Abstract: A neon transformer includes secondary windings 16 and 17, and a junction 20 therebetween is connected through a voltage detection circuit 41 to a ground terminal 18 of the transformer. A resistive element 54 is connected in shunt with the voltage detection circuit 41. The voltage detection circuit 41 may comprise a series circuit of a Zener diode 52 and a light emitting element 53L of a photocoupler, for example. When a ground fault occurs on the secondary side of the transformer, the location of the ground fault assumes a reference potential, whereby the potential of the junction 20 rises to cause a current flow through the Zener diode 52 and the light emitting element 53L, which renders a light receiving element 53P conductive to pass an energizing current through a relay drive coil 34, thus throwing a relay contact switch 13 from NC to NO, thus interrupting the supply of the a.c. power.