Patent ID: 6522033
Filing Date: 2003-02-18
Classification: H03H,H03K

Abstract:
A switching circuit for use with an electrical appliance operating from an a.c. supply having live and neutral connections and a reliable ground connection, the switching circuit comprising:a main feeder interruption means operatively coupled to an impedance measuring circuit and being responsive to the switching signal for opening a switching device connected in at least one of the live and neutral connections, first and second normally open switching contacts, and wherein said high sensitivity impedance measuring circuit is coupled to the first and second switch contacts for measuring an impedance therebetween, said high sensitivity impedance measuring circuit including a differential comparator circuit for comparing a fraction of the voltage between the live and neutral connections with a voltage at the first or second switch terminal of the switching circuit and producing at an output thereof a switching signal if a difference between said voltages exceeds a predetermined threshold; the differential comparator circuit comprising: a first input connected to the ground connection of the a.c. supply, and a second input connected to a common junction of a voltage divider including a pair of resistors connected in series between the live and neutral terminals of the supply feeder; the appliance being coupled to said output so as to be responsive to said switching signal; and the first switch contact being electrically floating relative to a ground feeder of the electrical supply, so that a â€œground faultâ€  between ground and the first switch contact induces the switching signal.