. This results in the opening of the breaker and disconnection of the faults circuit.