TYPE AND QUALITY OF CABLES
Unless otherwise specified in the Schedule of Quantities all wiring cables shall be P V C/XLPE insulated and P V C sheathed conforming to relevant IS Standard. The conductor of cable shall be of stranded wires of aluminium or copper as specified. All power cables shall be 1100 volts grade, PVC insulated, PVC.sheathed and armoured with stranded aluminium conductor. Materials should be obtained from the approved list of manufacturers/ brands as indicated in the document.
HANDLING OF CABLES
It shall be ensured that both ends of the cables are properly sealed to prevent ingress / absorption of moisture by the insulation.
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. While removing cables, the drum shall be properly mounted on jacks or on a cable wheel or any other suitable device, making sure that the spindle, jack, etc. are strong enough to take the weight of the drum.
DEFECTIVE CABLES
Cables with kinks and straightened kinks or with similar apparent defects like defective armouring, etc. shall not be installed.
BENDING RADIUS
Cable runs shall be uniformly spaced, properly supported and protected in an approved manner. All bends in runs shall be well defined and made with due consideration to avoid sharp bending and kinking of the cable. The minimum safe bending radius for all types of P V C cables shall be taken as 12 times the overall diameter of the cable. Wherever practicable, larger radius shall be adopted.
LENGTH OF CABLES
All cables shall be laid in one length. No joint shall normally be made at any intermediate point in through runs of cables, unless the length of the run is more than the length of the standard drum
supplied by the cable manufacturer. In such cases where jointing is unavoidable, the same shall be made by means of standard cable joint boxes/ kits. Before cutting the cables, the requisite length between terminals (including extra length required at loops) shall be carefully measured.
STRIPPING OF OUTER COVERING
While cutting and stripping the outer covering (i.e. sheathing of the cable, care shall be taken that the sharp edge of the cutting instrument does not cut or damage the inner insulation of the conductor. The protective outer covering of the cable shall be stripped off near the connecting terminal, the protective covering being maintained upto a point as close a possible to the connecting terminal.
CABLE LAID IN TRENCHES
Cables shall be laid generally in accordance with Indian Standard Code of Practice IS: 1255.
SIZE OF TRENCH
Unless otherwise mentioned in the Schedule of Quantities, the minimum width and average depth of trench for laying a single cable in ground shall be 460mm and 760mm for L.T. and 1000 mm for
H.T. cable respectively. For laying of multiple 11 KV and 6.6 KV grade power cables, horizontal axial spacing shall be 250mm. For 1100 volt grade power cables, the horizontal axial spacing
shall be 150mm. However, communication cable shall not be taken in a common trench. Where more than one cable are to be laid in the same trench in horizontal formation, the width of trench shall be increased according to the above stated inter-axial spacing between the cable, (except where otherwise specified). There shall be a clearance of at least 150 mm between the trench edge and axis of the end cable.
EXCAVATION OF TRENCH AND PREPARATION OF BED
The trench shall be excavated in reasonably straight line. Where there is a change in direction, suitable curvature shall be provided. Where gradients and changes in depth are unavoidable, these shall be gradual.
Adequate precautions shall be taken during excavation not to damage any existing cables, pipes or similar installations in the proposed route. Where bricks, tiles or protective covers or bare cables are encountered, further excavation shall not be carried out without the approval of the Engineer-in-Charge.
The bottom of the trench shall be level across the width and free from stone, brick bats, etc. The trench shall be then provided with a cushion of fine sand, the thickness of the cushion being not less than 75mm.
CABLE Laying - Direct in ground As per CPWD
1. Laying of one number PVC insulated and PVC sheathed / XLPE power cable of 1.1 KV grade of following size direct in ground including excavation, sand cushioning, protective covering and refilling the trench etc as required.
(For 100 Mtr.)
a. Common burnt clay F.P.S. (non modular) bricks class designation 7.5 = 874 + 44 (Wastage@ 5%) = 918 Nos.
b. Fine sand - 8.76 Cum
c. With Labour, Excavation including refilling
2. Laying of one number additional PVC insulated and PVC sheathed / XLPE power cable of 1.1 KV grade of following size direct in ground in the same trench in one tier horizontal formation including excavation, sand cushioning, protective covering and refilling the trench etc as required.
(For 100 Mtr.)
a. Common burnt clay F.P.S. (non modular) bricks class designation 7.5 = 874 + 44 (Wastage@ 5%) = 918 Nos.
b. Fine sand - 5 Cum
c. With Labour, Excavation including refilling
LAYING OF CABLES
All cables shall be tested for proper insulation prior to laying. The cable drums shall be transported on wheels to the place of work. The cables shall be laid out in proper direction as indicated on the drum using cable drum lifting jacks. In case of higher size cables, the laid out cables shall run over rollers placed at close intervals and finally transferred carefully on to the trenches and racks, care shall be taken so that kinks and twists or any mechanical damage does not occur in cables. Only approved cable pulling grips or other devices shall be used. The entire length of cable shall, as far as possible, be paid in one operation. However, if this is not possible, the remainder of the cable may be shifted from position by 'falking' i.e. by making one long loop in the reverse direction. For crossing water, gas or sewerage pipes, etc, cables shall be taken above the pipes where minimum 500 mm clearance is not available. The cable shall cross these pipes through RC/ GI pipes at a minimum depth of 750 mm from finished ground level keeping the distance between the utility pipes and pipe carrying cables 300 mm minimum.
