An apparatus, comprising: a leakage current sensor configured to measure leakage current from a source of AC power, which provides power to at least one load circuit; and a control circuit configured to: (i) receive measured leakage current values from the leakage current sensor, (ii) compare the measured leakage current values against one or more predetermined thresholds to establish status therefor; and (iii) perform a self test to determine whether the leakage current sensor is operative; characterized in that the source of AC power and the at least one load are on a primary side of an electrical isolation boundary; the control circuit is on a secondary side of the electrical isolation boundary; and the leakage current sensor includes: (i) a first circuit on the primary side of the electrical isolation boundary; (ii) a second circuit on the secondary side of the electrical isolation boundary; and (iii) one or more third circuits (T100, 204) that inter-couple the first and second circuits (202A, 202B) of the leakage current sensor yet maintain the electrical isolation boundary therebetween, wherein the one or more third circuits of the leakage current sensor include: a differential current transformer having: (i) 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, and (ii) a secondary winding formed by a plurality of turns of wire about the transformer core; and an isolator circuit having: (i) a primary side circuit configured 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 (ii) a secondary side circuit configured to produce the intermediate control signal in response to a test control signal produced by the control circuit. a leakage current sensor configured to measure leakage current from a source of AC power, which provides power to at least one load circuit; and a control circuit configured to: (i) receive measured leakage current values from the leakage current sensor, (ii) compare the measured leakage current values against one or more predetermined thresholds to establish status therefor; and (iii) perform a self test to determine whether the leakage current sensor is operative; characterized in that the source of AC power and the at least one load are on a primary side of an electrical isolation boundary; the control circuit is on a secondary side of the electrical isolation boundary; and the leakage current sensor includes: (i) a first circuit on the primary side of the electrical isolation boundary; (ii) a second circuit on the secondary side of the electrical isolation boundary; and (iii) one or more third circuits (T100, 204) that inter-couple the first and second circuits (202A, 202B) of the leakage current sensor yet maintain the electrical isolation boundary therebetween, wherein the one or more third circuits of the leakage current sensor include: a differential current transformer having: (i) 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, and (ii) a secondary winding formed by a plurality of turns of wire about the transformer core; and an isolator circuit having: (i) a primary side circuit configured 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 (ii) a secondary side circuit configured to produce the intermediate control signal in response to a test control signal produced by the control circuit. a differential current transformer having: (i) 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, and (ii) a secondary winding formed by a plurality of turns of wire about the transformer core; and an isolator circuit having: (i) a primary side circuit configured 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 (ii) a secondary side circuit configured to produce the intermediate control signal in response to a test control signal produced by the control circuit.