GB702230A - Improvements relating to electric lighting systems also adapted for signalling purposes - Google Patents

Abstract

702,230. Protective cut-out arrangements. VICTOR PRODUCTS (WALLSEND), Ltd. June 30, 1952 [July 17, 1951], No. 16946/51. Addition to 632,909. Class 38(5) [Also in Groups XXXVI and XXXVIII] In an underground lighting and signalling system of the kind described in the parent Specification comprising a lamp circuit controlled by a low voltage pilot circuit by means of electromagnetic switches or relays, having conductors of both the lighting and pilot circuits extending over a series of cable .sections connected by disconnectable couplings and a plurality of switches distributed along the cable capable of operation to extinguish the lamps for signalling purposes, the system is arranged so that the lamps are extinguished by temporary closure of one of the switches to interconnect the two conductors of the pilot circuit. Fig. 1 shows a system wherein the pilot circuit comprises conductors P1, P2 and the lighting circuit comprises conductors L1, L2 extended over sections of a 4<SP>1</SP>- core trailing cable intercoupled by plug and socket boxes B1, B2 such that the pilot circuit is always broken before the lighting circuit, and closed at the remote end by a cap D. Lamps L are bridged across the lighting conductors and circuit closing push-buttons P are bridged across the pilot conductors. The supply apparatus contained in a gate-end box GB comprises a mains transformer T having a winding SL energizing the lighting circuit through the contacts of a relay S, a winding SR energizing the relays through the contacts of a differential relay M, and a winding SP energizing the pilot circuit at a low voltage through the contacts of an earth leakage relay EL, a spark suppression resistance LR, a switch HS, and a full-wave metal rectifier bridge R. Pilot conductor P1 is earthed, pilot conductor P2 is connected to the junction point of the relay windings whose windings WD, WO are also connected across the remaining diagonal of the rectifier bridge, the winding of relay EL is connected between earth and the centre point of the transformer winding SL, and at the remote end the pilot conductors are interconnected through a half wave rectifier HR within the closure cap such that current can only flow from conductor P1 to P2 of the pilot circuit. Normally only alternate half waves (at which conductor P1 is positive to P2) pass through rectifier HR, and rectifier bridge R operates to short circuit winding WD of relay M so that only winding WO is energized, relays M and S are operative, and the lighting circuit is energized. Operation of any push-button shorts out rectifier HR, so that relay windings WO, WD are energized by alternate half cycles, relays M and S release, and the lights are extinguished for signalling purposes. Leakages to earth from either lighting circuit energizes relay EL to break the pilot circuit and extinguish the lights. On occurrence of a fault due to a push-button sticking down causing extinction of the lights, switch HS is operated to connect a half-wave rectifier MR in the pilot circuit in the same polarity as rectifier HR when the lights will be illuminated. Successive couplings are then broken starting from the remote end until the lights are extinguished. In the event of a break in the pilot circuit, the rectifier MR is switched in and the push-buttons are successively operated starting from the supply end until one is found which does not operate to illuminate the lights. In a modification, Fig. 2 (not shown), the full-wave rectifier and differential relay are replaced by a relay whose magnetizing winding is connected in the pilot circuit and which has a single short circuited turn wound on its core so that when energized by alternate half-cycles its core is magnetized undirectionally to hold its contacts closed and close the lighting circuit while when energized by both half-cycles its core is reversibly magnetized and the relay releases to open the lighting circuit.