A transformer control circuit which includes a ground fault detection circuit that detects ground fault currents in the transformer secondary and delivers a ground fault detection signal to the primary circuit. This circuit derives power from the secondary winding and transmits the ground fault detection signal through an optical isolation barrier, thus avoiding the use of an auxiliary transformer. The ground fault detection circuit further includes safety features for detecting whether AC power is being supplied without earth ground being connected, and/or whether there is an undesired electrical connection between a transformer output connection and earth ground, which would defeat the ground fault detection circuit.