PURPOSE: To detect disconnection automatically and reliably by connecting a plurality of capacitors or resistors in parallel with a grounding conductor between the secondary ground phase and the earth and producing a voltage divided disconnection detecting signal upon disconnection of the grounding conductor.

CONSTITUTION: Grounding conductor 2 is laid between the ground phase terminal b2 of the secondary winding of a transformer 1 and the earth E. Capacitors C1, C2 are connected, as a voltage supply means 3, in parallel with the grounding conductor 2 between point D of the ground phase and the earth E, then a PT is connected between the joint of the capacitors C1, C2 and the earth while a voltage detecting means 4 is connected to the secondary thereof thus forming a voltage detecting means 5. The capacitors C1, C2 may be replaced with resistors. Upon disconnection of the grounding conductor 2, ground phase voltage increases and a voltage divided by the capacitors C1, C2 is produced across the capacitor C1. Then the voltage is detected 5 and an alarm (not shown in the figure) is actuated. By such arrangement, disconnection can be detected automatically and reliably.