Most Construction projects requires underground cabling system for its power supply, some times High tension system too. The architects and builders are least bothered about how this cables routed to the premises and equipment. If electrical professionals pointed out the feasibility, the aesthetics of the project would touch to the intended level. Hence, in the planning stage itself the routes and size of the power cable trench to be incorporated in the project design.
Steps for Direct Buried Cable Laying
This drawing shows how to lay a Low Tension (LT) power cable, single core direct buried method. After taking trench keep clean the trench for the following steps:
- Make a sand bed or fine riddled soil bed as per the measurements given in the drawing.
- Lay the cable in the middle of the trench.
- Provide sand bed on top of the cable.
- Lay the protective tile just above the sand bed.
- Backfill the excavated soil leaving 300 mm from the ground level.
- Lay the Cable warning tape and again back-fill the remaining portion of the trench upto the ground level.
All the materials should be as per the relevant section of the project specification and standards.
Cable laying method for Road Crossing and Utility Service Crossing using uPVC Duct
Drawing below shows Installation of Underground cable in two environments. First one shows direct buried cable installation and the second one shows cable passing through duct mainly where road crossing comes. Another occasion for using duct due to crossing of other services like water pipe line, telecommunication lines or petrochemical pipe lines. Duct should be uPVC encased with concrete all along with the crossing of either road or other services.
Direct Buried method for Multiple cables using Single Cable Trench
Below drawing explain how to lay multiple cables in a single trench. Here cables run two tier (layer) and parallel as a matrix system. The sequence shall be compacted sand bed, first tier cable parallel run, sand fill, second tier cable parallel run, sand fill, protective concrete block, backfill of the excavated soil, warning tape, sand fill or backfill of the excavated soil, and finally the pavements.
Typical Specification for underground Cable laying or installation
Wherever the installation involves the laying of certain sections of cable direct in the ground trenches prepared within the Contract, the Contractor shall protect the cables with reinforced concrete or accepted PVC covers, all cables drawn into prepared pipes or ducts shall be effectively sealed where the cables enter substations or buildings to prevent the entry of water and vermin. Where required by the Engineer duct entries into manholes shall be suitably sealed, after the installation of all cabling.
The invert levels of all pipes or ducts entering a building shall be such that the pipes or ducts slope away from the building. Should the pipe or duct entry into a building be below the water table puddle flanges shall be incorporated.
Where cables are laid direct in the ground 75mm of sand or riddled earth shall be placed at the bottom of the trench to form a bed for the cables. When the cables have been laid they shall be inspected and accepted by the Engineer. They shall then be covered with similar riddled sand/earth which shall be punned around and over the cables to a level of 75mm above the top of the cables. Where more than one horizontal layer of cables is to be laid similar sand /earth bedding and cover shall be provided for each layer.
Protective covers shall be placed centrally over the cables, each cover being closely butt jointed and interlocked with the adjacent covers throughout the length of the cable. No final filling in of trenches shall be commenced until the Engineer has accepted the placing of the protective covers.
After any cable has been laid and until the whole of the cables to be laid in the trench have been covered with protective covers, no sharp tools or other appliances shall be used in the trench or left in such positions that might result in damage to the cables if they fell into the trench. Any rollers used for the cable laying operation shall be free from all sharp projections liable to cause damage to the cables.
The Contractor shall be solely responsible for ascertaining whether the soil is chemically active and for taking special precautions to protect the cables against chemical action. The Contractor shall take precautions to avoid electrolytic and/or electro-chemical action occurring in situations where the cable and accessories are likely to be installed in close proximity to other dissimilar metals in the presence of moisture.
The arrangement of cables, all methods of laying and installation shall be agreed with the Engineer, which shall be planned to provide an orderly formation, free from unnecessary bends and crossings, permitting the removal of any one cable without undue disturbance to adjacent cables.
Where ducts have been laid the Contractor shall ensure that they have been ridden and cleaned to remove all foreign matter before the cables are drawn in. After cables have been laid in accordance with requirements specified the trenches shall be filled in by the Contractor and the refilled trench shall be left and maintained in a thoroughly safe condition until the work of permanent reinstatement is carried out. Cable ducts shall be internally glazed vitrified clay with plastic flexible sleeve joints or PVC or pitch fibre type.
Cables shall be identified in all manholes and at equipment by an insulated tag or tape with the circuit identification inscribed thereon. Cables shall have the phases and the number and sizes of cores identified with a letter or numerical designation in all manholes and switchboards. The designation shall be on non-corroding material which shall also be resistant to chemical degradation.
Cables running through formed ducts shall be fixed with reinforced industrial nylon or aluminium alloy cable cleats bolted to vertical or horizontal mild steel channel supports embedded in the duct walls at 600mm fixing centres.
Cables passing under roads shall be drawn through full round ducts 600mm below ground level for cables up to 1 kV, 1100mm below ground level for cables up to 11 kV. Cables laid in unmade ground shall be buried minimum 600mm for 1 kV rating and 800mm for 11 kV rating below ground level. Marker posts shall adequately indicate the routes of all buried cables. They shall be reinforced concrete units fitted with lead inserts bearing cable sizes and references. The marker posts shall extend above ground level or shall be flush with the final ground level, as specified.