Underground Cables are used for transmission and electric poser in congested areas (Such as in cities, towns etc.) at comparatively low at medium voltages. Usually, the underground cables used in congested cities or towns, substations, railway crossings and where safety is very important. However, recent improvements in the design and manufacture have led to the development of cables suitable for use at high voltage. In this article, we will discuss about the definition of underground cable and construction of underground cables.
What is Cable?
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 cable must have proper thickness of insulation.
- The cable must be provided with suitable mechanical protection.
- The cables do not react with chemicals.
Construction of Cables
The cables have various parts. There are given below.
The cable consists of one or more than one core. It depends upon the typ0es of service for which it is intended. The conductors are made of tinned copper or aluminium . The stranded conductors 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 the 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 metarial 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.