Protecting generator is one of the most important safety activity in power plant, as the failure of generator is costly affair in terms of repair and production loss. During various time of technology development, different types of relays are in practice, typically electromagnetic relays are the first generation invention and rules the filed for nearly 50 years. Later stage of invention was static relays.

At present the state of art technology is numerical relays, which are in practice. The major advantages of numerical relays over electromagnetic relays and static relays are elimination of physical operating coils, moving disc, needle contacts, high burden CT and PT requirements etc. The added advantages of numerical relays are direct display of parameters, numeric setting method, pre-warning alarms, trip history with values, customized logics, PC connectivity for continuous monitoring and trending etc. Certain models of numeric relays also include additional functions like synchronizing, voltage / frequency adjustment commands and breaker ON/OFF commands.