An underground cable essentially consists 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 cable to be used will depend upon theworking voltage and service requirements. In general, a cable must fulfil the following necessary requirements:
(I)The conductor used in cables should be tinned stranded copper or aluminium of high conductivity.Stranding is done so that conductor may become flexible and carry more current.
(ii)The conductor size should be such that the cable carries the desired load current withoutoverheating and causes voltage drop within permissible limits.
(iii)The cable must have proper thickness of insulation in order to give high degree of safety andreliability at the voltage for which it is designed.
(iv)The cable must be provided with suitable mechanical protection so that it may withstand therough use in laying it.
(v)The materials used in the manufacture of cables should be such that there is complete chemical and physical stability throughout.
Construction Of Cables
Fig (1) shows the general construction of a 3-conductor cable.
The various parts are:
(i) Cores or Conductors: – A cable may have one or more than one core (conductor) dependingupon the type of service for which it is intended. For instance, the 3-conductor cable shownin Fig is used for 3-phase service. The conductors are made of tinned copper or aluminiumand are usually stranded in order to provide flexibility to the cable.
(ii) Insulation: – Each core or conductor is provided with a suitable thickness of insulation, thethickness of layer depending upon the voltage to be withstood by the cable. The commonlyused materials for insulation are impregnated paper, varnished cambric or rubber mineralcompound.
(iii) Metallic sheath: -In order to protect the cable from moisture,gases or other damaging liquids(acids or alkalies) in the soil and atmosphere, a metallic sheath of lead or aluminium is provided over the insulation as shown in Fig.
(iv) Bedding: – Over the metallic sheath is applied a layer of bedding which consists of a fibrous material like jute or hessian tape. The purpose of bedding is to protect the metallic sheath against corrosion and from mechanical injury due to armouring.
(v) Armouring: -Over the bedding, armouring is provided which consists of one or two layers ofgalvanised steel wire or steel tape. Its purpose is to protect the cable from mechanical injurywhile laying it and during the course of handling. Armouring may not be done in the case ofsome cables.
(vi) Serving: – In order to protect armouring from atmospheric conditions, a layer of fibrousmaterial (like jute) similar to bedding is provided over the armouring.This is known as serving.
- “Power system” by V.K. Mehta.