Underground Cables are used for transmission and distribution of an electric power in congested areas (Such as in cities, towns etc) at comparatively low or medium voltages. Usually, the underground cables are used in congested cities or towns, substations, railway crossings and where safety concern is most important. However, recent improvements in the design and manufacture, have led to the development of cables more suitable for the use at high voltage. In this article, we will discuss about definition of underground cable and construction of underground cables.
A conductor covered with a suitable insulation and protecting layer is called cable.
Requirement of Underground Cables
- The conductor used in underground cable should be standard in order to provide flexibility. The conductor should have high conductivity.
- The area of cross section should be such that the cable carries the desired load current without overheating.
- The voltage drop in conductor should be within permissible limits.
- The cables must have proper thickness of insulation.
- The cables must be provided with suitable mechanical protection.
- The cables do not react with chemicals.
Construction of Cables
The cable consists of one or more than on core. It depends upon the types of service for which it is intented. The conductors are made of tinned copper or aluminium . The stranded conductor are used in order to increase flexibility.
Each conductor or core covered with proper thickness of insulation. Basically thickness of insulation depends upon the voltage level. The commonly used insulating materials are impregnated paper, varnished cambric etc.
A metallic sheath is provided over the insulation in order to protect the cable from moisture, other liquids that are present in soil and atmosphere. The commonly used material for metallic sheath is lead or aluminium.
Over the metallic sheath bedding layer is provided which consists of a fibrous material like jute or hessian tape. Bedding protects the metallic sheath against corrosion and from mechanical injury due to armouring.
Armouring is provided over the bedding which consists of one or two layers of galvanized steel wire. It protects the cable from mechanical injury.
In order to protect armouring from atmospheric conditions, a layer of fibrous material provided over the armouring. This layer is called serving.