Electric power can be transmitted or distributed either by overhead system or by underground electrical cables. The underground cables have several advantages such as:

1.    Less liable to damage through storms or lightning
2.    Low maintenance cost
3.    Fewer chances of faults
4.    Smaller voltage drop
5.    Better general appearance

Such locations may be thickly populated areas where municipal authorities prohibit overhead lines for reasons of safety, or around plants and substations or where maintenance conditions do not permit the use of overhead construction so we have to use underground cables in these occasions. Cables for underground service may be classified in two ways according to;

1.    The type of insulating material used in their manufacture
2.    The voltage for which they are manufactured.

 However, the latter method of classification is generally preferred, according to which cables can be divided into the following groups:

1.    Low-tension (L.T.) cables — up to 1000 V
2.    High-tension (H.T.) cables — up to 11,000 V
3.    Super-tension (S.T.) cables — from 22 kV to 33 kV
4.    Extra high-tension (Fill cables — from 33 kV to 66 kV
5.    Extra super voltage cables — beyond 132 kV

A cable may have one or more than one core depending upon the type of service for which it is intended. It may be ;

1.    Single-core
2.    Two-core
3.    Three-core
4.    Four-core

Underground cable which are used is depending upon the
1.    Operating voltage
2.    Load demand.
3.    Type of service