A relay is automatic device which senses an abnormal condition of electrical circuit and closes its contacts. These contacts in turns close and complete the circuit breaker trip coil circuit hence make the circuit breaker tripped for disconnecting the faulty portion of the electrical circuit from rest of the healthy circuit.
The electricall quantities which may change under fault confition are voltages, current, frequency and phase angle. Through the change in one or more of these quantities, the fault signal their presence, type and location to the protective relays.
Having detected the fault, the relay operaes to close the trip circuit of the breaker. This reslut in the opening of the breaker and disconnection the the faulty circuit