PURPOSE: To remove the fault quickly if a decision is made that the fault is at a blind spot between a ZCT for taking in the zero-phase current and a distribution line switch when overvoltage due to ground fault appears at a transformer station having a plurality of distribution lines.

CONSTITUTION: Upon occurrence of a ground fault at point (b) on a distribution line 53, a directional ground relay 63 functions to interrupt a switch 43. But the fault is sustained at the point (b) even after interruption of the switch 43. A ground selector relay 2 makes a decision that the fault is located at point (b) based on the function of the relay 63, the interruption of the switch 43 and sustention of Vo at GPT. Subsequently, the distribution line switches 41-45 are not interrupted sequentially but the secondary switch 7 of main transformer is interrupted thus finishing the disconnection of fault point.