Abstract: A device for detecting and isolating a downed conductor in a three-phase four-wire multi-grounded distribution system. The device cooperates with a power supply protective device if the device has a breaking capacity less than the maximum prospective short-circuit current of the distribution line, and the device is installed at a tap point of the distribution line. The device comprises a current detector, a low-pass filter, a high-pass filter, an arcing ground fault current-detecting circuit, a first overcurrent-detecting circuit that delivers an output when the output current from the low-pass filter exceeds a given value, a second overcurrent-detecting circuit that detects an overcurrent exceeding the breaking capacity of a device which has a breaking capacity less than the maximum prospective short-circuit current of the distribution line, a no-voltage detecting circuit for sensing that the distribution line is not electrically charged, a main contact and a trip mechanism.