PURPOSE: To easily detect an earthing trouble generation section, by connecting a link plate for solid bonding and a current transformer to the metal sheath of an intermediate connection part.

CONSTITUTION: If it is assumed that an earthing trouble is generated in a cable section 15b, the current in a conductor 13 is flowed to an accident point when the earthing resistor of a link box L is sufficiently large as compared with earthing resistor of a terminal connection part 14. As a result, a sheath current is flowed to the metal sheath 12 as shown by the arrow and the current to the same direction is flowed to the link plates 17b1, 17b2 connected to the cable section 15b. Therefore, induction voltages induced in current transformers 18b1, 18b2 are mutually inverse polarities and, therefore, a loop current is flowed to a pilot cable 19b to fuse a fuse 20b. Therefore, by confirming the presence and absence of the fusion of fuses 20aW20d by opening the box L after an earthing trouble, the cable section, where the earthing trouble is generated, can be determined.