GB679789A - Improvements in and relating to earth leakage protection for single phase a.c. circuits - Google Patents

Abstract

679,789. Protective cut-out arrangements. BROOKE, W., and BALDWIN, H. T. Sept. 30, 1950. No. 23974/50. Class 38 (v). An earth-leakage protective system for single-phase A.C. circuits has an earth-leakage relay R1, which on the incidence of an earth fault is energized from a rectifier connected between earth and an artificial neutral point, a second relay R2 being energized from an auxiliary supply by the closing of the earth-leakage relay to open the contactor switch C controlling the protected circuit. In the Figure, plug points Z1, Z2 are supplied from a transformer T through the contactor C which has its operating coil F supplied from lines L1, L2 through a full-wave rectifier D2 and the normallyclosed contacts RC, RE of relays R1, R3. A transformer A<SP>1</SP> has its primary winding centretapped to provide an artificial neutral point AP which is connected to earth through a fullwave rectifier D1 which supplies the operating coil of the leakage relay R1. As soon as the transformer T is energized the contactor C closes and at C3 inserts an economy resistance G in series with the contactor operating coil F. If now an apparatus, such as a drill Y, connected to a plug point, should develop an earth fault as indicated at W, fault current will flow from line L2, through a conductor U, the fault W, earth conductors V and X, a connection 2, the rectifier D1 and the coil of relay R1 to the neutral point AP. The relay R1 closes its contacts RA to complete a circuit from secondary winding AS1 of the transformer A1 through a conductor 6, a rectifier D3, a connection 10, the coil of relay R2, the contacts RA, a connection 9, the rectifier D3 and a connection 5. The relay R2 now opens its contacts RC to deenergize the coil F of the contactor, which opens. The relay R2 also closes contacts RB, so that should the fault W persist, a circuit is completed from the secondary winding AS1 through the rectifier D3, a connection 101, a centretapped neutral point impedance A2, the conductor U, the fault W, connections V, X and 13, the relay contacts RB, a connection 12, the coil of relay R3, the rectifier D3 and the connection 5. The coil of relay R3 is thus energized by a D.C. current flowing through the fault and the relay R3 opens its contacts RE to prevent the reclosure of the contactor C so long as the fault remains. The relay R3 closes contacts RD to maintain a holding circuit for itself since the relays R1, R2 open when the contactor C opens to disconnect the plugs (and the fault) from the supply. The relay R3 when energized also closes contacts RF to light a fault indicating lamp E. When the fault is removed the relay R3 is de-energized and the contactor recloses automatically as previously described or under the control of a push button (not shown).