Cables laying method | Cable laying method is used in generating station |Cable laying method use for 3.3 Kv ,11 Kv,22 Kv , 33 Kv | Lv cable laying method
The reliability of the underground cable (UG) installation depends upon the proper laying and attachment of fittings (i.e) cable and boxes, joints, branch connectors etc. Methods of laying of UG cables
The following are the methods of laying underground cables
👉1 Laying direct in ground
👉2 Laying in ducts
👉3 Laying on racks in air.
👉4 Laying on racks inside a cable tunnel.
👉5 Laying along buildings or structures.
The choice of any of the systems given above depends on
(i) The actual installation conditions
(ii) Inital cost of laying
(iii) Maintenance and repair charges
(iv) Deisred care in replacement of any cable or adding new cables for the future.
As far as the possible cable should be laid along the roads and streets. Power and communication cables should cross at right angles.
During the preliminary stages of laying the cable, consideration should be given to a proper location of the joints position so that when the cable is actually laid, the joints are made in the most suitable places.
As far as possible water logged locations, carriage ways, pavements, proximity to telephone cables, gas or water mains in accessible places, ducts pipes, racks etc shall be avoided for joint position.
👉 Laying direct in ground:
This method involves digging a trench in the ground and laying cable(s) on a bedding of minimum 75 mm riddled soil or sand at the bottom of the trench, and covering it with additional riddled soil or sand of minimum 75 mm and protecting it by means of tiles, bricks or slabs.
Depth:
The desired minimum depth from ground surface to the top of cable is as follows:
a) High voltage cables, 3.3 KV to 11 KV rating : 0.9 m.
b) High voltage cables, 22 KV, 33 KV rating : 1.05 m.
c) Low and medium voltage and control cables : 0.75 m.
d) Cables at road crossings : 1.00 m.
e) Cables at railway level crossings (measured from bottom of sleepers to the top of pipe) : 1.00 m.
Width:
The width of a trench for laying a single cable should be minimum 35 cm. When more than one cable is laid in the same trench in horizontal formation, the width of the trench shall be so increased that the inter-axial spacing between two cables is 20 cm.
Clearance from the terminal cable to the sides of a trench should be 15 cm.
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| Fig 1 |
Cable is protected by sand or layer of brick as shown in Fig 1. Bricks should be second class bricks of a size not less than 20 cm x 10 cm x 10 cm and laid for full length for one cable (bricks to be laid breadthwise).
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| Fig 2 |
When more than one cable is to be laid in the same trench, this protective covering shall extend atleast 5 cm. over the sides of the end cables. An alternative to this covering can be earth ware or R.C.C. or fire-bricks of peaked covers section as shown in Fig 2.
It is good practice to leave about 3 metres of cable spare in a loop formation near poles and joints, so that in case joint fails, this additional cable comes to rescue. Cable should be laid 0.4 metre away from water and power mains.
For road crossings cast iron, or 2nd class RCC pipes or M.S/G.I. Pipe of medium class having an appropriate diameter should be laid during construction of the road to avoid damage to the road later on.
The top surface of the pipe should be at a minimum depth of 1m. Pipes provided for entry to a building shall slope upward to prevent entry of water into the building. After laying of the cable they should be sealed.
Advantages:
👉 1 It is a simple and less costly method.
👉 2 It gives the best conditions for dissipating the heat generated in the cables.
👉 3 It is a clean and safe method as the cable is invisible and free from external disturbances.
Disadvantages:
👉 1 The extension of load is possible only by a complete new excavation which may cost as much as the original work.
👉 2 The alterations in the cable network cannot be made easily.
👉 3 The maintenance cost is very high.
👉 4 Localisation of fault is difficult.
👉 5 It cannot be used in congested areas where excavation is difficult.
👉 Drawing the cables into duct pipes:
When drawing the cables through ducts, lack of space in the drawing pits usually restricts the distance from the cable drum to the duct mouth.
It is essential that the direction of curvature of the cables is not reversed as it enters the duct.
If the cable drum is on the same side of the drawing pit, as shown in Fig 3, this condition is fulfilled.
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| Fig 3 |
👉 Advantages
👉 1 Reparis, alterations or additions to the cable network can be made without opening the ground.
👉 2 As the cables are not armored, therefore, joints become simpler and maintenance cost is reduced considerably.
👉 3 There are very less chances of fault occurrence due to strong mechanical protection provided by the system.
👉 Disadvantages
👉 1 The initial cost is very high.
👉 2 The current carrying capacity of the cables is reduced due to the close grouping of cables and unfavourable conditions for dissipation of heat.
This method of cable laying is suitable for congested areas where excavation is expensive and inconvenient, for once the conduits have been laid, repairs or alterations can be made without opening the ground. This method is generally used for short length cable routes such as in workshops, road crossing where frequent digging is costlier or impossible.
👉 Laying cables on racks in air:
This method if cable laying used in generating stations.
Inside buildings, industrial plants, generating stations, substations and tunnels, cables are generally installed on racks fixed to the walls or supported from the ceiling.
Racks may be ladder or perforated type and may be either fabricated at the site or pre-fabricated.
Considerable economy can be achieved by using standard factory made racks.
The necessary size of the racks and associated structure has to be worked out taking into consideration the cable grouping and permissible bending.
Fig 3 shows the method of laying cables inside a tunnel on racks.
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| Fig 04 |
👉 Laying cables along buildings or structures:
Cables can be routed inside the building along with structural elements or with trenches under floor ducts or tunnels.
The route of the proposed cable should be such that intersection with other cables will be minimum.
The route should not subject these cables to any vibrations, damage due to heat or other mechanical causes. All adequate precautions should be taken to protect the cables.
👉 Precautions while handling cables:
1 Prevent the cable from dragging on the floor.
2 Prevent kinking of the cable.
3 After laying the cable in the ducts it should be immediately covered or suspended.
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