Underground cables are, of course, meant to be installed or laid under the ground. The reliability of underground cable network highly depends upon proper laying of cables, quality of cable joints and branch connections etc. There are three main methods of laying underground cables, which are - (i) direct laying, (ii) draw-in system and (iii) solid system. These three methods are explained below with their advantages and drawbacks.

This method is the most popular as it is simple and cheap. The cables to be laid using this method must have the serving of bituminised paper and hessian tape so as to provide protection against corrosion and electrolysis. The direct laying procedure is as follows.

Laying procedure

A trench of about 1.5 meters deep and 45 cm wide is dug.
Then the trench is covered with a 10 cm thick layer of fine sand.
The cable is laid over the sand bed. The sand bed protects the cable from the moisture from the ground.
Then the laid cable is again covered with a layer of sand of about 10 cm thick.
When multiple cables are to be laid in the same trench, a horizontal or verticle spacing of about 30 cm is provided to reduce the effect of mutual heating. Spacing between the cables also ensures a fault occurring on one cable does not damage the adjacent cable.
The trench is then covered with bricks and soil to protect the cable from mechanical injury.

Advantages

Simpler and cheaper than the other two methods
Heat generated in cables is easily dissipated in the ground.

Disadvantages

To install new cables for fulfilling an increased load demand, completely new excavation has to be done which costs as much as the new installation.
Alterations in the cable network are not easy.
Maintenance cost is higher.
Identifying the location of a fault is difficult.
This method can not be used in congested areas such as metro cities where excavation is too expensive.