Patent ID: 8907678

Claim:
An apparatus, comprising: a leakage current sensor configured to measure leakage current from a source of AC power on a primary side of an electrical isolation boundary, which provides power to at least one load circuit on said primary side of said electrical isolation boundary; said leakage current sensor including (i) a first circuit on the primary side of the electrical isolation boundary; (ii) a second circuit on a secondary side of the electrical isolation boundary; and (iii) a third circuit that inter-couples the first and second circuits of the leakage current sensor while maintaining the electrical isolation boundary therebetween, including at least (a) a differential current transformer having a primary winding formed by one or more equal numbers of turns of respective legs about a transformer core as such legs pass from the source of AC power to the at least one load circuit, and a secondary winding formed by a plurality of turns of wire about the transformer core; and (b) an isolator circuit having: (1) a primary side circuit operating to induce an unbalanced flux in the transformer core that produces a current in the secondary winding of the differential current transformer, the unbalanced flux being in response to an intermediate control signal crossing the electrical isolation boundary from the secondary side thereof; and (2) a secondary side circuit operating to produce the intermediate control signal in response to a test control signal produced by a control circuit, said secondary side circuit including a switch circuit that turns on and off in response to the intermediate control signal; a test winding of one or more turns on the differential current transformer; and a resistance; wherein the switch circuit, test winding, and resistance are coupled in series between the legs such that the unbalanced flux is produced in response to the intermediate control signal turning the switch circuit on; and a control circuit on a secondary side of the electrical isolation boundary configured to: (i) receive measured leakage current values from the leakage current sensor at least at some times other than during a self test of the leakage current sensor, (ii) compare the measured leakage current values against a plurality of predetermined thresholds at least at some times other than during a self test of the leakage current sensor; (iii) provide a status output indicating the status of leakage current with respect to said plurality of thresholds; and (iv) perform a self test to determine whether the leakage current sensor is operative by providing said test control signal.