GB433887A - Improvements in electric protective systems - Google Patents

Abstract

433,887. Protective cut-out arrangements. GENERAL ELECTRIC CO., Ltd., Magnet House, Kingsway, London, and GARRARD, C. C., c/o General Electric Co., Witton, Birmingham. March 26, 1934, No. 9427. [Class 38 (v)] An arrangement for giving protection in the event of damage to a cable in a mine or other place subject to explosive atmosphere, of the kind in which an auxiliary lead is incorporated in the cable and adapted to carry control current, has the cable provided with two insulated screens disposed around the conductors and located in the go and return paths for the control current. The screens are associated with control gear for disconnecting the main conductors on the occurrence of a direct electrical connection between the screens. The gate-end switch box 1 is connected to the machine-end box 2 by a threecore trailing cable having an earthing conductor 5 and screens 6, 7 carrying the current for controlling the main switch 13 from the reverser and control switch 10 at the machine end. The screens 6, 7 are interconnected over resistances 11, 37 and contacts 12 which are closed in operative positions of the reverser. One side of a rectifier 33 is connected to screen 7, whereas the other side is connected to 6 over contacts 29 of a relay 30, relay coils 26, 21, and contacts 27 of an overload relay and " stop press-button 36. With contacts 12 closed, insufficient current flows to energize 21, but by operation of 10 the resistance 11 is momentarily shorted, and 21 closes its contacts 19 to energize relay 17 which closes at 15 the circuit of contactor coil 14. If the cable gets damaged so that 6, 7 come in contact and short the resistances 11, 37, coil 26 closes contacts 25 to energize 24, which opens at 22 the circuit of 17, so causing 13 to disconnect the cable from the supply. Leakage from the main conductors causes coil 30 to open the control circuit and closure of test switch 44 simulates this faulty condition. A test switch 43 is provided for effecting a leakage test by connecting the screens and operating relay 26, and a " start " switch 40 and link 41 are provided for closing the main circuit at the gate-end. The relays in the control circuit are arranged to have low inductance, and the short-circuit current of transformer 18 is also small, so that the spark-energy at a point of short-circuit is too low to ignite combustible gas. The Provisional Specification refers also to two pilot conductors in the cable connected in parallel with the respective screens, and to auxiliary plugs and sockets which do not engage until the couplings are locked in position.