Patent ID: 8526143

Claim:
A device for use with an AC power system that includes a line conductor, a neutral conductor and a transformer, the line conductor and the neutral conductor each coupled between a source and a load, the neutral conductor coupled to ground at the source, wherein the transformer includes a first primary winding coupled in series with the line conductor, a second primary winding coupled in series with the neutral conductor, and a secondary winding, the device comprising: a first circuit coupled to the secondary winding, wherein the first circuit provides a first detection signal if a current from the line conductor to ground exceeds a first predetermined threshold; and a second circuit coupled to the secondary winding, wherein the second circuit includes an AC voltage source that provides an AC signal at a specified amplitude and a specified frequency to the secondary winding, and wherein the second circuit provides a second detection signal if an impedance between the neutral conductor and ground is less than a second predetermined threshold and a current in the secondary winding exceeds a third predetermined threshold.