Question

Draw and explain the construction of underground cables. 

Answer 1

Underground cables: 

 OR

Explanation:-

i) Core or conductor:

* It function is carry current.

* Cable may have single or more than single core conductor.

* Conductor are made up of copper or aluminium material

* Cross section of conductor is directly proportional to current. (Cross section of

conductor depends upon current carrying capacity)

* Conductor used is -

◘Annealed

◘Tinned

 ii) Insulation:

* Each core of conductor is provided with suitable thickness of insulation to avoid

short circuit between two conductors.

* The thickness of insulation layer depends on magnitude of voltage for which it

is designed.

* Commonly used materials for insulation are e.g.:-

◘PVC (Polyvinyl Chloride)

◘Polyethene

◘XLPE (Cross- linked polyethylene)

 iii) Lead (Metallic) Sheath:

* It is provided over insulation.

* To provide the protection of core from entry of moisture, gases or other

damaging liquids (acids & alkaline) in the soil & atmospheric.

* The metallic sheath is made up of lead or lead alloys recently aluminum is also

being used as a metallic sheath.

iv) Bedding:

* Over the metallic sheath there is layer of bedding.

* The function of bedding is protecting the metallic sheath against corrosion & from the mechanical injury due to armouring.

* It is made from fibrous material such as jute, hessian tape

 v) Armouring:

* This layer is over a bedding only for underground cable and not for over head

cable

* Its function is to protect the cable from mechanical injury.

* It covers the bedding, which consists of 1 or 2 layers of galvanized steel wire or

steel tapes

 vi) Serving:

* This layer is last layer which comes over armouring.

* Its function is to protect armouring against rusting and it also helps for easy

handling of cables.

* It is similar to bedding & consists of fiborous material such as jute.