Grounds in the field of a synchronous machine must be immediately detected, since the occurrence of a second ground might short circuit part of the field winding, and the resultant unbalance and vibration may damage the machine.
The DGF relay provides a reliable means for detecting grounds in the DC winding of the generator field. It uses a voltage divider consisting of a resistor and varistor connected across the field winding of the generator. The relay operating coil is connected from the mid-point of the voltage divider to ground. A ground in the generator field winding will cause current to flow through the moving coil of the DGF and operate the relay to trip or sound an alarm, as desired.