Underground Distribution Lines

Q. For what purpose metallic sheath is provided in underground cables?

Ans. The metallic sheath is provided around the insulation to protect it against ingress of moisture, gas or other damaging liquids (acid or alkalies) from the soil and atmosphere.

Q. Why is a layer of bedding provided over the metallic sheath in underground cables?

Ans. A layer of bedding is provided over the metallic sheath for the protection of metallic sheath against corrosion and from mechanical injury.

Q. Why are copper conductors not used in power cables?

Ans. Copper conductors are not used in power cables because of the scarcity of copper.

Q. What are the main requirements of insulating materials used for underground cables?

Ans. The main requirements of the insulating materials used for underground cables are high insulation resistivity, high dielectric strength, good mechanical properties, non-hygroscopic, non-inflammable, immune to attacks by acids and alkalies.

Q Why single core cables are usually not provided with armouring?

Ans. Single core cables are usually not provided with armouring in order to avoid excessive loss in the armour.

Q. What is the factor, which is to be considered in the selection of a cable for a particular service?

Ans. The factors to be considered in the selection of a cable are continuous current rating, voltage grade, permissible voltage drop and short circuit rating (in a case of high tension cable only) and operating cost.

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