The scope of work shall cover laying, connecting, testing and commissioning of low voltage power cables complete with all cable support, cable, cable trays, clamps and joints. Cables to be purchased from who regularly engaged in the manufacturer of electrical cable and wire products of the types and ratings required, whose products have been in satisfactory use in similar service not less than 5 years. The standard of the cables shall comply with National Electrical Manufacturers Association of local standards or any other approved.
Low Voltage Cables (Armoured and Unarmoured)
All cables shall be XLPE/PVC insulated and of the type and size indicated on the drawings and shall have standard copper conductors (unless stated otherwise). Complying with the local and international applicable standard.
The rating of all power cables for 220 V system voltage shall be 600/1000 V grade.
XLPE/PVC insulated cables shall consist of plain XLPE/PVC copper stranded conductors, circular or sector, shaped insulated laid up together and wormed circular, further insulated with an extruded inner sheath of PVC. Any necessary fillers or warnings shall be extruded over sheath of black PVC applied directly over the inner sheath.
XLPE/PVC armored cables shall conform to the specification and sizes indicated on the drawings and shall be in accordance with the latest requirements and specification of local standard or any other approved equivalent.
The armored cables shall be galvanized steel wire or tape.
Colour Coding of LV Power Cable
- Phase-1: Red Colour
- Phase-2: Yellow Colour
- Phase-3: Blue Colour
- Neutral: White Colour
- Ground: Green or Green/Yellow Colour
Durable marking shall be provided on the surface of the cable or intervals not exceeding 610 mm. Marking shall include manufacture name, insulating material conductor size and voltage class.
Cable Tray For LV Power Cables
Cable trays shall be of the following types:
1. Heavy Duty Cable Tray
2.0 mm minimum thickness perforated galvanized steel with return flange cable tray of 50 mm minimum height on each side the tray shall either be tapered over its complete length or supplied with a reduced width into the wider end. Bolts shall be used to secure each joint. All cable tray tees, intersection units, bends, turns and sets shall wherever possible, be purposed made by the manufacturer and shall be matching design to the main section of cable tray. The appropriate joint shall be secure as applicable. On installations where “Purpose made” accessories are not available or otherwise impracticable, these may be made up on site only with the written approval of the architect.
2. Cable Tray Supports
Supplied by the manufacturer of made up on site shall be of ample strength to maintain rigid support to the fully laden cable tray along its entire length and shall ensure that the deflection of any one section does not exceed 15 mm at mid span.
Where proprietary channel section cable support racks being used elsewhere on the contract with cleats, then any such channel used for support of cable trays shall be of the same type and make.
3. Cable Tray Finish
Subject to the environmental conditions of the installation, cable trays and accessories specified shall be dipper galvanized. Cable tray supports shall be of a compatible finish to the associated cable tray or, where manufactured on site, be given a protective finish of “Metagalv” “Galafroid” or other suitable approved paint or aerosol sprayed with PVC as applicable.
4. Installation of Cable Tray
Wherever possible, cable trays shall be installed in full length without cutting. Should it be necessary to cut or drill a length or tray then for galvanized trays, the bared ends or damaged section of the tray shall immediately be given a coat of zinc rich cold galvanizing paint such as “Metagalv” or “Galvafroid” or similar approved paint.
All site manufactured accessories, supports and metal fittings required to ensure correct installation of the cable trays shall be similarly treated.
All cables shall be firmly secured to the tray using purpose made saddles, as approved by the consultant together with nylon fastener and/or cable cleats.
Following installation of cable, the tray shall remain rigidly supported and the deflection of any one section shall not exceed 15 mm at mid span.
The cable tray shall be supported horizontally by galvanized steel U channels hinged to the ceiling by 16 mm diameter minimum of threaded steel galvanized rods. The spacing between the fixation points shall not exceed 100 cm, as to the cable tray shall be supported by rigid supports such as U steel galvanized channels fixed to the ceiling to the rigid frame at every 9 meter minimum spacing. In case of cable tray installed vertically the spacing between fixation points shall not exceed 120 cm.
