Cable Laying Procedures

??????????????????????????????? Technical specifications of cable work

Applicable Standards are:

Cross-linked polyethylene insulated, PVC sheathed (XLPE), conforming to IS: 7098 (Part-1)- 1988.

Cross-linked polyethylene (XLPE) insulated, PVC sheathed conforming to IS:7098 (Part-2)- 1985.

Note:

?Both ends of the cables should be properly sealed to prevent ingress/ absorption of moisture by the insulation during storage

Handling:

(i)????????????????? When the cable drums have to be moved over short distances, they should be rolled in the direction of the arrow marked on the drum.

(ii)??????????????? ?For manual transportation over long distances, the drum should be mounted on cable drum wheels, strong enough to carry the weight of the drum, and pulled using ropes. Alternatively, they may be mounted on a trailer or a suitable mechanical transport.

(iii)?????????????? ?For loading into and unloading from vehicles, a crane or a suitable lifting tackle should be used. Small-sized cable drums can also be rolled down carefully on a suitable ramp or rails, for unloading, provided no damage is likely to be caused to the cable or the drum.

General:

Cables with kinks, straightened kinks or any other apparent defects like defective armoring, etc. shall not be installed.

(ii) Cables shall not be bent sharp to a small radius either while handling or in installation. The minimum safe bending radius for XLPE (MV) cables shall be 12 times the overall diameter of the cable, The minimum safe bending radius for XLPE (HV) cables at present is 400mm2 Al HV XLPE cable, at joints and terminations, the bending radius of individual cores of a multi-core cable of any type shall not be less than 15 times its overall diameter.

(iii) The ends of lead-sheathed cables shall be sealed with solder immediately after cutting the cables. In the case of PVC cables, a suitable sealing compound/tape shall be used for this purpose, if likely exposed to rain in transit storage. Suitable heat-shrinkable caps may also be used for the purpose.

Route:

?Before the cable laying work is undertaken, the route of the cable shall be decided

(i)????????????????? While the shortest practicable route should be preferred, the cable route shall generally follow fixed developments such as roads, footpaths, etc. with proper offsets so that future maintenance, identification, etc. are rendered easy. Cross-country runs merely to shorten the route length shall not be adopted.

(ii)??????????????? ??Cable route shall be planned away from drains, subject to any special local requirements that may have to be necessarily complied with.

(iii)?????????????? ?As far as possible, the alignment of the cable route shall be decided after taking into consideration the present and likely future requirements of other services including cables en route, the possibility of a widening of roads/lanes, etc. (iv) Corrosive soils, the ground surrounding sewage effluent, etc. shall be avoided for the routes.

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Trenching (i) Width of trench the width of the trench shall first be determined on the following basis (a) The minimum width of the trench for laying a single cable shall be 35cm

?(b) Where more than one cable is to be laid in the same trench in horizontal formation, the width of the trench shall be increased such that the inter-axial spacing between the cables, except where otherwise specified, shall be at least 20cm.

? There shall be a clearance of at least 15cm between the axis of the end cables and the sides of the trench. (ii) Depth of trench The depth of the trench shall be determined on the following basis (a) Where the cables are laid in a single-tier formation, the total depth of the trench shall not be less than 75cm for cables up to 1.1KV and 1.2m for cables above 1.1KV.

?When more than one tier of cables is unavoidable and vertical formation of laying is adopted, the depth of the trench in (ii) above shall be increased by 30cm for each additional tier to be formed.

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Excavation of trenches

(a)??? The trenches shall be excavated in reasonably straight lines. Wherever there is a change in the direction, a suitable curvature shall be adopted complying with the requirements.

(b)??? ??Where gradients and changes in depth are unavoidable, these shall be gradual.

(c)???? ? The bottom of the trench shall be level and free from stones, brickbats etc.

(d)??? ??The excavation should be done by suitable means-manual or mechanical. The excavated soil shall be stacked firmly by the side of the trench such that it may not fall back into the trench.

