GB727320A - Improvements relating to electric control systems - Google Patents

Abstract

727,320. Protective cut-out arrangements. METROPOLITAN VICKERS ELECTRICAL CO., Ltd. Sept. 23, 1952 [Sept. 26, 1951], No. 22516/51. Class 38 (5). In a remote control arrangement in which the control of a circuit-breaker or contactor (not shown) is effected by a relay having a winding 1 connected in series with a low voltage A.C. supply 2, a pilot line 3, a remote switch 4, a half-wave rectifier 5 and an earth return, a local circuit is arranged to be completed when the relay and circuit-breaker close to cause a backing current to flow in the winding or in a further winding of the relay. This reduces the holding power of the relay to make the system more sensitive to changes in resistance in the pilot circuit without making it more sensitive to changes in the supply voltage. In Fig. 1, the local circuit comprises a resistor 6 in series with auxiliary contacts C of the relay or circuit-breaker and a half-wave rectifier 7 of opposite polarity to rectifier 5. When the switch 4 is closed " forward " half-wave rectified current flows through the relay winding 1 to close the relay. The resulting closure of the contacts C causes half-wave rectified backing current of opposite polarity to flow through the relay coil to produce an opposing effect so weakening the holding power of the relay. In Fig. 2 (not shown) the backing current may flow through a separate additional winding on the relay, the additional winding being connected in series with the local circuit. In either arrangement the relay may be provided with a further shortcircuited winding as described in Specification 700,472. In a further arrangement, Fig. 3, the relay has parallel-connected relay coils 1a, 1b, associated with rectifiers 8a, 8b, 9a, 9b. When the switch 4 is closed current flows through the .rectifier 9a, coil la and rectifier 5 to close the relay. Then on the closure of contacts C the backing current flows through the rectifier 7, the coil 1 b and the rectifier 9b to reduce the holding power of the relay.