While laying cables parallel to building, railway track, utility pipe lines, drainage, sewerage, etc. the minimum clearance shall not be less than 1000mm.
Adequate length of cables shall be pulled inside the switch boards, control panel terminal boxes, feeder pillar etc. so as to permit neat termination of each core.
SURPLUS CABLE
At the time of original inspection, approximately l meter of surplus cable (in the form of a loop or otherwise) shall be left at each entry or exit of the cable at a pole or at the pillar box, or near any terminal as may be directed by the Consultant / Engineer-in-Charge.
PROTECTIVE COVER FOR CABLES DIRECTLY BURIED IN GROUND
Except where otherwise directed by the Consultant/Engineer-in-Charge, the cable (for the entire length in trench) shall be protected by a layer of bricks laid flat on top and shall be provided at least by 75 mm sand cushioning both at top and bottom. This brick protection shall cover all the cables in the trench (single cable or multiple cables, in horizontal formation). In case of a single cable, the brick protection shall consist of one brick flat (with the length along the width of the trench) and supported on two lines of brick-on-edge, one on each side of the cable (with the length of the bricks along the length of the trench).
For multiple cables in horizontal formation, in addition to the two outer lines of brick-on-edge, there shall be additional lines in between adjacent cables. The top cover of brick flat shall extend to cover all the cables, each brick being supported on the lines of brick-on-edge.
BACK FILLING OF TRENCH
After laying of cables the remaining portion of the trench shall be back filled with good excavated soil and well rammed in successive layers not exceeding 300 mm depth each and duly compacted to the satisfaction of the Consultant/ Engineer-in-Charge. Surplus soils of excavation shall be removed or disposed of as per direction of the Engineer-in-Charge.
All material like sand, brick and clamp, etc. shall be supplied by the contractor. The cable laying rate shall be inclusive of all these items.
CABLES LAID THROUGH PIPE SLEEVES
Entry of cable from underground trenches to the building or tunnel shall be through pipe sleeves. Necessary precaution shall be taken to make entry point fully water tight by properly sealing the pipe sleeves in a manner approved by the Engineer-in-Charge.
Where cables are required to cross roads, railway tracks and surface drains, they shall be taken through pipe sleeves at a minimum depth of 1000 mm.
LAYING OF CABLES ON RACK/ TRAY/ BRACKET/ HOOKS/ MASONRY TRENCH
Where cables are required to be laid directly along structure walkway, walls, ceiling, they shall generally be taken exposed on brackets, cable racks, trays, hooks laid along building structure. Spacing of saddles/ hooks shall be such that the cables are straight and shall not exceed 750 mm.
The cable rack/ trays shall be ladder type / pre-fabricated perforated type and bends / curvature shall be smooth and suitable for bending the largest cable running in the rack / tray. The cable rack/ trays shall be suitably installed on the building structure with proper support at regular intervals. Cable rack/ trays shall be so arranged that they do not obstruct or impair clearance of passage way.
Where there is possibility of mechanical damage cable racks / trays shall be adequately protected by
sheet steel cover.
Unless otherwise specified in the schedule of quantities the rack/ trays shall be painted with corrosion resistant paint and finished with enamel paint of shade battleship grey or any other colour shade acceptable to Consultant/ Engineer-in-Charge.
CABLES TERMINATION
Power cable termination shall be carried out in such a manner as to avoid strain on the terminals by providing suitable clamp near the terminals. All power cables shall be terminated to the circuit breaker, switch fuse units, busbars, etc. by means of suitable sizes crimping type or soldering type cable socket / lugs / ferrules and empire tape upto palm of the cable lug. PVC tape shall not be used directly, because of its poor thermal stability. It may however, be used over the empire tape. Control cables shall be terminated by crimping or directly clamped in the terminal blocks by screws.
When pinching the smaller size conductor directly in the terminal bore of the switches, the individual strands shall be fanned out and cleaned by wire wool or emery paper and the cleaned surface shall be coated with a thin layer of oxide inhibiting grease. The conductor shall be tightened fully to the terminal bore but over tightening shall be avoided.
For connection to busbars and other terminals, brass or cadmium plated nuts/ bolts and washers shall be used. Copper cables shall never be terminated directly on aluminiumbusbar. Suitable measure shall be taken to avoid heating due to bimetallic contacts.
A selection chart of crimping type cable lugs for various combination of cables/ busbar/ fuse switch terminals is shown below :
Material of busbar/switch terminals |
Material of Cables |
Material of crimping lug |
|
Aluminium |
Aluminium |
Aluminium over tin plated copper |
|
Copper |
Aluminium |
Aluminium lug with copper plated palm |
|
Silver/tin plated copper |
Aluminium |
Aluminium or tin plated copper |
|
Aluminium |
Copper |
Tin plated copper |
|
Copper |
Copper |
Copper or tin plated copper |
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