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CHAPTER -XV
CABLES
15.1 GENERAL
Railway signalling circuits should normally be carried on cables. Overhead lines may be used on branch lines. For new works, signalling circuits shall only be carried on cables. In 25 KV AC electrified areas, overhead lines shall not be used.
PART I - CABLES SECTION 'A' - REQUIREMENTS
15.2 TYPE OF CABLES
Cables used for carrying signalling, circuits shall conform to relevant approved specification. The conductors used shall be of annealed copper and of approved size.
Power cable used for carrying power supply for signalling circuits shall conform to relevant approved specification. The size of conductor shall be so selected as to suit the electrical load.
15.3 PLANNING
15.3.1 While planning for cabling on a route, the number of conductors required, depending upon the circuits should be first determined. Recommended core sizes as per specifications shall be used.
15.3.2 Adequate spare conductors to a minimum of 20% of the total conductors used shall be provided for in each main cable up to the farthest point zone, beyond this there should be a minimum of 10% spare conductors of the total conductors used. No spare conductors are required if the total number of conductors used is 3 or less. The spare conductors shall be provided on the outermost layer.
15.3.3 Where a number of cables have been laid along a route, the circuits shall be so distributed that cables can be disconnected for maintenance purpose with the least possible dislocation to traffic. Line wise and, if necessary; function wise cable shall be provided. Auxiliary signals shall be taken in different cables.
15.3.4 After deciding the size and the number of conductors in the different types of cables to be used on a route, a foot survey along the track should be done to determine the best route for the cable.
15.3.5 The route shall be shown clearly on a cable route plan showing the actual alignment of track, giving offsets from permanent way or permanent structures. The diagram should indicate the various road and track crossings, crossing with power cables, water and sewage mains and other points of importance. It is preferable to chart the route on a route plan on which the existing routes of power cables, etc. are shown. Changes if any should be incorporated in the chart/plan.
15.3.6 Cable route plan shall also be approved by Engineering and Electrical department.
15.3.7 As far as possible low lying areas, platform copings, drainages, hutments, rocky terrains, points and crossings, should be avoided
15.3.8 Separate cables of suitable size shall be laid for point operation.
15.4 LAYING CABLES ABOVE GROUND
Signalling cables for out door circuits should not normally be laid above ground. In exceptional cases where it becomes unavoidable, the following precautions should be taken.
i) The cable should be suspended in wooden cleats, from cable hangers or in any other approved manner so that no mechanical damage occurs to the cable even under exposed condition.
ii) The cable supports shall be so spaced as to avoid sag.
iii) In station yards, cable shall be laid in ducts suitably protected.
iv) Indoor signalling cable should normally be laid on ladders, channels or in any other approved manner. The cables should be neatly tied/laced.
Note: In AC electrified areas cables shall be laid underground only. For laying cables in RE area instructions laid down in chapter XXII of SEM shall also be followed.
15.5 CABLES LAID UNDERGROUND
Cables may be laid underground, either in the trench, in ducts, in cement troughs, in pipes or in any other approved manner.
15.6 LAYING THE CABLES IN DUCTS
15.6.1 RCC, masonry or any other approved type of ducts may be used for laying the cable.
15.6.2 The ducts shall have suitable covers.
15.6.3 The ducts shall be of such design as to prevent water collecting in the duct.
15.6.4 When cables are laid in rocky area, it is desirable to protect them with split RCC ducts of suitable design.
15.6.5 Where it is necessary to take the cable between the tracks, it shall be carried in trunking kept sufficiently below the ballast level.
15.7 Where several cables of different categories have to be laid in the same trench, they shall be placed as far as possible in the following order starting from the main track side, so that in the event of failures, the maintenance staff may easily recognise the damaged cables.
i) Telecommunication cable
ii) Signalling cable
iii) Power cable
15.8 Cables belonging to the Posts & Telegraphs Department or the Electrical Department must not be laid in the same trench along with Signal & Telecommunication cables. A distance of approximately 10 cm must be maintained between telecommunication cable and signalling cables. The signalling cables must be separated from power cables by a row of bricks between them.
15.9 STORING AND TRANSPORTATION OF CABLE.
15.9.1 Cable drums shall not be stacked on flat side. Suitable stoppers shall be placed for stability.
15.9.2 Cable drums shall have easy access for lifting and moving.
15.9.3 When rolling the cable drum either for unloading or transportation, the drum shall always be rotated in the direction of the 'arrow' which is marked on the drum.
