PURPOSE: To search the position of a fault point while the supply of a current is kept by detecting the high frequency ground current and ultrasonic waves generated in a distribution line at the time of an arc ground fault to utilize the speed difference between them.

CONSTITUTION: Ultrasonic detection elements 5-7 are respectively attached to three- phase high voltage distribution lines 1-3 and the outputs thereof and the output of a zero phase current transformer 16 are respectively inputted to the adders 17-19 connected to the input terminals 8-10 of a memory scope 15. For example, when it is assumed that the distribution line 1 generates an arc ground fault accident at a contact point in this state, a high frequency ground fault current is detected in a zero phase transformer 16 by the shock of said accident to display a high frequency ground current signal 11. At the same time, ultrasonic waves are generated in the distribution line 1 at the fault point and propagate through the distribution line 1 to be received by an ultrasonic element 5 but the wave motion thereof is slow in speed as compared with the high frequency current and an ultrasonic signal 14 is displayed as a delay signal. Since the high frequency ground fault current signal 1 propagates at the velocity of light to instantaneously reach the memory scope 15, an arc ground fault point 4 can be estimated as the distance calculated by multiplying the delay time by the propagation velocity of ultrasonic waves.