Underground Cables
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.
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