Under Ground Electric Cables

Under Ground Electric Cables Points : under ground electric cables, testing, cable conductor, handling of cables, cable route, laying of cables, laying in pipes / closed ducts, laying on surface, cable identification tag, completion report Testing The cable shall be tested for continuity and insulation between conductors and conductor and earth before and after installation. Test results shall be submitted in the proforma given in Appendix 1. Cable Conductor The cable shall have either solid or stranded aluminum conductor and shall be P.V.C. insulated and P.V.C. sheathed and l.S.I. certified.

The cables may be armoured or unarmoured. Unarmoured cables may used only when the runs are short and the cables are laid in pipes or closed masonry trenched or protected and secured enclosures. Single core cables armoured with steel wire or tape shall not be used for alternating current supply. Selection of Cables: The final current carrying capacity of the cable, after allowing for derating, shall not be less than the current ills required to carry. The current carrying capacity of P.V.C. insulated and P.V.C. sheathed cables and the derating factors are given in Appendix 5.

The cables should be able to withstand the minimum short circuit current for the period of short circuit. Cable insulated with P.V.C. or any thermoplastic material should not be overloaded even for a short period. Handling of cables The cable should be bandied carefully so as to avoid formation of kinks, any injury in conductor, insulation, sheathing, armoring etc which may result in earth fault or discontinuity of conductor or both. Damaged cables, cables with kinks and straightened kinks or with similar apparent defect shall not be installed.

The cable drums shall not be stored on water logged and loose surface. Both the ends should be sealed to prevent ingress or absorption of moisture by insulation. The cable drum shall be rested on flanges. The drums should either be rolled in the direction of arrow or mounted on cable drum wheels and pulled by means of ropes. The cable shall note be bend to a radius of less than 12 times the overall diameter of cable. Cable route The cable shall be run, as far as possible parallel to roads, footpaths or other fixed development items. Cross country runs to shorten the route length should be avoided.

The cable shall be laid away from drains, storm water drains, kerb lines, existing cables, private property etc. The cable shall not be laid in corrosive soil.

Cables of different voltage rating shall be laid in different trenches with adequate separation. Cables of higher voltage shall be laid at a lower level than the cable of lower voltage. The alignment of the cable route should take into consideration the interest of other authorities and also future expansion like widening of roads etc. Laying of Cables Cables shall be laid directly in ground, in pipes, in open/closed ducts or on surface as per requirement at site. Joints in cables shall not be provided. When the distance exceeds the standard length supplied by manufacturer, kiosks shall be provided. The location of the kiosks shall be decided before laying the cables. The knosks shall not be provided in water logged locations, carriage ways, pavements, proximity of telephone cables, gas and water mains, in accessible places, ducts, rocks etc.

Laying direct in ground: This method shall be adopted where frequent excavations are not encountered and re-excavation is easily possible without affecting other services. The cables shall be laid in reasonable straight trench such that its top is at a depth of 5 cm from the ground level.

For laying single cable, the trench shall be 45 cm wide. For laying two or more cables in one trench, the width of the trench shall be increased so as to provide an inter axial distance of at least 30 cm and end clearance of 15 cm from the side walls. The bottom of cable trench shall be leveled and cleared of all rubbish, stone and hard materials etc. And covered with 7.5 cm thick layer of clean dry sand. punned smoothly with hand throughout.

The cables shall be laid on the layer of sand and covered with at Least 7.5 cm thick layer of clean dry sand and covered on the sides and top with well burnt bricks to provide protection. Pre—cast cement concrete can be laid at the lop of sand instead of bricks. In case of vertical tier formation, and sand cushion, of 30 cm shall be provided between each tier. In addition, a partition layer of brick shall be laid between layers. Extra length of cable in the shape of open mouthed loop shall be left at each termination and kiosk.

In the case of long runs of cable, loose cables may be left at suitable intervals also. The trench shall be back tilled with soft earth, rammed solidly and dressed properly to the satisfaction of Engineer Incharge.

Route markers: If desired, cable route marker marked “CABLE” shall be provided along route of the cable and location of loops. The route markers may be of 100 x 100 x 5 mm G.I. plate welded or bolted on to 15 mm dia MS. rod or of 600 x 600 x 100 mm block of 1:2:4 cement concrete. Plate marker should be mounted parallel to and 50 cm away from the edge of the trench. The concrete marker shall be laid flat over be cable, trench projecting over the surrounding surface. Laying in Pipes / closed ducts In locations such as road crossings, entry to buildings, paved areas, on poles etc. the Gables shall be laid in G.I. cast iron or spun reinforced concrete pipe, as specified. For a single and more. Separate pipe for each cable shall be preferred. Unless specified otherwise, the pipe shall be laid directly in ground without any bed such that its torn at a depth of at least one meter from the ground. level. The pipes on road crossing shall be laid 1on the skew. The pipes shall be continuous and shall be cleared of debris or concrete before the cable is drawn.

If specified manholes of adequate size shall be provided to facilitate drawing in of cables. The mount the pie entering a building shall be scaled to avoid entry of the water and the pipe shall slope downwards outside.

Laying in Open ducts: In location such as sub-stations, Switch room, workshops, plant rooms etc, the cables shall be drawn in-open ducts of suitable dimensions, with easily removable covers of cement concrete cheqered plates. The cables shall not cross each other, unless unavoidable. The cables shall have no joints or splice inside the duct. The cable shall be fixed with clamp’4 On the wall of the duct, taken in trough in duct, support on racks In the duct or laid direct in trench over suitable spacers as directed at site. The duct may be filed with dry sand, covered with easily removable covers of cement concrete or cehquered plates or finally finished in cement plaster. Laying on surface The cable maybe laid on surface in switching stations, factories, tunnels, rising mains, overhead busbars etc. The cable shall be laid in trough or on bracker or fixed with mild steel clamps, such that there is no undue sag in the cable. The cable shall be laid at least 25 cm clear of the wall. Cable identification tag Wherever more than one cable are existing, suitable marker tags inscribed with cable identification details shall be permanently attached to all cables in the manhole, pulpits, joints, open ducts, under ground cables etc., at suitable intervals. Completion Report After completion, test result on the prescribed proforma (Appendix E) and cable route duly marked on the building site plan shall be submitted. The portion of the building etc. damaged during erection of installation shall be repaired properly to original finish and color of the building etc.