Installation of the Low Voltage Power Cables
Cables shall be run between termination points in continuous lengths. Joints will be allowed only where specified or with approval.
1. Installation Below Ground - Buried Directly Underground.
Cables laid direct in the ground shall be laid at a depth of 650 mm and blinded with a radial thickness of at least 100 mm of sand around. A layer of concrete tile is laid over the sand continuously along the cable route. The remaining portion of the trench shall be back filled and consolidated. The cable markers as described elsewhere shall be provided at regular intervals not greater than 75 m in straight runs and at the point where change of descriptions occurs. Also a marker tape as described elsewhere shall be laid. Cables shall not be bent during installation smaller radius than 12 x dia of cable.
Cables which follow the same route and are laid in the ground shall be in horizontal formation with spacing between cables where possible of not less than 150 mm with the exception of single core cables which shall be run in trefoil formation and touching along their entire length. Each group of cables shall be indicated by separate cable markers.
Adequate cable markers of approved design shall be used to indicate the route of buried cables at intervals of not more than 75 meters and at points where change of direction occurs. Where markers are of free standing post type, these shall be not less than 150 mm wide x 75 mm shall be exposed above ground.
Cables shall be marked at each end at access points by a label indicating the cable size and circuits.
2. Cables Buried Underground in PVC Pipe
The cables shall be installed in PVC pipes at the depth specified in the drawing. The minimum depth shall be 60 cm below finished floor level. Wherever necessary for installation and maintenance of cables, suitable size manholes with heavy duty covers shall be provided.
Cables under road crossing shall be run in PVC conduits encased in concrete. The conduits shall have minimum diameter of 150 mm, have bends to suit the cable and extend 1 m on either side of the road.
Cables under floor or through wall shall be run in conduits of either PVC conduits or vitrified clay having a plastic sleeve to form a push fit joint with glazed interior on PVC. The conduits shall have a minimum diameter in conformity with IEE regulation and have bends to suit the cables.
Cable conduits and ducts which terminate in building shall be sealed with permanently plastic weather proof sealing compound in conjunction with hardwood bushes to prevent the ingress of water, foreign matters and vermin. Any spare conduits and ducts shall be similarly sealed.
3. Installation of Low Voltage Power Cables - Above Ground
1. All cables shall run from point to point without joints and where they change direction the radius of the bend shall not be less than the minimum state in the IEE regulations. Unless otherwise specified all cables shall be laid on cable trays or ladder trays or PVC pipes.
2. Unless otherwise specified, cables run in air, including engineering service ducts, shall be fixed with aluminium alloy or approved non flammable claw cleats with galvanized back straps using galvanized bolts conforming to BS 1490 with maximum spacing between supports as detailed in Table B2M of the IEE regulations. Cleats for cables upto and including 50 mm dia shall be fixed by a single bolt and fixing that the stud does not extend appreciably beyond the tightened nut. Excess pressure of cleats on PVC insulated cables shall be avoided to prevent deformation of the plastic sheathing. Suitable supporting steel work and/or galvanized cable tray shall be provided where cables cross open spaces. Such steel work shall be protected by a rust inhibiting paint.
Cables installed above ground shall be supported by one of the following methods:
- Supported in cleats, clips or hangers fixed to walls or ceilings.
- Laid in cable trays supported from walls or ceiling.
- Vertical runs of cables in the shafts shall be laid in cable ladders supported from the walls.
3. Cable cleats shall be manufactured from the die cast aluminium nylon or other approved material and shall incorporate an approved type of clamping device capable of holding the cable securely without damaging it in any way. They shall be of the correct size to suit the overall diameter of the cable with which are to be used.
4. Cable hangers shall be the ‘J’ type either rectangular or semi circular, and shall be manufactured from mild or cast steel or die cast aluminium. They shall be of a size appropriate to the cable(s) with which they are to be used.
5. All steel cable cleats, clips, saddles and hangers shall be hot dip galvanized after manufacture.
6. Where prepared means of fixing have not been provided, cable supports, whether the single or multi way type, shall be fixed to brickwork or concrete walls or ceilings with ‘Rawl Bolts’ or other approved fixing devices of adequate size. In the case of single cable fixing for cables of not more than 30 mm overall diameter, however, wood screws and masonry plugs may be used as an alternative.
