GB246893A - Improvements in or relating to protective arrangements for alternating current electric circuits - Google Patents

Abstract

246,893. Reyrolle & Co., Ltd., A., Clothier, H. W., and Burgess, W. A. A. Oct. 3, 1924. Electromagnetic switches; automatic circuitbreakers; cut-out arrangements. - Relates to a protective arrangement for use in conjunction with a trailing or like cable between an electromagnetically-operated switch and electrical apparatus (for example a portable coal-cutter used in mines), in order to ensure that, when the earthed pilot circuit is incomplete, the gateend switch automatically opens. An electromagnetic relay or other tripping device has a magnetizing winding and an auxiliary winding normally short-circuited by the sound field wire circuit so as to vary the magnetic or inductive effect of the electromagnet. As shown in Fig. 1, the core A has a magnetizing winding B connected to the supply mains and tending to attract the armature A<1> and close the relay. The core, however, has an additional winding D the ends of which are connected to the normally-earthed pilot wires so that the winding is short-circuited and acts to weaken the flux through the armature with the result that, until the earthed pilot circuit becomes incomplete, the armature is not attracted to close the tripping circuit. In a modification, the additional winding is arranged upon a magnetic shunt path A<2>, Fig. 2. When short-circuited, the current induced in the auxiliary winding weakens the flux in the shunt path and therefore increases that through the armature which is therefore attracted to open the holding circuit. The adaptation to a corebalance leakage system for a three-phase, trailing cable supply is shown in Fig. 3. In the event of earth leakage the current in the secondary E<2> trips the gate-end switch G in the usual way. In order to provide for tripping in the event of discontinuity of the earthed pilot circuit K, the transformer is provided with a magnetizing winding E<1> connected across two of the phases and with an additional winding D<5> connected to the earthed field wires which normally shortcircuit it. The short-circuited winding damps the effect of the primary winding E<1> which is then ineffective to energize the secondary E<2>, adequately to trip the circuit-breaker. If, however, the earth circuit becomes incomplete, the circuit-breaker is tripped. Specification 240,574 is referred to.