Ground fault interrupter with pulsed neutral-to-ground fault detector

Claim 1

Apparatus for monitoring an alternating current supply circuit having at least one line conductor, a neutral conductor, and means forming a ground-return, wherein said conductors extend from supply terminals to load terminals and wherein the neutral conductor has a ground connection to the ground-return, the monitoring apparatus including neutral-to-ground fault monitoring means having a differential current transformer having a core and primary windings forming series segments of said line and neutral conductors so arranged that load current carried thereby tends to produce mutually cancelling flux in said core, the differential current transformer being disposed between said ground connection and said load terminals, resonant means resonant at a higher frequency than that of the alternating current supply circuit including a further winding linking said core and a capacitor connected to said further winding, means for periodically pulsing said resonant means, and fault-evidencing means coupled to said transformer for responding thereto in one manner to indicate normal operation when the resonant means rings at least at a minimum amplitude for a prescribed period after each pulse of the pulsing means and for responding thereto differently to indicate abnormal operation in the event of a fault between the neutral conductor and the ground-return means at the load-terminal side of said transformer causing damping of the resonant means and decay of the ringing below said minimum amplitude during said period.