General Information on Track Circuits - RGS Online

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Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page D6 of 20 6 Operating Times Withdrawn Document Uncontrolled When Printed <strong>General</strong> <strong>Information</strong> on Track Circuits The monitoring device will usually be housed at the nearest interlocking, but this will largely be governed by the routine and out–of–hours fault finding cover arrangements which exist in the vicinity. The transmission of information to the monitoring device may be achieved by additional FDM channels, a low–cost FDM system approved for use in signalling or telecommunications cables or direct wire circuits. 6.1 Time Delays There can be significant differences between the drop–away and pick–up times of different types of track circuit, such that the rear track may register clear before the forward one registers occupied. The detection of the vehicle is therefore momentarily lost, resulting in a wrong side failure, which could permit the irregular release of vital interlocking. To overcome this, additional time delays must be built into the pick–up time of track repeaters, the precise requirement being dependent upon the combination of track circuit types involved. The indication circuits to the signalman may be transmitted via a TDM or FDM link. Therefore the transmission system reaction times must also be considered to ensure that the signalman does not observe an apparent loss of train detection. 6.2 Operating Categories And Conditions In order to simplify the number of possible permutations, track circuits are assigned to operating categories as follows: Track Relay Operating Characteristics Operating Category Slow to Pick Up - Quick to Drop Away A Medium to Pick Up - Medium to Drop Away B Quick to Pick Up - Slow to Drop Away C Track Circuit Type Operating Category TI21 A UM71 (French A D.C. (all types) B Phase sensitive a.c. (50Hz, 75Hz & 83.3Hz) B Aster B Reed with adjustable track filter B Western Region “Quick Release” (a.c./d.c.) B Diode B Coded B GEC Alsthom High Voltage Impulse (HVI) C Reed without adjustable track filter C D6 RAILTRACK
<strong>General</strong> <strong>Information</strong> on Track Circuits Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page D7 of 20 With Geographical systems, the differing combinations of abutting categories of track circuits need to be examined and dealt with specially, according to the original design principles. With free wired relay interlocking and SSI, they must be dealt with as follows: Category A When used with a free wired relay interlocking, these track circuits do not require a slow to pick up TPR. Therefore, the TR may be used directly in controls. When used with an SSI, standard track circuit data must be used. Category B When used with a free wired relay interlocking, these track circuits require one slow to pick up TPR, in accordance with Figure D4 When used with an SSI, standard track circuit data must be used. Category C and Category B abutting Category C When used with a free wired relay interlocking, these track circuits must be provided with two slow to pick up TPRs, in accordance with Figure D5. The TR and TPR must be in the same location case or equipment room. The T2PR must be controlled directly by contacts of both TR and TPR to prevent the drop– away of T2PR from being unnecessarily delayed whilst still achieving the delayed pick–up required. When used with an SSI, track circuit data with “extra delay” must be used. Where the time delay is achieved by relay cascade, it is important that other contacts of the TR and any intermediate repeater relays are not used for control indication purposes. To prevent inadvertent subsequent use, a suitable note must be made on the Contact Analysis Sheet. A schedule must be provided listing all TPRs, the individual sections repeated by each TPR and the type of track circuit (including the frequency in the case of a jointless track circuit). B50 N50 Figure D4 B50 N50 Withdrawn Document Uncontrolled When Printed Figure D5 EG TR EG TR BR 933 EG TPR RAILTRACK D7 EG TR EG TR EG TPR EG TPR BR 933 BR 933 EG TPR EG T2PR
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