Relay

Functional Requirements of the Relays:

i)   Reliability   :    The most important requisite of protective relay is reliability since they supervise the circuit for a long time before a fault occurs; if a fault then occurs, the relays must respond instantly and correctly.

ii) Selectivity  :  The relay must be able to discriminate(select) between those conditions for which prompt operation is required and those for  which no operation, or time delayed operation is required.

iii) Sensitivity :  The relaying equipment must be sufficiently sensitive so that it operates reliably when required under the actual conditions that produces least operating tendency.

iv)  Speed       :    The relay must operate at the required speed. It should neither be too slow which may result in damage to the equipment nor should it be too fast which may result in undesired operation.