Abstract: A ground fault interrupter employs a single differential transformer that is made of a ferrite toroid with an eleven turn secondary winding. The transistorized fault detector circuit is for the most part formed on a silicon integrated chip. The hot-wire-to-ground-fault detector portion is an impedance-transforming circuit that when energized presents a low impedance, i.e. virtually a short circuit, to the secondary winding. A multiplexing circuit periodically de-energizes the hot-wire-to-ground-fault detector causing it to present a high input impedance during periods when neutral-wire-to-ground faults are sensed via the transformer.

Abstract: A differential transformer having a core of low permeability ferromagnetic material, a pair of identical primary windings and a secondary winding of relatively few turns capable of producing a large voltage output for a small magnitude ground fault current. A cross-field winding is utilized to periodically saturate a portion of the flux path in the transformer core to produce sudden changes in magnetic flux, which flux changes vary in magnitude proportional to the amount of fault current. During the time interval when the cross-field portion of the sensor is not saturated by a pulse of sampling current, a second pulse of current is introduced into the sensor secondary winding for detecting a grounded neutral wire by sensing the output voltage produced in response thereto.