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Underground Cable

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The underground cable which is laid under the ground and consists of one or more conductors which is covered with different layer of insulation for protection purpose, that type of cable is called underground cable.

The different layers of underground 3 core cable are describing below:-


The Core is a conducting parts of a cable. This core may be one or more than one depending upon the type of service. A 3(three) core cable is shown in fig. The conductors are made of tinned copper or aluminum. The thickness of the core depends upon the rating of current and voltage rating.


Insulation of cable is a layer of insulating materials which is provided over the core (conductor ). The thickness of insulation depends upon the voltage to be withstood by the cable, Commonly Impregnated paper is used for insulation. And also varnish cambric or rubber mineral are used for insulation.

Metallic sheath:

A metallic sheath of lead or aluminum is provided over the insulation of cable for protecting the cable from moisture, gases or other liquids (like) acids or alkalies in the soil and atmosphere. The metallic sheath is shown in the figure.

Bedding :

Bedding is a layer in between metallic sheath and armoring. It consists of fibrous materials like jute or hessian tape. It protects the metallic sheath against corrosion and from mechanical injury due to armoring.

. Armouring:

Armoring is provided over the bedding which consists of one or two layers of galvanized steel wire or steel tape. It protects the core from mechanical injury which laying and also used as earthing purpose of the cable. Some cable has not provided armouring.


A layer of fibrous materials is provided over the armouring to protect the armouring. This layer is known as serving of the cable. Serving of cable is shown in Fig.

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Multiple Choice Question (MCQ) of Transmission and Distribution page-2:
11. In overhead transmission lines the effect of capacitance can be neglected when the length of line is less than
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1. Which of the following is not the transmission voltage in India ?
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B) 264 kV
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