Abstract: An apparatus for detecting a ground fault in an ungrounded DC power supply system having a DC power supply connected by positive and negative lines to a load, an AC voltage is selectively applied between a positive line and ground or between a negative line and ground. A current sensor including an electrical coil surrounding the positive and negative lines is responsive to significant alternating current in one of the lines, when the AC voltage is applied to the line and the line has a ground fault, to thereby indicate the fault.

Abstract: A circuit interrupter responsive to ground-fault conditions is disclosed that is rendered relatively immune to many possible failures of components and connections. A differential current transformer has a pair of differential primary windings in series with the conductors of the protected load circuit (or a ground-fault-current primary winding in a modification), and a secondary winding in control of an AC-energized SCR that controls a "hold-in" coil of a circuit interrupter, and an AC - excited biasing winding. The arrangement is such that, in the absence of ground fault current, the hold-in coil is energized, whereas in case of a ground fault the AC biasing flux is opposed so as to be cancelled or reversed, releasing the circuit interrupter for opening.