GB823046A - Improvements relating to track circuits for railway signalling systems - Google Patents

Abstract

823,046. Railway signalling track circuits. WESTINGHOUSE BRAKE & SIGNAL CO. Ltd., DUCKITT, H., and TILLOTSON, P. Oct. 26, 1956 [Nov. 3, 1955], No. 31465/55. Class 105. In a coded D.C. track circuit having one, viz. 11, of the two rails only used as the return for A.C. traction power current and the other, viz. 12, divided into insulated sections by insulated joints 13, 14, the track relay unit includes a transductor 23 composed of a magnetic circuit linked with windings 26, 27 in an A.C. circuit and with control windings 24, 25 connected across the track rails, the latter windings presenting a high impedance to the traction return current so that the saturable effects in the magnetic circuit are controlled predominantly by the code impulses of direct current received from the track feed via the rails 11, 12, the A.C. circuit being connected to the code following relay 30 through a full-wave rectifier 28. By this means the relay 30 is isolated from the effects of intruding traction return currents. The control windings 24, 25 present to the A.C. traction return currents an impedance which is as high as is compatible with the sensitivity of the relay 30 to the resulting distortion of the code impulses. As shown, the decoding circuit comprises first and second relays 31, 32 each in series with a condenser 33a, 33b and each being shunted by a rectifier 36a, 36b so that each condenser can be charged independently of the operation of its associated relay alternately under the control of the code following relay 30, by means of front contacts 34 and back contact 35 respectively and when not being charged, when the front contacts 34 or back contact 35 open, the condensers discharge through their associated relays to maintain them operated. The relay 31 has a front contact 31a in series with the back contact 35 of the code following relay 30 in the circuit of the relay 32 so that the condenser 33b can only charge when the relay 31 is energized. The relays 31, 32 have contacts 31b, 32a in series in the circuit of the clear track indication lamp 37 and contacts 31c and 32b in parallel in the circuit of the occupied track indication lamp 38. In the track feed set the coding contact 16a operated by the coding relay 16 is inserted between the input side of the fullwave rectifier 17 and the source of alternating current and to suppress sparking a resistance 21 and condenser 22 are connected in series across the primary winding of the transformer 15. To reduce the effects of build-up voltages across the rectifier 17 by the traction return currents in the rails, a protective choke 18 is connected in series in the positive feed output circuit of the rectifier 17 and a variable resistance 19 regulates the magnitude of the code impulses. A lowimpedance path 20 to the traction frequency currents is connected across the output circuit of the rectifier to limit the A.C. voltage developed across the output terminals by the traction current.