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.
