Abstract: Electrical potentials are induced in the rotating shaft of many rotary electrical machines because of stray flux that exists in the machines. Unless provided with adequate ground insulation to interrupt the circuit, currents can flow through the machine bearings and other parts to ground and thereby expose the machine to severe damage. The device of this invention provides a sandwich-like ground insulation with a conductor therebetween and measures the voltage across that portion of the insulation between the conductor and either the bearing housing or ground to indicate if there is a circuit for excessive current leakage from the bearing to ground.