The relays used in the power system operate by virtue of the current or voltage supplied by the  current and voltage transformers connected in various combinations to the system element that is  to be protected. Through the individual or relative changes in these two quantities, faults signal  their presence, type and location to the protective relays can be found. Having detected the fault  the relay operates the trip circuit which results in the opening of the circuit breaker and hence in
the disconnection of the healthy circuit.
Most  of  the  relays  are  of  electro-mechanical  type.  They  work  on  the  following  two  main  operating principles:
(i) Electromagnetic attraction (ii) Electromagnetic induction

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