The general construction of an underground cable is given below:
The underground cable employed for transmission of power at high voltage consists of one central core or a number of cores (two, three or four) of tinned stranded copper or aluminium conductors insulated from each other by paper or varnished cambric or vulcanized bitumen or impregnated paper.
A metallic sheath of lead or alloy or of aluminium is provided around the insulation to protect it against ingress of moisture, gases or other damaging liquids (acid or alkalies) in the soil and atmosphere. A metal is essential for sheath because no organic material is sufficiently impervious to moisture.
The main advantages of lead sheaths are the comparative ease with which they are made in lead presses, their flexibility and high corrosion resistance while they have got drawbacks of large specific gravity, low mechanical strength, fluidity and small resistance to vibrations.
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The hardness, mechanical strength and resistance to vibrations of lead sheaths can be considerably improved by adding alloying mixtures (about 1 per cent of copper, tin, bismuth, etc.). Aluminium sheaths being cheaper (costing 3.5 times less than the lead sheaths) and having much greater mechanical strength than lead sheaths, low weight and low fluidity are now being increasingly employed.
For the protection of metallic sheath against corrosion and from mechanical injury a layer of bedding consisting of paper tape compounded with a fibrous material (such as jute, cotton or hessian tape) is provided over the metallic sheath. The compound used should be of such a nature that it does not react with the material of the armouring and the lead sheath but simultaneously it is adhesive enough so as to stick both to the lead sheath and the armouring. Bedding is employed in paper insulated lead covered cables. It is not required in PVC cables.
Over the layer of bedding armouring consisting of one or two layers of galvanized steel wire or steel tape is provided to save the cable from mechanical injury and over the armouring a layer of fibrous material similar to that of bedding, known as serving, is provided in order to protect the armouring.
Steel tape armour consists of two steel tapes coated with preservative compound, applied helically in the same direction over fibrous bedding, the outer layer covering the spaces between the turns of the inner layer. This provides sufficient protection against mechanical damage but steel wire armour is recommended where additional longitudinal stresses may occur during installation or in service. Double wire armours are employed for high degree of mechanical protection.
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Single core cables for ac systems are not provided with armouring because eddy currents induced in the steel armour causes additional power losses. In such cables mechanical protection is provided by plastic wrap. Aluminium sheathed cables, owing to comparative hardness of sheath, need not to be armoured in the same way as the lead sheathed cables but for special conditions armouring is provided for these cables too.