15.9.4 The drums shall not be rolled over objects that could cause damage to the protective battens of the cable.
15.9.5 When unloading is carried out from the vehicle the drum shall not be dropped on the ground directly to avoid damage due to impact. Fork lifter or ramp shall be used.
15.9.6 During all stages of storage, it is essential that the ends of the cable are effectively sealed by end cap or in any other approved manner to avoid water entry into the cable.
15.9.7 It is desirable that cable drums are stored in covered shed to protect against direct exposure to sun.
SECTION 'B' INSTALLATION
15.10 PAYING OUT AND TESTING THE CABLE
15.10.1 For paying out cables, the cable drums shall be mounted on cable wheels. It should be ensured that no kink is formed while paying out the cable.
15.10.2 The drum on the wheel shall be brought to one end of the trench and the end of the cable freed and the cable shall be laid along the trench.
15.10.3 A party of labourers shall move along the trench carrying cable at suitable intervals so that cable is not damaged due to dragging along the ground or bent unduly.
15.10.4 Before the cable is laid in the trench, a visual inspection of cable shall be made to see that there is no damage to the cable. It shall be tested for insulation and continuity of the cores. Thereafter the cable shall be laid into the trench. Record of insulation and loop resistant must be maintained.
15.10.5 In cases where the wheels are not available, the drum shall be mounted on an axle at one end of the trench and cable paid out and carried by labourers.
15.10.6 In no case, shall the drum be rolled off on to the road for laying the cable and the cable dragged on the ground for laying purposes.
15.10.7 Whenever mechanized equipment is used, the work shall be carried out by a trained operator under the supervision of SSE/SE/JE (Signal) incharge of the work.
15.10.8 Where the cable drum is in damaged condition the cable may be placed on a horizontal revolving platform and the cable paid out in the same manner as given in paras above.
15.10.9 Paying out of cable should be done by rotating the cable drum and not by pulling the cable with excessive force.
15.11 LAYING
Cables shall be laid generally as per instructions given in this chapter. However, special precautions to be taken in the station yards etc where a number of other utilities may be existing, may be detailed in a joint circular issued by the Civil Engineering, Signalling and Electrical Department (where applicable) of the Railway.
15.12 The cable laid parallel to the track shall normally be buried at a depth of 0.80 meters from ground level. While those laid across the track must be 1.0 metre below the rail flanges. However, in case of rocky soil, the depth may be reduced suitably. When it concerns the laying of tail cables which serve the track apparatus etc the depth should not be less than 0.50 metres. In theft prone areas the cables may be laid at a depth of 1.2 m with anchoring at every 10m.
15.13 The width of manually made cable trenches should commensurate with number of cables. The minimum width shall be kept as 0.3 meters. The bottom of the cable trench should be levelled and got rid of any sharp materials. In the soft ground, the cable should be laid at the bottom of the trench previously levelled. In the rocky ground, the cable should be laid on a layer of sand or sifted earth of 0.05 meter thickness previously deposited at the bottom of the trench. In both the above cases, the cable should be covered with a layer of sand or sifted earth of 0.10 metre thickness and thereafter a protective cover of trough or a layer of bricks should be placed.
15.14 CABLE CROSSING
15.14.1 When a cable has to cross the track, is should be ensured that-
i) The cable crosses the track at right angles;
ii) The cable does not cross the track under points and crossings and
iii) The cable is laid in concrete / Gl / Cl / PVC pipes, suitable ducts or in any other approved manner while crossing the track.
15.14.2 Wherever practical, the cable may be taken underground across the drain bed at a suitable depth for crossing small culverts with low flood level.
15.14.3 When cables have to cross a metallic bridge, they should be placed inside a metallic trough which may be filled, as an anti-theft measure, with sealing compound. The cable should be supported across the bridge in a manner which would involve minimum vibrations to the cable and which will facilitate maintenance work. Adequate cable length to the extent of 2 to 3 meters shall be made available at the approaches of bridge.
15.15 Cable markers wherever provided should be placed at suitable interval and at diversion points.
15.16 While laying the cables in accordance with the above instructions, the following instructions should be adhered to for the safety of the track.
i) Outside the station limits, the cables should generally be laid at not less than 5.5 m from the centre of the nearest track.
ii) Within the station limits, the trenches shall preferably be dug at a distance of not less than 3 m from the centre of the track, width of the trench being outside the 3m distance.
iii) At each end of the main cable an extra loop length of 6 to 8 m should be kept.
15.17 It is desirable that the excavation of the trenches is not done in long lengths and does not remain uncovered for long period. It is preferable that cables are laid and refilling done on the same day.
15.18 Backfilling of the trenches should be done properly. The earth excavated shall be put back on the trench, rammed and consolidated.
