PURPOSE: To judge the conduction of a circuit detecting a voltage generated in the neutral conductor with the ground with a local panel board by operating an earth relay mounted on a secondary side wire of a main switch gear at a substration with the adjustment of a variable resistance connected to the panel board while the power source wire is connected to various earth resistances through a resistance.

CONSTITUTION: In order to examine which light panel board, for example, the No.3 main switch gear 4 in a substation is connected to, first, a zero-phase current transformer 10 and an earth relay 9 is mounted to the secondary side of the main switch gear 4 and a circuit switch 7 is closed. Then, a variable resistance 11 of a local light panel board 6 is adjusted and it is observed if a voltometer 14 indicates when the value of an ammeter 12 is shifted to an integer value of the earth relay 9. When said test is implemented in the No.3 light panel board 6 connected to the main switch gear 4, the earth relay 9 operates and the voltometer 14 indicates only a drop in the voltage of, for example, a second- grade earth resistance. This makes it clear that the main switch gear 4 is connected to the No.3 light panel board 6.