There are following methods of cable laying
( a ) Direct laying
- In this method of cable laying, a trench of about 1.5 meter deep and 45 cm wide is dug.
- The trench is covered with layer of sand of about 10 cm thickness and cable is laid over it.
- The sand prevents the entry of moisture from bottom side or ground.
- Once cable is laid over the sand, another layer of sand about 10 cm thickness laid above the cable.
- The trench is covered with brick in order to protect cable from mechanical injury.
- If more than one cable is laid in the trench, it is placed horizontally or vertically.
- There is axial spacing of 30 cm is provided between cables in order to reduce heating between them and fault on one cable does not affect other cable.
- The minimum depth of laying of cable from ground surface to the top of the cable is given below.
Cable |
Clearance from top of ground to top
of cable |
High voltage cable : 3.3 kV to 11 kV |
0.9 m |
High voltage cable : 22 kV , 33 kV |
1.05 m |
Low voltage cable |
0.75 m |
Control cable |
0.75 m |
Cable at road crossings |
1.00 m |
Cable at railway crossings |
1.00 m |
Power cable to control cable |
0.2 m |
Power cable to gas / water main |
0.3 m |
Power cable to communication cable |
0.3 m |
Advantages
- Simple method
- Less costly
- Better condition for dissipation of heat generated in the cable
- Cable is free from adverse effect of external atmosphere conditions
Disadvantages
- Localization of fault is difficult
- Extension of load is only possible by costing of new excavation and cable laying
- High maintenance cost
- It cannot be used in the congested area where excavation is inconvenient and expensive.
- Cable may be damaged due to frequent digging at the place of cable laying
- The lead sheath of cable may be damages due to chemical changes in the soil.
( b ) Draw in system
- This method is adopted in the congested area where direct laying of cable is expensive.
- In this method of cable laying, a line of conduits, duct or tube made of either iron or stone / cement concrete are laid in the trenches side by side.
- The cable is pulled in the manholes.
- Figure shows four way underground duct line for cable.
- Out of four duct, three duct carries main power cable and fourth duct carries control cable for protection and switchgear equipment.
- The diameter of duct pipe should always be larger than the diameter of cable in order to pull cable safely.
- The duct always made straight but sometimes it is made curved if any obstacles.
- When duct is made curved, the diameter of pipe is made kept larger to facilitate drawing cable through pipe.
- The vertical distance between two adjacent pipes should be 0.25 m – 0.75 m.
- The cable need not be provided armouring but provided with serving of jute / hessian tape in order to protect them during draw in duct.
- Repair / maintenance of cable is possible without excavation of ground
- Maintenance cost is considerably reduced as compared to direct laying of cable method
- Less chances of fault occurrence between cable due to mechanical protection provided by the duct pipe.
- Long life of cable due to less chances of fault on cable
Disadvantages
- High initial cost
- The current carrying capacity of cable is reduced due to less heat dissipation and close grouping of cables.
( c ) Cable lying on racks inside tunnel
- The laying of cable on racks is shown in the figure.
- If the tunnel is unventilated, the heat generated in the cable is dissipated only through the wall of tunnel.
- Therefore there is an increase in the temperature of cable inside tunnel resulting current carrying capacity of cable is derated in the tunnel.
- The increase in temperature in the tunnel is given by
T = W / 3P
Where
T = Temperature in 0C ( above ambient
temperature )
W = Heat loss per meter length of cable ( watt )
P = Perimeter of tunnel cross section in meter
Cable suspended from catenary wire
- In some area, the cable is suspended by means of aluminum / steel / leather suspenders.
- The suspender is hung from catenory wire by means of galvanized iron wire.
- The catenary wire is made of galvanized steel of adequate strength.
- The two ends of catenary wire should be connected to rigid supports. The rigid supports may be of steel poles.
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