The impedance, is the ratio of the bus voltage and fault current (V/I), between the point where the relay is located and the point of fault will become less than Z and hence the relay operates. It is a distance relay that measures the distance by equating the fault current with voltage (which equates to impedance) across the fault loop and thus trips. Impedance Relay Definition: An impedance relay, also known as a distance relay, is defined as a device that triggers based on the electrical impedance measured from a fault's location to the relay. Here the prefix word distance mentions that impedance is nothing but an electrical measurement of distance along a transmission line. It is a voltage controlled equipment.