Abstract: A device for detecting and isolating a downed conductor in a three-phase four-wire multi-grounded distribution system 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 lateral of the distribution line. The device comprises a current detecting device, a high-pass filter, a band-pass filter, two arcing ground fault current-detecting circuits, a first overcurrent-detecting circuit that delivers an output when the output current form the current detecting device exceeds a predetermined value, a second overcurrent-detecting circuit that detects an overcurrent exceeding the breaking capacity of the 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, and a trip mechanism.