GB894409A - Improvements in or relating to protective systems for electrical equipment - Google Patents

Abstract

894,409. Protective arrangements. ASQUITH ELECTRICS (COLNE) Ltd. May 2, 1960 [May 2, 1959], No. 15075/59. Class 38 (5). A protective arrangement, e.g. for a portable tool, comprises an earth loop formed by two earth circuits, one circuit including the winding W1 of a first relay R1, a transformer secondary winding b1 and a winding W3 of a second relay R2, and the other circuit including a choke A, a transformer secondary winding b and a further winding W2 of the relay R2. Under normal conditions, current circulates in the earth loop and the windings W2, W3 oppose each other so that the contacts C2 of the relay R2 remain closed. The circulating current causes relay R1 to close its contacts C1 so that an operating circuit for the winding W4 of a third relay R3 is completed on the operation of a push-button switch PB. Relay R3 closes contacts C4, C5 in the supply circuit to the tool PT and closes contacts C3 to provide a holding circuit for itself. A break in either earth circuit causes the relay R1 to open and interrupt the operating circuit for the relay R3. Earth leakage currents flowing from the tool through either or both of the earth circuits cause the energizing of the relay R2 which opens its contacts C2 to interrupt the operating circuit of the relay R3. A resistance M6 is connected in the supply circuit to the tool to increase the impedance of the live side of the supply to prevent a dangerous voltage arising between the tool casing and earth should a short-circuit occur between the live side of the supply and the casing. Specification 892,594 is referred to.