GB734436A - Improvements in or relating to the protection of electric cables - Google Patents

Abstract

734,436. Protective cut-out arrangements. ETABLISSEMENTS MERLIN & GERIN SOC. ANON. Feb. 2, 1953 [Feb. 1, 1952; March 5, 1952], No. 2873/53. Class 38 (5). A cable, having insulated phase conductors and an insulated pilot wire which are associated with a safety screen having a metallic leakage collector, is protected against screen to pilot wire faults by connecting the pilot wire and leakage collector to opposite poles of a condenser. The invention is described as applied to the cable of Specification 716,516 which has power conductors a, b, c, an earthing wire and a pilot wire 1, the latter having a coaxial leakage collector 2 connected to a conductive screen 5 in which all the conductors are embedded. If a fault occurs between the collector 2 (i.e. the screen 5) and the pilot 1, the following circuit is made: collector 2, conductor 62, relay coil 20, negative terminal of a condenser 60, the positive terminal thereof, earth, a rectifier 17 and pilot 1. The coil 20 is thereby energized to open contacts 21 and close contacts 22, thereby de-energizing a relay 13, the contactor coil 12 and an indicator lamp 54. The relay 13 is connected across a condenser 14 in the pilot circuit 1 and is normally traversed by a low rectified A.C. derived from a transformer 19, but if a pilot-to-earth fault occurs, e.g. at A, the rectifier 17 and a resistance 18 are shortcircuited so as to pass a greater current through a rectifier bridge 53 to relay coil 20 which is thereby energized. The positive side S of rectifier 53 is connected to earth through a circuit 57, 58 and a parallel circuit containing a neon lamp 56 and rectifier 55. The negative side t of rectifier 53 is connected to earth over the condenser 60 and a resistance 61. Faults from phase to screen or screen to earth also complete circuits including condenser 60 and coil 20. If, after a plug 68 is removed, a fault occurs between conductors 1, 62 (which are normally connected to the pilot and screen), A.C. flows through the transformer 51 and conductor 65 to earth, via point s and circuit 57, 58 so that coil 20 is energized.