Then the trench is covered with a 4" 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 4" thick.
When multiple cables are to be laid in the same trench, a horizontal or verticle spacing of about 4"Inch 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.
Simpler and cheaper than the other two methods
Heat generated in cables is easily dissipated in the ground.
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.
In this method, cast iron or concrete pipes or ducts are laid underground with manholes at suitable positions along the cable route.
The cables are then pulled into the pipes from the manholes. Usually, an additional pipe/duct is also provided along with the three cable ducts for carrying relay protection connections and pilot wires.
Distance between the manholes should be such that pulling in the cables is easier. At corners or while changing the direction of route, radius of the corners must be longer.
The cables that are to be laid in this way need not be armoured but must be provided with the serving of hessian and jute in order to protect them when being pulled.