Underground cable


An underground cable essentially consists of conductors covered with suitable insulation and surrounded by a protecting cover.

 There are several types of cables are available, the type of cable to be used will depend upon the working voltage and service requirements. Some important points about cable
  1. The cable must have proper thickness of insulation in order to give high degree of safety and reliability at the voltage for which it is designed.
  2. The conductor size should be such that the cable carries the desired load current without overheating.
  3.    The conductor used in cables should be tinned stranded copper or aluminium of high conductivity to become flexible and carry more current.
  4. The cable must be provided with suitable mechanical protection so that it may withstand the rough use in laying it.
  5.   The materials used in the manufacture of cables should be such that there is complete chemical and physical stability throughout  Construction of Cables


Construction of a 3-conductor cable

  •     Cores or Conductors. A cable may have one or more than one core (conductor) depending upon the use for which it is intended. For instance, the 3-conductor cable is used for 3-phase service.
  •     Insulatian. Each core or conductor is provided with a suitable thickness of insulation, the thickness of layer depending upon the voltage to be withstood by the cable. The commonly used materials for insulation are impregnated paper, varnished cambric or rubber mineral.
  •     Metallic sheath. In order to protect the cable from moisture, gases or other damaging liquids like  acids or alkalies in the soil and atmosphere, a metallic sheath of lead or aluminium is provided over the insulation.
  •     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.
  •     Armouring. Over the bedding, armouring is provided which consists of one or two layers of galvanised steel wire or steel tape to protect the cable from mechanical injury while laying it. Armouring may not be done in the case of some cables.
  •     Serving. To protect the armouring from the atmospheric conditions, a layer of fiber as same as bedding used.

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