(e)??? ??Adequate precautions should be taken not to damage any existing cable(s), pipes, or any other such installations in the route during excavation. Wherever tricked, tiles protective covers or bare cables are encountered, further excavation shall not be carried out without the approval of the Execution team.

(f)????? ??Existing property, if any, exposed during trenching shall be temporarily supported adequately as directed by the Engineer-in-Charge. The trenching in such cases shall be done in short lengths, necessary pipes laid for passing cables therein, and the trench refilled by standards

(g)??? ?It there is any danger of a trench collapsing or endangering adjacent structures, the sides may be left in place when backfilling the trench.

(h)??? Excavation through lawns shall be done in consultation with the Department concerned.

Sand cushioning:

? The trench shall then be provided with a layer of clean, dry sand cushion of not less than 8cm in depth, before laying the cables therein. (Bottom: 150mm Two sides: 50mm Top: 300mm)

Testing before laying:

?All the time of issue of cables for laying, the cables shall be tested for continuity and insulation resistance?

?The cable drum shall be properly mounted on jacks, or a cable wheel at a suitable location, making sure that the spindle, jack, etc. are strong enough to carry the weight of the drum without failure, and that the spindle is horizontal in the bearings to prevent the drum creeping to one side while rotating.

?The cable shall be pulled over on rollers in the trench steadily and uniformly without jerks and strain. The entire cable length shall as far as possible be laid off in one stretch.

After the cable has been so uncoiled, it shall be lifted slightly over the rollers beginning from one and by helpers standing about 10m apart and drawn straight. The cable shall then be lifted off the rollers and laid in a reasonably straight line.

Sand covering Cables:

?laid in trenches in a single-tier formation shall have a covering of dry sand of not less than 17cm above the base cushion of sand before the protective cover is laid. In the case of vertical multi-tier formation, after the first cable has been laid, a sand cushion of 30cm shall be provided over the base cushion before the second tier is laid. If additional tiers are formed, each of the subsequent tiers also shall have a sand cushion of 30cm as stated above. Cables in the topmost tiers shall have a final sand covering not less than 17cm before the protective cover is laid.

?Inter-tier spacing should be maintained as in (b) above with soft soil instead of sand between tiers and for covering.

Extra loop cable:

?At the time of original installation, approximately 3m of surplus cable shall be left on each terminal end of the cable and each side of the underground joints. The surplus cable shall be left in the form of a loop. Where there are long runs of cables such loose cables may be left at suitable intervals

Where it may not be practically possible to provide separation between cables when forming loops of several cables as in the case of cables emanating from a substation, measurement shall be made only to the extent of the actual volume of excavation, sand filling, etc.

Mechanical protection over the covering:

The cables shall be protected by second-class brick of nominal size 22cmX11.4cmX7 cm or locally available size, placed on top of the sand (or, soil as the case may be). The bricks shall be placed breadth-wise for the full length of the cable. Where more than one cable is to be laid in the same trench, this protective covering shall cover all the cables and project at least 5cm over the sides of the end cables.

Where bricks are not easily available, or are comparatively costly, there is no objection to using locally available materials such as tiles or slates or stone/cement concrete slabs. Where such an alternative is acceptable

Backfilling

The trenches shall be then back-filled with excavated earth, free from stones or other sharp-ended debris, and shall be rammed and watered, if necessary in successive layers not exceeding 30cm depth.

?Unless otherwise specified, a crown of earth not less than 50mm and not exceeding 100mm in the center and tapering towards the sides of the trench shall be left to allow for subsidence. The crown of the earth, however, should not exceed 10 Cm, so as not to be a hazard to vehicular traffic.

?The temporary re-statements of roadways should be inspected at regular intervals, particularly during wet weather and settlements should be made good by further filling as may be required.

