PURPOSE: To make it possible to take out a zero-phase voltage signal readily at the time of ground fault by fixing a potential detector lead wire for taking out a voltage to the detecting part, i.e., the voltage terminal part, of a T-branch constituting a T-joint.

CONSTITUTION: An insulator 14 for insulating a voltage induced at a voltage terminal 5 is formed around a lead wire insulator 9, and a semiconductive layer 15 for electrically connecting the outer semiconductive layer 4a of a T- branch and a lead wire shield 10 is further formed thereabout. Upon charging of a conductor 1a, a voltage is induced at the voltage terminal 5. Since thus induced voltage is applied between a lead wire conductor 8 and the lead wire shield 10, the induced voltage Ex can be measured across an inserted capacitor 28. When the induced voltage Ex is detected for each of U, V and W phases, phase signal of zero-phase voltage can be transmitted to a ground directional relay at the time of ground fault.