The battery earth fault relay is used to detect earth faults and deterioration of wiring insulation in either pole of battery. The scheme consists of a centre tapped resistor, a measuring relay, plug setting bridge, auxiliary relay and rectifier bridge to provide unidirectional supply to the measuring relay coil. For different battery voltages, different values of centre tapped resistors are used. Variable sensitivities are provided by means of the tapped coil whose taps are connected to the plug setting bridge. The centre tap of resistor is brought to one terminal of the relay and this terminal is either directly earthed or earthed through a centre zero milliammeter. Under healthy condition no current flows through the measuring relay coil and in any pole of the battery or wiring insulation failure, current flows through the measuring relay coil and the relay operates.