?After the subsidence has ceased, trenches cut through roadways or other paved areas shall be restored to the same density and materials as the surrounding area and –re-paved by the relevant building specifications to the satisfaction of the Engineer-in-Charge.

? Where road beams or lawns have been cut out of necessity, or curb stones displaced, the same shall be repaired and made good, except for turfing /asphalting, to the satisfaction of the Engineer and all the surplus earth or rock shall be removed

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Route markers

(i)????????????????? Location Route markers shall be provided along the runs of cables at locations generally at intervals not exceeding 100m. Markers shall also be provided to identify changes in the direction of the cable route and at locations of underground joints.

(ii)??????????????? ?Plate type marker Route markers shall be made out of 100mm X 5mm GI/aluminum plate welded/bolted on 35mm X 35mm X 6mm angle iron, 60cm long. Such plate markers shall be mounted parallel to and at about 0.5m away from the edge of the trench.

Laying in pipes / closed ducts

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?In locations such as road crossing, entry into buildings, paved areas, etc. cables shall be laid in pipes or closed ducts.

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Road crossings:

?The top surface of pipes shall be at a minimum depth of 1m from the pavement level when laid under roads, pavements, etc. (ii) The pipes shall be laid preferably askew to reduce the angle of bend as the cable enters and leaves the crossing

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When pipes are laid cutting an existing road, care shall be taken so that the soil filled up after laying the pipes is rammed well in layers with watering as required to ensure proper compaction. A crown of earth not exceeding 10cm should be left at the top.

Manholes shall be provided to facilitate feeding/drawing in of cables with sufficient working space for the purpose. They shall be covered by suitable manhole covers

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Cable entry into the building:

Pipes for cable entries to the building shall slope downwards from the building. The pipes at the building end shall be suitably sealed to avoid the entry of water after the cables are laid.

JOINTING

Before laying a cable, proper locations for the proposed cable joints, if any, shall be decided, so that when the cable is laid, the joints are made in the most suitable places. As far as possible, waterlogged locations, carriageways, pavements, proximity to telephone cables, gas or water mains, inaccessible places, ducts, pipes, racks, etc. shall be avoided for locating the cable joints.

?Joints shall be staggered by 2m to 3m when joints are to be done for two or more cables laid together in the same trench.

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Joints pits:

Joint pits shall be of sufficient dimensions to allow easy and comfortable working. The sides of the pit shall be well protected from loose earth falling into it. It shall also be covered by a tarpaulin to prevent dust and other foreign matter being blown on the exposed joints and jointing materials.

?Sufficient ventilation shall be provided during jointing operation to disperse fumes given out by fluxing.

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Jointing materials

(i)????????????????? Jointing materials and accessories like conductor ferrules, solder, flux, insulating and protective tapes, filling compound, jointing boxes, heat shrinking joint kit etc. of right quality and correct sizes, conforming to relevant Indian Standards, wherever they exist, shall be used.

(ii)??????????????? ??The design of the joint box and the composition of the filing compound shall be such as to provide an effective sealing against entry of moisture in addition to affording proper electrical characteristics to joints.

(iii)????????????? ?Where special types of splicing connector kits or epoxy resin spliced joints or heat shrinkable jointing kits are specified, materials approved for such application shall be used. Storing as well as jointing instructions of the manufacturer of such materials shall be strictly followed.

Testing after laying

After laying and jointing, the cable shall be subjected to a 15-minute pressure test. The test pressure shall be as given. DC pressure testing may normally be preferred to AC pressure testing.

? In the absence of facilities for pressure testing as above, it is sufficient to test for one minute with 1000V megger for cables of 1.1KV grade and with 2500/5000V megger for cables of higher voltages.

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Clearances

All cables and ducts crossing roads shall be installed with a minimum horizontal clearance of 150mm from other services

Cable supports

The solid cleating support system in which the cleating distance is such that the cable is maintained in a straight line with no sagging may be employed. ?