Economy of using concrete transmission line poles

Prof I Tomba Singh *

With the increasing tempo of industrialisation coupled with the demand of rural electrification, the task of expanding the network of electric transmission line grids has been given priority to all the States and Union Territories in India. Rural electrification activities are yet to be extended to a number of villages in rural and hill areas in Manipur.

For power transmission purposes especially for 33/11 KV transmission systems, wooden poles, steel poles and pre-stressed concrete poles are generally used. In most of the States, Concrete poles are used extensively for transmission of HT and LT lines in view of its economy, of less maintenance cost, durability and other properties.

Prestressed concrete transmission line poles are replacing the steel and wooden poles because of its manifold advantages such as;

1. Concrete poles are fire proof and not affected by chemical reactions and soil.
2. Concrete strength does not reduce drastically with age and life is longer than steel poles
3. Effect of electric current on animals and plants through concrete poles are negligible in
4. No further cost for replacement of concrete poles will be needed
5. No extra cost for encasing below the ground around the concrete poles is needed, unlike wooden and steel poles that need strengthening with cement concrete around the poles to protect erosion from soil and chemical reactions.

The concrete poles are classified into 11 (eleven) classes under the ISI specifications depending upon the length of the poles ranging from 6 to 17 metres, conforming to minimum transverse load. Maximum overall length (metres) ranging from 7.5 metre to 17.0 metre varies the minimum ultimate transverse load of 200 kg to 3000 kg.

The minimum depth of planting of poles below ground level, in accordance with the direction of experts based on soil conditions varies from 1.20 metres to 2.40 metres for the poles having the length of 6 metres to 17.0 metres. Concrete poles are of two types (i) Prestressed concrete poles (ii) Reinforced concrete poles.

The advantage of prestressed concrete poles over the ordinary reinforced concrete poles are as follows;
1. Prestressed concrete poles are much lighter, being more slender in section and therefore easier to transport
2. Due to the use of high tensile steel, there is saving in steel consumption.
3. Due to the use of a more slender section, there is saving in concrete.
4. As the concrete always remains in compression in the pre-stressed concrete poles and therefore more durable.

Normally, the poles are so designed not to fall owing to failure of concrete compression. Planting direct to the ground or planning in the ground with a kicking block are done depending upon the soil conditions. The poles are so designed that when it is vertical, its strength in transverse direction shall be sufficient to take the horizontal load due to wind on wires.

As prescribed by Experts, poles used for power transmission lines, the strength of the poles in the direction of the line shall not be less than one-quarter of the strength required in the transverse direction. Normally, sizes of concrete poles of 7.5 metres, 8.5 metres and 9.75 metres are commonly used in rural electrification purposes of 33/11 KV system.

Two methods known as 'Long line method' and 'Unit mould method' are adopted in production of prestressed concrete poles. Higher tensile steel wires are used and tensile initially in a platform known as 'Pre-tensioning bed'. The poles are then moulded using concrete and vibrators. After the concrete is sufficiently hardened and attained certain strength, the wires are released.

The period between the time of casting concrete and the time of releasing wires known as 'Stress transfer period' should be carefully checked up before starting new casting cycles'. The same unit can be used for manufacturing a wide range of products like bridge beams upto 35-40 ft span, floor beams, purling, slender columns, prestressed planks, fencing posts, railway sleepers.

The basic raw materials needed for the plant are concrete, high tensile wires, sand, stone chips. Machineries required for the plant will include concrete mixture, vibrators, moulds, testing equipment, anchor grips, jacked, pre-tensioning bed etc.

Commissioning a plant to manufacture such poles, the state Electricity Companies particularly the Manipur State Power Distribution Company Ltd. (MSPDCL) will be benefited in various aspects. Lack of proper data on the month/year of installation, strengthening of the poles with encasting concrete around the poles, effective study on the cost benefit analysis cannot be taken up.

Proper marking of the month/year of installation of poles, strengthening activities of the poles used will enable the researchers or Engineers to study the economy of different types of poles depending upon different soil conditions. In view of its importance, all kinds of railway sleepers are marked with month/year of installation and also in electric or communication poles in other States.

Introduction of prestressed concrete transmission line poles in intervals of steel or wooden poles may be considered to enable the Engineers to study, compare their strength, cost-benefit analysis and other effects in the near future.

A unit for manufacturing prestressed concrete poles in ISI specifications will not only provide sufficient return on capital employed, provide employment opportunities to local educated persons but also will contribute to the state economy.