Relay: - It is an automatic device by means of which an electrical circuit is indirectly controlled (opened or closed) and is governed by a change in the same or another electrical circuit.

Protective relay: - It is an automatic device which detects an abnormal condition in an electrical circuit and causes a circuit breaker to isolate the faulty element of the system. In some cases it may give an alarm or visible indication to the alert operator. 

Operating force or torque: - A force or torque which tends to close the contacts of the relay.

Restraining force or torque: - A force or torque which opposes the Operating force or torque.

Pick-up: - The threshold value of the actuating quantity (current, voltage etc.) above which the relay operates.

Reset or drop out (level): - The threshold value of the actuating quantity (current, voltage etc.) below which the relay is de-energised and returns to its normal position or state.

Operating time: - It is the time which elapses from the instant at which the actuating quantity exceeds the relay's pick-up value to the instant at which the relay closes its contacts.

Reset time: - It is the time which elapses from the moment the actuating quantity falls below its reset value to the instant when the relay comes back to its normal (initial) position.

Back-up relay: - A back-up relay operates after a slight delay, if the main relay fails to operate.

Back-up protection: - The back-up protection is designed to clear the fault if the primary protection fails. It acts as a second line of defence.

Primary protection: - If a fault occurs, it is the duty of the primary protective scheme to clear the fault. It acts as a first line of defence. If it fails, the back-up protection clears the fault.

Measuring relay: - It is the main protective relay of the scheme, to which energising quantities are applied. It performs measurements to detect abnormal conditions in the system to be protected.

Instantaneous relay: - It has no intentional time delay in its operation. It operates in less than or equal to0.1 second.

Inverse time relay: - A relay in which the operating time is inversely proportional to the magnitude of the operating current.

Definite time relay: - A relay in which the operating time is independent of the magnitude of the actuating current.

Inverse definite minimum time relay (IDMT): - A relay which gives an inverse time characteristic at lower values of the operating current and definite time characteristic at higher values of the operating current.

Reach: - It is the maximum length of the line upto which the relay can protect. This term is related to distance relay.

Overreach: - Sometimes a relay may operate even when a fault point is beyond its present reach i.e. its protected length.

Underreach: - Sometimes a relay may fail to operate even when a fault point is within its reach, but it is at the far end of the protected line.

Burden: - The power consumed by the relay circuitry at the rated current.

Selectivity or discrimination: - It is the ability of a relay to discriminate between faulty conditions and normal conditions or between a fault within the protected section and outside the protected section. In other words, it is the quality of the protective system by which it distinguishes between those conditions for which it should operate and those for which it should not.

Reliability: - A protective relay must operate reliably when a fault occurs. A typical value of reliability is 95 %.

Sensitivity: - A protective relay should be sensitive enough to operate when the magnitude of the actuating quantity exceeds its pick-up value.

Stability: - This is the ability of the protective system to remain inoperative under all load conditions and also in case of external faults. The relay should remain stable when a heavy current due an external fault is flowing through it.

Fast operation: - A protective relay should be fast enough to cause the isolation of the faulty section as quickly as possible to minimise the damage and to maintain the stability.