1. Wooden Poles
These are made of seasoned wood (sal or chair) and are suitable for lines of the moderate cross-sectional area and of relatively shorter spans, say up to 60 meters. Such supports
are cheap, easily available, provide insulating properties and, therefore, are widely used for distribution purposes in rural areas as an economic proposition. The wooden poles
generally tend to rot below the ground level, causing foundation failure. In order to prevent this, the portion of the pole below the ground level is impregnated with preservative compounds like creosote oil. Double pole structures of the ‘A’ or H’ type are often used to obtain a higher transverse strength that could be economically provided by means of single poles.
The main disadvantages of wooden poles are
(i) The tendency to rot below the ground level
(ii) Comparatively smaller life (20 25 years)
(ii) Cannot be used for voltages higher than 20 kV
(iv) Less mechanical strength.
(v) Require periodical inspection.
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2. Steel poles
The steel poles are often used as a substitute for wooden poles. The steel poles have the great mechanical strength that’s why it uses for longer spans (approximate 50 to 80
meters). These are costlier and the steel poles required the foundation of concrete and paint to protect them from a chemical reaction. These poles are generally used for
distribution purpose in urban areas. The steel poles may be used for the distribution voltage up to 33 kV. Steel poles are heavier as compared to wooden poles. The average
life of steel poles is 40 years.
The steel poles are of three types:
(a) Rail poles
(b) Tubular poles
(c) Rolled steel joints.
3. RCC Poles (Reinforced Cement Concrete)
The reinforced concrete poles have become very popular as line supports in recent years reused for system up to 33 kV, and have greater mechanical strength, longer life permit longer spans (80-200 m) than steel poles. Moreover, they give a good outlook, and require little maintenance and have good insulating properties.
The figure shows R.C.C. poles for the single and double circuit. The holes in the poles facilitate the climbing of poles and at the same time reduce the weight of line supports
The main difficulty with the use of these poles is the high cost of transport owing to their heavyweight. Therefore, such poles are often manufactured at the site in order to avoid
the heavy cost of transportation.
4. Steel towers
In practice, wooden, steel and reinforced concrete poles are used for distribution purposes at low voltages. However, for long-distance transmission at a higher voltage (66 kV or above), steel towers are invariably employed. Steel towers have greater mechanical strength, longer life, can withstand most severe climatic conditions and permit the use of longer spans (300 m or above). The risk of interrupted service due to broken or punctured insulation is considerably reduced owing to longer spans
Tower footings arc usually grounded by driving rods into the earth. This minimizes the lightning troubles as each tower acts as a lightning conductor. These towers are used for transmitting huge power at high voltages levels, i.e., 132 kV, 220 kV, 400 kV, 765 kV, etc. The figure shows a single circuit tower. However, at a moderate additional cost, double circuit tower can be provided as shown in the figure. The double circuit has the advantage that it ensures continuity of supply. In case there is a breakdown of one circuit, the continuity of supply can be maintained by the other circuit.
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