Current Dependent Switch
You should be able to do this with something similar to a GFCI circuit.  If you run your supply and return wires through the core of a non-contact DC current sensor, you will normally read zero current since the individual flows in the wires will balance each other out.  If you have another path to ground outside the normal circuit, the supply wire will provide more current and the difference will be picked up by your sensor, which can be set for whatever level of sensitivity you want.