METALLIC SHEATHING IN POWER CABLE TECHNOLOGY

Metallic Sheathing In Power Cable Technology - metallic sheating is provided over the paper isulation and should have intimate contact with the external layer of insulation. the metallic layer provided must be electrically contionous. thus, the sheath used not only provide waterproof protection, but also provides an electrical earth shield the potential of which is zero and the potential of the conductor is reffered to this.

Metallic Sheathing In Power Cable Technology

the metallic sheath used may be either of lead or lead alloy or of aluminium.

Lead and lead alloy

are the mostly used materials as the facilitate the manufacturing of cables and is pliable. also lead is generally immune to corrosion except under secial conditions (by electrolytic action and organic acids). however, lead may crack due to inter-crystalline corrosian and vibration. it has been observaed that the cables laid underground where heavy traffic passes and near the railway lines, bridges etc. the cables fail due to such cracks.

Aluminium Sheath

In recent years aluminium has also been used a sheath, since it decreases the weight of the cable (lead is about four times heavier than aluminium) and its conductivity is about seven times that of lead which make such cable much adaptable for mining works. It may be pointed out that the sheath loss increases due to increases in sheath current. the current rating for the multicored aluminium sheathed cables is the same as that for lead-sheathed cables for the same conductor temperature but the single core aluminium sheathed cables used on a.c system are derated due to increase in sheath current.

The following are the advantages of the aluminium sheathed cables over the lead sheathed cables:

• Increase in the tensile strenght.
• The Weight  is decreased which facilitates in handling of the cable in its installation.
• since the cable is of light weight, so the fixtures will also be light.
• The cable is free of cracking due to vibration etc.
• It improves the mechanical properties of the cable and reduces the need of heavy armouring.

Sometimes with lead sheathed cables an earth continouly conductor is run which is in good contact with the sheath throught its length to ensure adequate earthing.

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