Supports - Overhead Lines
A support is a column of concrete, wood, steel or a few other material supporting overhead conductors through means of arms or brackets. The supports used for overhead line construction vary in design and the reasons they have to perform. The different kinds of supports for overhead lines are: concrete poles, wood poles, steel poles and lattice type towers.
1. Wood poles: Chemically treated wood poles are used for distribution lines. The benefits of using wood poles are in which they are low in cost. Therefore, they are susceptible to decay. The specification for wood poles are covered by IS: 876 and IS: 5978. According to this standard, the timber appropriate for poles has been categorized into three groups depending upon its strength. For example, IS 6056 for jointed wood poles for overhead lines specifies that sal, deodar, chir, kail, wood be used. Jointed wood poles along with wire bound lap joint are considerably less expensive and found to be extremely appropriate for LT and HT lines in rural areas.
2. Concrete poles: Concrete poles are more expensive than wood poles but cheaper than steel tubular poles. Concrete poles are of three types:
- Pre-cast cement concrete poles (PCC) made of cement concrete;
- Reinforced cement concrete poles (RCC);
- Pre-stressed cement concrete poles (PSCC).
The low maintenance, competitive price and aesthetic appearance of PCC poles makes them superior to wood or steel for use in electric lines. Ease and speed of installation means faster project completion and lower installation cost. RCC poles have a very long life and required little maintenance but they are bulky in size and comparatively heavy. They have shattering tendency while hit through a vehicle. PSCC poles take care of these shortcomings to some extent. Therefore, the handling, transportation and erection of these poles are more difficult since of their heavy weight.
3. Steel poles: The steel poles are of the following categories:
- Steel tubular poles whose specifications are covered through IS: 2713-1967. Because of their light weight, high strength to weight ratio and long life, they possess distinct benefits over other types of poles. The use of a pole cap at the top, concrete muff in the ground and regular coating of paint prolongs their life.
- Old and second hand rails and Rolled Steel (RS) joists are often used as supports for overhead lines. The portion embedded within the ground should be protected through concrete muff and the remaining portion through regular paint unless galvanised steel is used.
4. Lattice type supports: These are fabricated from narrow base steel structures. They are light in weight and economical and could be assembled at site if bolted construction is used. Generally both welded and bolted kinds are used.