A detection circuit detects a difference between a source current and a return current. The detection circuit includes a constantly biased magnetic core through which conductors of the source and return currents extend, and a sense winding around the core. The sense winding carries an AC signal. A sensing circuit connected with the sense winding senses a change in small signal impedance of the sense winding relative to the core. This detection circuit can be used to protect people and equipment against accidental contact with high-voltage DC systems.