1. A cable is basically an insulated conductor, externally protected against mechanical injury, moisture entry and chemical reaction.

Construction of Cables

  1. Cores or conductors:- conductor are made of tinned conductor or aluminium.
  2. Insulation:– materials are insulations for impregnated paper, varnished cambric or rubber mineral compound.
  3. Metallic sheath: – a metallic sheath of lead or aluminium is provided over the insulation.
  4. Bedding:– fibrous material like jute or hessian tape is used as a bedding.
  5. Armouring:– it consists of one or two layers of galvanized steel wire or steel tape.
  6. Serving:– a layer of fibrous material (like jute) similar to bedding is provided over the armouring. This is known as serving.

Insulating materials for cables

  1. Rubber
  2. Vulcanised India Rubber (V.I.R.)
  3. Impregnated
  4. Varnished
  5. Polyvinyl chloride (PVC)

Classification of cables

Low-tension (L.T.) cables

upto 1000V

High-tension (H.T.) cables

upto 11,000V or 11KV

Super-tension (S.T.) cables

from  22KV to 33KV

Extra high-tension (E.H.T.) cables

from 33KV to 66KV

Extra super voltage cables

beyond 132KV

 

Cables for 3-phase service

  1. Belted cables – upto 11KV
  2. Screened cables – from 22KV to 66KV
  3. Pressure cables – beyond 66KV

Laying of underground cables

  1. Direct laying
  2. Draw-in-system
  3. Solid system

Grading of Cables

  1. Capacitance grading
  2. Intersheath grading

Types of cable faults

  1. Open-circuit fault:- when there is break in the conductor of a cable, it is called open circuit fault.
  2. Short-circuit fault:- when two conductors of a multi-core cable come in electrical contact with each other due to insulation failure.
  3. Earth fault:- when the conductor of a cable comes in contact with earth, is called earth fault or ground fault.

Loop test for Location of faults in Underground cables

  1. Murray Loop test
  2. Varley Loop test