Distance protection is a widely used scheme for the protection of high and extra high voltage transmission and subtransmission lines. This scheme employs a number of distance relays which measures the impedance or some component line impedances at the relay location. The measured quantity is proportional to the line length between the location of the relay and the point where the fault has occurred. As the measured quantity is proportional to the distance along the line, the measuring relay is called distance relay.

A distance protection scheme is a non unit system of protection. A single scheme provides both primary and back-up protection. The most important and versatile family of relay is a distance relay group. It includes the following types.