Patent GB-826553-A (GB)
Title: Improvements relating to alternating current track circuits for railway signalling systems
IPC: B61L1/18
Filed: 1955-11-03
Abstract: 826,553. Railway signalling track circuits. WESTINGHOUSE BRAKE & SIGNAL CO. Ltd., DUCKITT, H., and TILLOTSON, P. Oct. 26, 1956 [Nov. 3, 1955], No. 31464/55. Class 105. In a railway A.C. track signalling system in which one or both rails may be used as a return for A.C. traction current and in which filter circuits are used to pass the A.C. signalling current of frequency different to that of the traction current through a rectifying device to a D.C. relay, in order to avoid at the relay end of the section, due e.g. to a break in a rail or impedance bond, the possibility of current of traction power frequency through the relay of " critical " strength such as to hold the relay when a vehicle enters the section, a saturable transformer is connected between the filter circuit and the track rails at the relay end and is designed to become saturated when the builtup voltage of traction power frequency attains a predetermined value below the " critical " value. The saturable core may be of " Mumetal " (Registered Trade Mark) or of " H.C.R." metal the magnetization curve of which has a sharp cut-off. In cases where one continuous track rail is used, a voltage build-up of traction power frequency may occur due e.g. to the presence of a train in an adjacent section, the saturable transformer may become saturated and release the track relay without a vehicle in the section and to avoid this a second filter to pass the signalling frequency is inserted between the transformer and the track rails (Fig. 3, not shown). To provide protection against failure of rail-joint insulation resulting in false energization of the relay, adjacent track circuits may be operated with A.C. signal currents of different frequencies, the filter network of a circuit responding only to the signal frequency for that circuit or alternatively by employing an A.C. circuit adjacent a D.C. circuit or in the case of coded A.C. track circuits using double rail traction power return sections with impedance bonds, by using different code or modulation frequencies for adjacent track circuits. The filter circuits may have a single stage (Fig. 1, not shown) or a plurality of seriescoupled stages (Fig. 2, not shown). When coded signalling current is used, the relay constitutes a code-following relay controlling a decoding apparatus which in turn controls a track signal (Fig. 4, not shown). As shown in Fig. 5 the relay equipment is made phase-sensitive. The saturable transformer 8 feeds, through a filter 11 tuned to the signal frequency, a centretapped transformer 28 preferably of small magnetizing current, the secondary winding of which feeds two full-wave rectifiers 30, 31 of a modulator type circuit, the outputs of the rectifiers being fed respectively to the identical opposing windings of a code following relay 32 together with an A.C. voltage from a generator 35 of signal current frequency in phase with that at the feed end of the track circuit. This generator 35 is connected between the centre tap of the secondary winding 29 of the transformer 28 and a common connection 34 of the two rectifiers 30, 31, so as to be in phase with the input of one rectifier and in phase opposition to the input of the other rectifier. The relay 32 is of the direct-current polar kind it only pulls up when the lower winding has the stronger energization to control the decoding apparatus network 26 the selective output of which controls the signal 36. The track feed apparatus 24 may also be protected against the traction current by feeding the track rails through a saturable transformer 6, the core material of which need not have such a sharp cut-off in its magnetization curve as that of the relay end transformer 8. Specification 823,045 is referred to.