Patent GB-572151-A (GB)
Title: Improvements relating to coded track circuit system of railway signalling
IPC: B61L23/16
Filed: 1942-10-30
Abstract: 572,151. Railway signalling systems. TOMLINSON. A. V. (Union Switch & Signal Co.). Oct. 30, 1942, No. 15271. [Class 105] In an approach controlled coded trackcircuit system for railway signalling using a code-following track-relay TR at the entrance end of an insulated track section such as D.E. and a detector relay KR at the exit end, a source of direct current KB is permanently connected across the track in parallel with the relay TR and a second source of direct current TB at the exit end is connected track through a code transmitter so that the potential supplied thereby is opposed to and periodically neutralizes or reverses the potential due to the source KB. The steady "positive" potential supplied Ay, source KB is illustrated in Fig 2 at KB, the negative code impulses supplied from source TB at TB-CR, and the resulting positive impulses of code frequency at KB-TB. Relay TR picks up only energization, but may be replaced by a polar stick relay the contact of which remains in the position to which it is moved by one polarity of current until moved to qpposite position by current of opposite polarity. Relay KR also picks up with positive current, but is of slow release type so that it remains picked up by positive impulses of any of the frequencies used, so long as the track section is unoccupied as shown. Contact 9 of relay KR thus cuts off the source of signal current from the signal Se of the next section in advance, until section D.E. becomes occupied, thus providing approach control for a signal in advance. Signal Se, when energized, is selectively illuminated under coutrol of decoding apparatus DAe of known type according to traffic conditions in advance, this apparatus also controlling the frequency of the'code transmitted to section D.E. under control of contact 5 of coding relay CR. With the threeindication signal shown, the usual clear and caution frequencies of 180 and 75 cycles per minute respectively are given. During an "off'' period of the code supplied by source TB, that is with contact 5 of relay CR picked up, relay KR is connected across the track and during this period source KB is effective to pick up relays TR, KB in During an "on" period, with contact 5 released, source TB is connected to the track and during this period relay TR releases, but relay KB remains picked up. Relay TB thus follows the code to control signal Sd at the entrance of the section through decoding apparatus DAd. Impedances 12, 13, 15, 16 connected in the circuits as shown, have values selected in accordance with the minimum ballast and rail resistance conditions of section D.E., so that during "on" code period current through relay TR is zero, or is reversed when desirable to wipe out foreign current or storage battery effect. Fig. 3 shows a circuit arrangement in which false operation of the detecting relay by "kick-back" energy is avoided by disconnection of source TB from the e track when the section is occupied, and provision for cab signalling-is made by connecting an alternating current source of coded energy to the track during occupation. The arrangementdiffers from that of Fig. 1 in that slow release relay KR is replaced by a code-following relay KR1 having contact 18 controlling energization from a local source of a slow-release repeater relay AR, having the approach controlling contact 9 and also a contact 19 which in picked-up condition (track-section D.E. unoccupied) connects source TB to track through coding contact 5. Occupation of the section releases contact 19 to disconnect source TB and substitute the secondary of a transformer TT so that coded A.C derived from source B.C is supplied to track. An altermative arrangement is suggested in which relay KR1 is slow-acting and has contact 19, relay AR being omitted.