CABLE LAYING AND INSTALLATION
All cables shall be handled, laid and installed according to this specification, BS 7671, cable manufacturer’s recommendations and ERA Reports by using proper installation equipment and tools. All cables shall be supplied in complete length to suit the circuits they serve, and no straight through joints shall be used.
Straight through joints in the cable will only be permitted in very exceptional circumstances such as arising from unavoidable limitations in manufacturing length. If straight through joints or other approved joints is permitted by the S.O.’s Representative, the cost of such joints shall be borne by the Electrical Contractor. No joints in the cable will be allowed unless approved in writing by S.O.’s Representative.
The minimum internal bending radius of the cable shall be not less than 12 times the overall diameter of the cable. Wherever cables are cut, the ends shall be immediately sealed in an approved manner unless; it is intended to proceed with cable jointing for termination straight away.
For groups of more than one cables laid in the same trench, they shall be spaced by the horizontal clearance between adjacent cables of at least twice the overall diameter of the adjacent largest size cables. Unless otherwise permitted by the S.O.’s Representative no cable shall be laid and covered up in the absence of the S.O.’s Representative.
8.1 CABLE LAID DIRECT IN GROUND
Before cable is laid, the trench shall be thoroughly inspected and any debris and sharp objects shall be removed. The bottom of the trench shall be covered with a layer of 75 mm of clean sand. The cables shall then be laid on this bedding in an orderly manner without overlapping and crossing each other. After laying the cables, a layer of 75 mm of clean sand shall cover the cables and carefully spread over the trench before placing the
cable protective covers.
Unless otherwise specified, the cable protective covers shall be of clay bricks. The bricks shall be new, well burnt and in complete pieces. They shall be laid length wise from end to end along the entire route of the underground cable if the cable size is more than 120 sq. mm. For cable size more than 120 sq. mm., more than one row of the bricks shall be laid.
Each cable shall be separately protected by these bricks and the cover shall have at least 25 mm overhang on each side of the cable. If uPVC cable protective covers are specified, the covers shall be of polyvinlcloride without plasticiser type with specific density between 1.37 g/cm3 to 1.42 g/cm3 as shown in Appendix A approved by the S.O.’s Representative. The uPVC cover shall be resistant to aggressive soils and of dimension 150 mm wide and 1000 mm long. The covers shall be single coloured orange and top side shall be embossed with standard danger sign and bold letters “BAHAYA! KABEL ELEKTRIK DI BAWAH”. The covers shall be provided with male and female interlocking device.
The covers are laid together lengthwise from end to end along the entire cable route and they are held together tightly by the interlocking device. At least 25 mm overhang on each side of the cable shall be provided to protect the cable underneath. The trench shall then be backfilled with earth and shall be consolidated after every 150 mm of backfilling using a mechanical rammer.
An orange coloured, multi-strand nylon rope of minimum 6 mm diameter shall be laid at a depth of 300 mm along the trench to identify the cable route. At every 10 metres interval, an extra 2 metres length of nylon rope shall be coiled and laid. The finished surface shall be left proud by 50 mm to allow for subsidence and the Electrical Contractor shall be responsible for the removal of any surplus to a position indicated by the S.O.’s Representative.
8.2 CABLE INSTALLED IN PRE-CAST CONCRETE TRENCHES
The surface of the refilled trench shall be temporarily reinstated and maintained in a thoroughly safe condition until complete consolidation of the soil is achieved. As soon as the soil has consolidated, the trenches shall be made good to the original conditions to the satisfaction of the S.O.’s Representative.
Method of installation of cables in pre-cast concrete trenches shall be in accordance with Method 12, 18, 19 or 20 of Table 4A BS 7671. However, if the method is not specified, the cables shall be installed in accordance with Method 18 and/or as directed by the S.O.’s Representative.
Cables laid at the bottom of the trenches shall be in accordance with Method 18 of Table 4A BS 7671. Cables installed on the trench wall shall be in accordance with Method 12, 19 or 20 of Table 4A BS 7671 and the cables shall be secured on the cable tray by means of saddles at suitable intervals. In the case of single core cable, non ferrous saddle shall be used.
The cable trays shall be fabricated from perforated hot dipped galvanized sheet. The minimum thickness of the sheet steel used shall be 1.5 mm for cable tray with width up to 300 mm and shall be 2.0 mm for width exceeding 300 mm. The cable tray shall be supported at least 25 mm from the trench wall by mild steel brackets at 600 mm intervals.
The bracket shall be hot dipped galvanised. All brackets shall be securely fastened with steel raw bolts and nuts. Samples of cable tray and bracket shall be submitted to the S.O.’s Representative for approval prior to installation. To provide electrical continuity, all cable tray joints shall be bridged by means of tinned copper tape of dimension not less than 25 mm x 3 mm.
All saddles for cables on cable trays shall be installed by bolts, washers and nuts. All cable tray tees, intersection units, adaptor units etc. shall be factory manufactured. The trenches inside the buildings shall be filled with clean sand up to a
level above the cable ducts.
8.3 CABLE RUN ON WALLS AND UNDER FLOOR SLABS
Cable run on walls and under floor slabs shall be mounted on perforated hot dipped galvanised sheet steel cable trays. Method of installation of the cables shall be in accordance with Method 11 and 12 of Table 4A BS 7671. The construction and finished of the cable trays and the way of installation of the cables on the cable trays shall be as described in 8.2 above.
The cable trays shall be suspended from floor slabs by hangers or mounted on wall by brackets at 600 mm interval. The material and finishes of the hangers, brackets and other suspending and supporting structures shall be as that described for brackets in 8.2 above. Where cable trays pass through floors or fire resistant walls, the surrounding hole shall be sealed to full thickness of the floor or wall with non-hygroscopic fire-resisting material of minimum 2-hour fire rating approved by Jabatan Bomba Dan Penyelamat Malaysia.