Underground cables are the electric power transmission cables. Because of their reliability on transmitting, they used in congested urban areas and in thick population areas, where overhead transmission is dangerous. Underground cables have low maintenance cost, less chances of faults, smaller voltage drop. In recent improvements in the design and manufacture have led to development of cables suitable for use at high voltages. Electric power can be transmitted or distributed either by overhead system or by underground cables. The underground cables have several advantages then overhead system, so we use underground cables A cable may have one or three core (Conductor) depending upon the type of service for which it is intended. For instance, the three conductor cable is used for 3-phase service. The conductors are made of tinned copper or aluminium and are usually stranded in order to provide flexibility to the cable.
These cables are used for voltages up to 11kv but in extraordinary cases, their use may be extended up to 22kv. The cores are insulated from each other by layers of impregnated paper. Another layer of impregnated paper tape, called paper belt is wound round the grouped insulated cores. The gap between the insulated cores is filled with fibrous insulating material so as to give circular cross-section to the cable. The belt is covered with lead sheath to protect the cable against ingress of moisture and mechanical injury.
CORE/CONDUCTORS : A cable may have one or three core (Conductor) depending upon the type of service for which it is intended. For instance, the three conductor cable is used for 3-phase service. The conductors are made of tinned copper or aluminium and are usually stranded in order to provide flexibility to the cable.
INSULATION : Each core or conductor is provided with a suitable thickness of insulation, the thickness of layer depending upon the voltage to be withstood by the cable. The commonly used materials for insulation are impregnated paper, varnished cambric or rubber material compound.
METALLIC SHEATH : In order to protect the cable from moisture, gases or other damaging liquids in the soil & atmosphere, a metallic sheath of lead or aluminium is provided over the insulation.
ARMOURING : Armouring is provided which consists of one or two layers of galvanised steel wire or steel tape. It purpose is to protect the cable from mechanical injury while laying it and during the course of handling. Armouring may not be done in the case of some cables.
SERVING : In order to protect armouring from atmospheric conditions, a layer of fibrous material is provided over the armouring. This is known as serving. It may not be out of place to mention here that armouring and serving are only applied to the cables for the protection of conductor insulation and to protect the metallic sheath from mechanical injury.
RUBBER : Rubber may be obtained from milky sap of tropical trees or it may be produced from oil products. It has relative permittivity varying between 2 and 3, dielectric strength is about 30kv/mm and resistivity of insulation is 10^17 Ω cm. Although pure rubber has reasonable high insulating properties, is suffers from some major drawbacks viz., readily absorbs moisture, maximum safe temperature is low (about 38̊ C), soft and liable to damage due to rough handling and ages when exposed to light. Pure rubber cannot be used as insulating material.
IMPREGNATED PAPER : It consists of chemically pulped paper made from wood chippings and impregnated with some compound such as paraffinic or napthenic material. This type of insulation has almost superseded the rubber insulation. It is because it has the advantages of low cost, low capacitance, high dielectric strength and high insulation resistance. The only disadvantage is that paper is hygroscopic and even if it is impregnated with suitable compound, it absorbs moisture and thus lowers the insulation resistance of the cable. For this reason, paper insulated cables are always provided with some protective covering and are never left unsealed. If it is required to be left unused on the site during laying, its ends are temporarily covered with wax or tar. It absorbs moisture so it provided with protective covering & does not left unsealed. It was high dielectric strength, high insulation resistance.
VARNISHED CAMBRIC : It is a cotton cloth impregnated & coated with varnish. This type of insulation is also known as empire tape. The cambric is lapped on to the conductor in the form of a tape and its surfaces are coated with petroleum jelly compound to allow for the sliding of one turn over another as the cable is bent. As the varnished cambric is hygroscopic, therefore, such cables are always provided with metallic sheath. Its dielectric strength is about 4kv/mm and permittivity is 2.5 to 3.8.