A power system consisting of generators, transformers, transmission and distribution lines. So sooner or later some failure can occur on any part of the system. When any fault occurs on any part of the system, it must be detected and the unhealthy part of the system must be disconnected from the healthy circuit.

The reason of disconnection is to provide uninterrupted power supply to the consumers and secondly, a rapid disconnection limits the damage spreading into the system. The detection of a fault and disconnection of a faulty circuit can be achieved by using fuses, relays and circuit breakers. A fuse performs both detection and disconnection but only in low voltage systems. For high voltage system (above 3.3kv) relays and circuit breakers are employed to serve the desired functions of automatic protection