Here I am going to explain you about the different methods of laying underground cables.The reliability of underground cable network depends to a considerable extent upon the proper laying and attachment of fittings i.e., cable end boxes, joints, branch connectors etc.
There are three main methods of laying underground cables viz., direct laying, draw-in system and the solid system.
1.Direct laying: This method of laying underground cables is simple and cheap and is much favoured in modern practice.In this method of laying underground cables, a trench of about 1·5 metres deep and 45 cm wide is dug.The trench is covered with a layer of fine sand (of about 10 cm thickness) and the cable is laid over this sand bed.The sand prevents the entry of moisture from the ground and thus protects the cable from decay.After the cable has been laid in the trench, it is covered with another layer of sand of about 10 cm thickness.
The trench is then covered with bricks and other materials in order to protect the cable from mechanical injury.When more than one cable is to be laid in the same trench, a horizontal or vertical interaxial spacing of at least 30 cm is provided in order to reduce the effect of mutual heating and also to ensure that a fault occurring on one cable does not damage the adjacent cable.Cables to be laid in this way must have serving of bituminous paper and Hessian tape so as to provide protection against corrosion and electrolysis.
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Advantages:
(i) It is a simple and less costly method.
(ii) It gives the best conditions for dissipating the heat generated in the cables.
(iii) It is a clean and safe method as the cable is invisible and free from external disturbances.
Disadvantages:
(i) The extension of load is possible only by a completely new excavation which may cost as much as the original work.
(ii) The alterations in the cable netwok cannot be made easily.
(iii) The maintenance cost is very high.
(iv) Localisation of fault is difficult.
(v) It cannot be used in congested areas where excavation is expensive and inconvenient.This method of laying cables is used in open areas where excavation can be done conveniently and at low cost.
2.Draw-in System: In this method of laying underground cables, conduit or duct of glazed stone or cast iron or concrete are laid in the ground with manholes at suitable positions along the cable route.The cables are then pulled into position from manholes. Fig. below shows a section through four-way underground duct line.Three of the ducts carry transmission cables and the fourth duct carries relay protection connection, pilot wires.
Care must be taken that where the duct line changes direction; depths, dips and offsets are made with a very long radius or it will be difficult to pull a large cable between the manholes.The distance between the manholes should not be too long so as to simplify the pulling in of the cables.The cables to be laid in this way need not be armoured but must be provided with serving of hessian and jute in order to protect them when being pulled into the ducts.
Advantages:
(i) Repairs, alterations or additions to the cable network can be made without opening the ground.
(ii) As the cables are not armoured, therefore, joints become simpler and maintenance cost is reduced considerably.
(iii) There are very fewer chances of fault occurs due to the strong mechanical protection provided by the system.
Disadvantages :
(i) The initial cost is very high.
(ii) 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 laying underground cables 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 crossings where frequent digging is costlier or impossible.
3.Solid System: In this method of laying underground cables, the cable is laid in open pipes or troughs dug out in the earth along the cable route.The troughing is of cast iron, stoneware, asphalt or treated wood.After the cable is laid in position, the troughing is filled with a bituminous or asphaltic compound and covered over.Cables laid in this manner are usually plain lead covered because troughing affords good mechanical protection.
Disadvantages :
(i) It is more expensive than direct laid system.
(ii) It requires skilled labour and favourable weather conditions.
(iii) Due to poor heat dissipation facilities, the current carrying capacity of the cable is reduced.
