GB823045A - Improvements relating to direct current track circuits for railway signalling systems - Google Patents

Abstract

823,045. Railway signaling track circuits. WESTINGHOUSE BRAKE & SIGNAL CO. Ltd., DUCKITT, H., and TILLOTSON, P. Oct. 26, 1956 [Nov. 3, 1955], No. 31463/55. Class 105. In a D.C. track relay circuit having one, viz. 11, track rail continuous as the return for the traction A.C. current, the other rail 12, being divided into sections by insulating joints 13, 14 the current feed set comprises a feed full-wave rectifier 31, 32 fed from an A.C. source 18 as through a transformer 17 and connected at its output terminals across the rails 11, 12 in series with a choke 22 designed to limit the flow of A.C. of traction current frequency and having across the said output terminals a low resistance 20 as a low-impedance path for currents of traction current frequency; and a reference fullwave rectifier 33, 34 preferably fed by the source 18 and means such as a relay 26 connected between the positive terminals of both rectifiers for comparing the output voltages of the rectifiers and for disconnecting, as by the relay contact 26a, the feed rectifier 31, 32 from the track rails when the output voltage of the feed rectifier attains a predetermined magnitude in relation to the reference rectifier output voltage whereby the continued operation of the feed set is prevented before the sensitivity of the track circuit to trains entering the section is reduced due to the half-wave rectification taking place in the feed rectifier on account of an A.C. traction current frequency voltage developing across the feed rectifier output terminals. The two rectifiers may be fed from the same transformer having two secondary windings so adjusted that the output voltage of the reference rectifier 33, 34 is normally greater than that of the feed rectifier so that the relay 26 is normally energized and picked up with its contact 26a in the feed circuit closed whereby an increase in feed rectifier output voltage above a predetermined value releases relay 26 and opens the track feed circuit. For proving the operation of the reference rectifier a relay 27 is connected across the output terminals and is arranged to act as a shunt path of low impedance to the A.C. current of traction power frequency to protect the rectifier from increased output due to the traction power frequency currents being half-wave rectified. The pick-up circuit of relay 27 includes a front contact 26c of relay 26 and a front contact 27e in its holding circuit. The relay 27 has a front contact 27b in the track feed circuit so that this circuit is closed only if the reference rectifier set is correctly energized. A back contact 27d of relay 27 is in the pick-up circuit of relay 26 over resistance 30 to be energized by the reference rectifier after an interruption of the feed circuit which relay 26 picks up relay 27 over contact 26c, should the relay 27 have also dropped away owing to the interruption in the A.C. supply 24.