An arrangement is made in a transformer such that voltages induced on the opposite sides of a mid-point 17 of a secondary winding 16 are unbalanced by an amount on the order of 5%. When a ground terminal 18 is not grounded while a mid-point of a load is grounded, a voltage VUE is developed between the mid-point 17 and a non-active line terminal 15 and is rectified to provide a rectified output which turns a transistor 28 on, causing a light emitting element 33L to emit light. The resulting light renders a light receiving element 33P conductive, whereby a relay 37 is operated to turn a switch 13 off. When a ground fault occurs on one side of the secondary winding 16 while the ground terminal 16 is not grounded, a ground fault detection circuit 38 fails to detect the ground fault, but the non-grounding protection circuit 30 detects the ground fault and turns the switch 13 off.