A device for detecting a ground fault in a winding mounted to rotate with the rotor of a machine, in which an electric potential is present across the winding during machine operation, the rotor is grounded and the winding is normally insulated from ground, the device including: a current responsive light emitting element mounted to rotate with the rotor and electrically connected between the winding and the rotor and a light responsive detector fixed in a position relative to the machine and having a field of view which is traversed by the light emitting element during a portion of each revolution of the rotor.
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