1.1 Introduction
       The transmission and distribution of an electrical power can be with the help of overhead transmission lines or by underground cables. It has been mentioned that in thickly populated are like towns and cities, the use of overhead lines is not practicable. In such cases electrical energy is transmitted and distributed with the help of underground cables. In its basic form, an underground cable is a conductor provided with proper insulation. As the voltage level increases, the cost of the insulation increases rapidly and thus the use of underground cables is restricted to low and medium voltage distribution.

1.2 Comparison of Underground cables and Overhead lines
       Compared to overhead lines, the underground cables have the following advantages,
1. It ensures non-interrupted continuity of supply. The possible supply interruptions due to lighting, storms and other weather conditions are eliminated because of underground cables.
2. It requires less maintenance.
3. The accidents caused due to breakage of overhead line conductors are eliminated due to use of underground cables.
4. The voltage drop in the underground cables is less.
5. The life of underground cables is long compared to overhead lines.
6- The beauty of cities and town get maintained due to underground network of cables.
7- The overhead lines use bars conductors which is unsafe in thickly populated area. Hence from safety point of view, the underground cables are more advantageous.
       The only drawbacks of underground cables are the extremely high initial cost and insulation problems at high voltages. In India, the big cities have adopted the system of underground cables for the transmission and distribution.
       Thus the use of underground cables is mainly for the distribution of an electrical power at low and medium voltages. Its use is almost compulsory at the location where use of overhead lines is not practicable due to the safety reasons such as congested urban area, crossing of wide roads, near gas plants and refineries, near substation etc.
       Still the overhead lines also have some advantages compared to the underground cables which are,
1- Long distance transmission is possibly by the overhead lines.
2- The conductors in overhead lines is less expensive.
3- The size of the conductor in overhead lines is less than underground cables due to good heat dissipation in overhead lines.
4- The insulation cost is very less as the air itself acts as an insulation between the conductors. The gas or oil is not required for overhead lines. For high voltage levels, spacing in air can be easily adjusted in case of overhead lines to obtain proper insulation.
5- The erection cost is much less for the overhead lines. The underground cable laying is difficult and complicated.
       Depending upon the situation and the requirements, the underground cables or overhead transmission system can be used.


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