Ground fault circuit interrupter with trip level adjustment

Claim 1

A ground fault circuit interrupter comprising: a differential current transformer including a core, primary windings on said core that comprise a hot line conductor and a neutral conductor, a secondary winding on said core for sensing a differential current between said primary winding; a sensing amplifier responsive to said differential current to produce a signal for operating circuit interruption means upon a differential current being of predetermined magnitude, said sensing amplifier being characterized by responding only to a given polarity of current differential between said primary windings, sensitivity adjustment means for causing an effective change in the sensitivity of said sensing amplifier comprising a continually closed circuit branch connected between a first point on said hot line conductor on a first side of said core and a second point on said neutral conductor on a second side of said core, said circuit branch containing a resistance for causing additional current flow vectorially added to differential current otherwise occurring between said primary windings; said sensing amplifier being responsive only to positive differential currents through said hot line conductor from the supply side of said core to the load side of said core; and said first and second points of connection of said circuit branch being respectively on the supply side and the load side of said windings in relation to said core and said additional current flow being in opposition to said normal differential current and effectively causing said sensing amplifier to be less sensitive.