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The scheme consists of a centre tapped resistor, a measuring relay, a plug setting bridge, an auxiliary relay and a rectifier bridge. For different battery voltages different values of centre tapped resistors are used. For ratings above 250V a standard 250V relay is used with a suitable external resistor board. Variable sensitivities are provided by means of the tapped coil whose taps are connected to the plug setting bridge. The centre tap of the resistor is brought to one of the terminals of the relay and this terminal is either directly earthed or earthed through a centre zero milliammeter. The schematic diagram shown (Figure 1) gives the general arrangement of the scheme. Under healthy condition no current flows through the measuring relay coil and in the event of an earth fault in any pole of the battery or a wiring insulation failure, current flows through the measuring relay coil and relay operates. The Alstom Battery Earth Fault Relay operates through the measuring relay and initiates the alarm scheme. If a milliammeter is provided in the scheme its deflection also indicates the faulted pole of the battery