15.19 During excavation, the earth of the trenches should not be thrown on the ballast. The earth should be thrown by the side of the trenches away from the track.
15.20 In places where cables are to be laid within 1 metre from sleeper end, digging beyond 0.50 meter shall be done in the presence of an official from Engineering Department, and the laying of the cable and refilling of trench should be done with least delay.
15.21 Cable joints of approved type shall only be used.
15.22 The work shall be supervised at site personally by an official of the Signal and Telecommunication Department not below the rank of a JE / SE / SSE (Signal).
SECTION 'C - MAINTENANCE
15.23 TESTING OF CABLES
15.23.1 All conductors in signalling cables must be tested for their insulation in dry weather every year as per instructions in annexure '12'. A comparison of the test results between successive tests carried on a cable under similar conditions will give an indication of the trend towards deterioration of the insulating material over a period of time. If a sudden fall in insulation is observed, the cause should be investigated and immediate steps taken up to repair or replace the cable.
15.23.2 In addition to the regular testing of cables in dry weather, random tests in wet weather may also be carried out, where considered necessary, to localise any sudden deterioration in insulation of cables.
15.24 EARTH LEAKAGE DETECTORS
The use of this device is desirable in all electrical signalling installations, such as Route Relay Interlocking, panel interlocking, centralized traffic control and SSI.
15.25 All cables termination devices, pillar boxes, cable heads and glands shall be kept clean and dry. These parts shall be frequently inspected and any tendency for moisture or water leak shall be immediately attended to.
15.26 No excavation in the vicinity of underground signal cables shall be undertaken without a representative of the signal & telecommunication department being present and without taking suitable precautions for the safety of the staff. Suitable instructions in this regard shall be issued by the Railway in the Joint circular as stipulated in para 15.11.
15.27 The protective works provided for the cables at places like track crossings, culverts, bridges etc shall be regularly inspected by the maintenance staff. Special attention being paid to these protective works soon after the monsoon.
PART II - OVERHEAD LINES
The requirements, installation practices and maintenance of Overhead lines carrying signalling circuits shall be in accordance with and be followed as laid down for telecom circuits and detailed in IR Telecom Manual.
Annexure 12
Para 15.23.1
INSTRUCTIONS FOR INSULATION RESISTANCE TESTING OF SIGNALLING CABLE
1.1 General
These instructions apply only to cables used for Railway Signalling and do not cover open line wires and internal wiring.
1.2 Insulation Resistance tests should be made in such a manner that safe operation of trains is not affected. It should be ensured that no unsafe conditions are set up by the application of test equipment.
1.3 All conductors in signalling cables must be tested for their insulation at the time of commissioning and thereafter in dry weather every year preferably during the same part of the year.
1.4 The insulation resistance tests should be made when conductors, cables and insulated parts are clean and dry.
1.5 In addition to regular testing of the cables in dry weather, random tests in wet weather may also be carried out where considered necessary.
1.6 The conductors of the cables possess appreciable electrostatic capacity and may accumulate electrostatic charge. The cable conductors should be shorted or earthed to completely discharge any accumulated charge (i) before connecting the insulation tester while commencing the test (ii) before the insulation tester is disconnected when the test is completed. This is in the interest of safety of personnel and protection of equipment
1.7 A 500V insulation tester should be used for insulation testing. The fact that the cable has capacitance means that it has to be discharged before a measurement of the insulation resistance can be made. The insulation resistance should therefore be recorded after the test voltage has been applied for one minute or so when the indicator of the insulation tester shows a steady reading.
1.8 Any metallic sheath or metal work of any rack or apparatus case should be bonded to earth during test.
PROCEDURE
2.1 Disconnect all cores of a cable at both ends. The disconnection may be made through links of ARA terminals, if provided.
2.2 Connect one terminal of the insulation tester to the conductor under test and other terminal to all the other conductors being bunched together and connected to earth.
2.3 Similarly test remaining conductors of the cable one by one as in 2.2
2.4 Insulation Resistance so measured should not be less than 5 mega ohms per kilometer at buried temperature. If the insulation resistance is found to be lower than 5 mega ohms, the cause should be investigated and immediate steps taken to repair or replace the cable to prevent any malfunctioning of the equipment and circuits.
2.5 The results of the insulation resistance tests should be recorded in approved proforma. A comparison of test results between successive tests carried out on a cable under similar conditions will give an indication of the trend towards deterioration of the insulation resistance of the cable. If sudden fall in the insulation resistance is observed the cause should be investigated and immediate steps taken to repair or replace the cable.
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