Under ground cables

 Electric power can be transmitted or distributed either by overhead system or by underground cables. The underground cables have several advantages such as less liable to damage though storm or lightning, low maintenance cost, less chances of fault, smaller voltage drop and better generation appearance. However, their major drawback is that they have greater installations cost and introduced insulation problem at high voltage compared with equivalent overhead system. For this reason, underground cables are employed where it is impractiable to use overhead lines. Such location may be thickly populated areas where 'municipal authorities prohibited over head lines for the reason of safety, or under plants and substation or where' maintenance condition do not permit the use of overhead construction.

 

The chief use of underground cables for many years has been for distribution of electric power in congested urban areas at comparatively low or moderate voltage s. However, recent improvements in the design and manufacture have led to development of cables suitable for use at high voltages. This has Mada it possible to employ under ground cables for transmission of electrical power for short or moderate distance. In this chapter, we shall focus our attention on the various aspect of underground cables and their increasing use in power system.

An underground cables essentially consist of one or 'more conductors covered with suitable insulation and surrounded by a protecting cover. 

Although several types of cables are available, the type of cables to be used will depend upon the working voltage and service requirement. In general, a cables' must fulfil the following necessary requirements.

(1). The conductor used in cables to should be tinned stranded copper or aluminum of high conductivity. Stranding is done so that conductor may become flexible and carry more current.

(2).the conductor should be such that the cable carries the desired load current without without overheating and causes voltage drop within permissible limits. 

(3)The cables must have proper thickness of insulation in order to give high degree of safety and reliability at the voltage for which it is designed.

(4).the cables must be provided with suitable mechanical protection so that it will be may withstand the rough use in laying it.

(5).The 'material used in the manufacture of cables should be such that there is complete chemical and physical stability through out.

Construction of Cables. 

Shows the general construction of conductor cables. The various parts are :

(1) Core of Conductors. The A Cables may have one or more than one core (conductor) 7 depending upon the type of service for which it is intended. For instance, the 3conductor of the cables shown. The conductors are made of tinned copper or aluminum and are usually stranded in order to provide flexibility to cable.

(2). Insulation. Each core or conductor is provided with a suitable thickness of insulation, the thickness of layer depending upon the voltage be withstood by the cables,. The commonly used material for insulation are impregnated paper, the varnished cambric or rubber mineral compound. 

(3) Metallic sheath.In order to protect the cables from moisture the gases or damaging liquids (acid or alkalies) of in the soil and atmosphere a metallic sheath of lead or aluminum is provided over the insulation. 

(4).Bedding. Over the metallic sheath is applied a layer of bedding which consist of a fibrous material like jute or hessian sheath tape. The purpose of bedding is to protect the metallic shealth against corossion and from mechanical injury due to armouring.

(5).Armouring. Over the bedding, armouring is provided which consist of one are two layers of galvanized steel wire or steel tape. It's purpose is to protect the cable from mechanical injury while laying it and during the course of handling. Armouring may not be done in the case of some cables.

(6).Serving. In order to protect armouring from atmospheric condition, a layer of fibrous materials like jute similar to bedding is provided over the armouring. This is known as serving. 

It may not be out of place to mention here that bedding, armouring and serving are only applied to cables for the protection of conductor insulation and to protect the metallic sheath from mechanical injury.