7. Wherever cables pass through walls, ceilings partitions and the like the sub contractor shall provide and fix in position a heavy gauge PVC sleeves having an internal diameter greater by at least 12 mm, but not more than 25 mm than diameter of the cable.
Joints and Terminations of Low Voltage Power Cables
The jointing and terminating of all cables shall be performed by an experienced cable joiner, trained and fully conversant with modern techniques and once the cable is cut for the purpose of making a joint or termination the work involved shall be completed without interruption; if for any reason the work cannot be completed, then the cable ends shall be sealed immediately with PVC tape.
All cable joints and terminations shall be supplied complete. All switch gear supplied, shall be completed with cable boxes and termination materials.
1. Low Voltage Cable Termination
The termination and joints for the XLPE cables shall guarantee a satisfactory stress relief at the cable end.
The outdoor terminations shall be able to withstand air pollution and bad weather without any sign of surface current tracking. The cable splice should be electrically equal to the cable, ensuring a completely reliable cable system. The conductors must be securely connected, the conductor shield must have smooth surface for a good contact with the insulation, which in turn must be free from voids and contamination. The splice insulation must be sufficiently thick and the outer semi conductor must adhere closely to it.
Where cables are terminated outside buildings or in damp situations compound filled boxes shall be used.
2. Low Voltage Cable Joints
The joints shall be made with compression type connectors.
The XLPE insulation of both cable ends are tapered down. The conductors are normally jointed by crimp connectors. For the larger cable sizes and higher voltages the conductors shall be welded. The conductor joint is trimmed and it is smoothly covered by a semi conductive sleeve bonding to tape. The insulation is then built up by wrapping insulating tape, making sure that no voids one formed. For voltages upto 72 KV the tape shall be of self bonding FP type on the splice insulation surface, which must be smooth, on insulation shield is applied in intimate contact with the insulation. The screen wires are collected and jointed with a compression sleeve. The whole splice is normally covered by copper netting. The jacket is then replaced by a plastic crimping sleeve or by PVC tape.
All joints shall be made in molded plastic boxes purpose made for PVC/SWA/PVC sheathed cables, the joints being wrapped with PVC tape to give a degree of insulation at least equal to that of the original insulation and the boxes filled with an approved compound.
Particular attention shall be paid to ensure earth continuity of the cable armory, molded plastic boxes with armor clamps and a copper strip. After completion of the joint, cast iron boxes shall be painted with two coats of bituminous paint.
The cores of low voltage cables shall be jointed color to color or number to number as appropriate.
Core which are identified by numbers shall be connected to that:
- 0 is neutral in any multi core cable.
- 1 is red phase in a 3 core or 4 core cable or the phase conductor in a 2 core cable.
- 2 is yellow phase in a 3 core or 4 core cable.
- 3 is blue phase in a 3 core or 4 core cable.
Joints shall be of the indented compression type.
The indented compression type shall be made by means of a hydraulic compression round head and suitable compression dies according to the core size of the cable, with strict adherence to the cable manufacturers recommendations, on accessories, ferrules, compression pressures and jointing techniques. Cable armour shall be neatly arranged and clamped to a suitable gland being protected with a PVC shroud.
Identification of LV Cables
- Each cable shall be provided with a means of identification fixed adjacent to the termination at each end. It shall be in the form of a plastic or brass label securely fixed to the sealing box or tied to the cable and provide details of the cable size, function and cable reference number.
- The cores of all mains cables shall be individually identified at terminations means of colors in accordance with Table B.4 of the IEE regulations. Where the core insulation is not itself colored, bands, of self adhesive PVC tape in the appropriate color shall be affixed to each core over the insulation.
Testing of Low Voltage Power Cables
As soon as possible after the completion of installation all cables shall be subject to the following test:
- An insulation resistance test, to measure the insulation resistance between conductors and between each conductor and earth, shall applied at 1000 volts.
- An earth continuity test to verify that the cable armory has been properly bonded to earth.
- Phase rotation test to prove that the cables have been properly connected.