Underground Cables have several advantages over the conventional Overhead Lines like less maintenance cost , less chances of fault , less chances of getting being damaged due to environment. As well as underground cable provides better appearance. But the underground cables have higher installation cost and requires more insulation for same working voltage.
Underground Cables are generally employed where it is not possible to use Overhead Lines. Such areas may be thickly populated areas where safety is an important issue.
Underground Cables Construction
An underground cable when used at high voltage consists of one or more central conductor (core) of stranded aluminium. over which number of layers are provided in order to provide necessary insulation, mechanical strength and protection.This article is about Construction of Underground Cables
Following figure shows the general layout of a 3-core underground cable. It basically consists of the following parts.
Major Parts of Underground Cables
i) Core or Conductors
iii) Metallic sheath
The above parts may vary depending on the operating voltage and the application.
Core is the central conductor/conductors which carries the actual current. A cable may have one or more cores depending on the application. It is usually a stranded conductor made up of copper or aluminium. Stranding is done in order to increase current carrying capacity and flexibility.
Each core is provided with suitable thickness of I insulation in order to prevent the leakage of current. The thickness of insulation depends on the operating voltage of the cable. Higher the voltage, more will be the thickness of insulation. The most commonly used insulating materials are butyl rubber, impregnated paper, PVC, XPLE, varnished cambric etc.
(iii) Metallic Sheath
A metallic sheath is provided around the insulation. The sheath is usually made up of lead or aluminium. The function of the metallic sheath is to protect the insulation from direct contact with the soil. This is done because the soil contains moisture and many other chemicals, which will deteriorate the insulation and also may cause some chemical reactions.
Bedding is nothing- but a layer of paper tape compounded with fibrous material provided over the metallic sheath. Bedding serves two purpose i.e., it protects the metallic sheath from corrosion due to moisture and it acts as adhesive to stick the metallic sheath and armouring. It also protects the metallic sheath from damages due to armouring.
Armouring is provided over bedding in order to provide good mechanical strength to the cable and to protect the cable from injuries during erection. It consists of layers of galvanized steel wire. In order to reduce the sheath loss, the wire is made up of high resistance material.
(vi) Serving – It is a layer of fibrous material (like jute) which protects the armouring from atmospheric conditions. It is like bedding, bedding protects the metallic sheath and serving protects the armouring.
PROPERTIES OF UNDERGROUND CABLES
1. Conductor used in the cable is generally made up of stranded copper or aluminium. Stranding is done because conductor may become flexible and carry more current.
2. Cable is provided with proper thickness of insulation in order to give high degree of safety and reliability. This avoids the leakage current.
3. Underground Cable have high mechanical strength to withstand the rough use in laying it.
4. Underground Cables are Non-Hygroscopic which means that it does not absorbs the moisture from soil because moisture decreases the insulation resistance and fastens the breakdown of the cable therefore, cables are enclosed in a water proof covering.
5. Underground Cables are non-inflammable.
6. Underground Cables remains unaffected by acids and alkalies present in the soil.