Electric Poles

Overhead lines run on wooden, concrete, steel and rail poles. The choice of pole depends on load importance, location, cost and maintenance. Low-voltage lines typically carry all phases on a single pole. The main types of poles in the electrical system are:

1. Wooden Electric Pole
2. Concrete Electric Pole
3. Steel Tubular Electric Pole
4. Rail Electric Pole

Wooden Electric Pole

Wooden poles were once widely used on 400 V and 230 V low-tension (L.T.) lines and on 11 kV high-tension (H.T.) lines, occasionally even on 33 kV lines. They are cheap to buy and install, and with proper maintenance and treatment they last a long time.

For these reasons, wooden poles were widely used in the past. Shaal wood was the most common choice, with an average weight of 815 kg per cubic meter. Masua, Tik, Chir and Debdaru were also used, depending on availability. Today, to protect forests and maintain ecological balance, wooden pole use has nearly stopped. Wooden poles fall into three categories by their load-bearing capacity.

1. Breaking strength above 850 Kg/cm²: for example, Shaal and Masua wood.
2. Breaking strength between 630 Kg/cm² and 850 Kg/cm²: for example, Tik, Seishun and Garjan wood.
3. Breaking strength between 450 Kg/cm² and 630 Kg/cm²: for example, Chir, Debdaru and Arjun wood.

Pole wood must be free from defects, and straight timber is strongly preferred. Fully straight, defect-free timber of the required length is rare, so slightly curved wood is acceptable. If needed, two shorter lengths can be joined together.

Treatment of Wooden Pole

Seasoning comes first: the wood must be dried properly. Fungus damages wood, termites do the worst damage, and heat and moisture decay it. This damage concentrates in the portion of the pole at and below ground level. Chemical treatment protects the wood against moisture and termites: tar mixed with Creojet Oil, or copper chrome arsenic (CCA), is applied. The next treatment is the Ascu (CCA pressure) treatment. The poles are placed in a cylindrical airtight tank and immersed in the CCA chemical, and a hydraulic pressure of about 10 kg per square centimetre is applied. That pressure drives the chemical deep into the pores of the wood, so moisture and termites cannot attack it for a long time.

If the wood was not properly treated, apply two coats of Creojet oil over the entire pole surface before erection. Use bituminous Creojet Oil on the buried portion and up to 50 cm or 20 inches above the soil. Where that is not possible, at least apply tar to those surfaces. If no chemical treatment is feasible, char the bottom outer surface of the pole for up to two meters to resist termites and moisture.

The pole top is cut into a sharp cone so that water cannot collect there. Grooves are then cut in the upper portion to fit the cross-arms tightly, and holes are drilled for the same purpose, with diameters from 17 mm to 20 mm. For a D-shaped iron clamp, grooves are unnecessary: a drilled hole at the required position is sufficient. The top hole should sit at least 200 mm or 8 inches below the pole tip. Cut every hole and groove before treatment, and avoid cutting them afterwards. If holes or grooves must be made after treatment, coat them with creosote oil or bitumen.

Concrete Electric Pole

There are two types of concrete poles:

1. R.C.C. Poles
2. P.C.C. Poles

P.C.C. poles now serve 11 kV and 400/230 V systems on a large scale, and they also appear on 33 kV H.T. lines. They cost more than wooden poles but less than steel poles. They last longer and need negligible maintenance. Their strength sits well above a wooden pole but below a steel pole. Their disadvantages are heavy weight and brittleness.

Concrete electric poles are made from cement concrete with iron bars or rods for reinforcement to increase strength. For earthing, a copper strip (25mm × 3mm) is placed inside the pole during concreting, or a hollow channel is included for the earthing wire. The poles also have 20 mm diameter holes for fixing different fittings.

The cross-section of the pole is always larger at the bottom than at the top. PCC poles have a rectangular cross-section, not square.

By lateral load capacity and height, concrete poles are divided into 11 classes

Note: the excavation depth also depends on soil quality and varies from place to place.

Steel Tubular Electric Pole

Steel tubular poles carry far more load than wooden or concrete poles, and with proper maintenance they last a long time. Because of their high cost, though, their use is gradually declining. These poles serve 400/230 V low and medium current systems and 11 kV H.T. lines, and in some cases higher voltage systems such as 33 kV.

Types of Tubular Poles

There are two types of tubular poles:

1. Stepped Pole
2. Swaged Pole

The 1^st type is called ISTP and the 2^nd ISWP. Either type may be made from a single tube or from several tubes welded together. A tubular pole should be straight, clean, rust-free and defect-free, and it comes in different sizes for different load capacities. H.T. lines use a thicker gauge than L.T. lines. The inside and outside are coated with bitumen up to the length that will be grouted under the soil; the rest is painted with red oxide. A 14 mm diameter earthing hole is made 300 mm above ground level, and a pole cap on top keeps rainwater out.

The pole safety factor is 2.5, and the weight of steel is taken as 7.85 grams per cubic centimetre.

Rail Electric Pole

Rail poles are the strongest and the costliest. Their high weight also raises transport, loading and unloading costs. They are not used on 400/230 V lines, but they serve 11 kV and 33 kV systems. Rail poles used in overhead lines generally come in four sizes.

1. 30 kg per metre
2. 37 kg per metre
3. 45 kg per metre
4. 52 kg per metre

An 11 kV line normally uses the 45 kg per metre rail pole, while 33 kV lines use the 45 and 52 kg per metre sizes. The different sizes run from 9 to 13 meters long. Before erection, the rail pole receives at least one coat of red oxide; a tar coating on the bottom portion, up to a set height above ground, increases its durability. A rail pole carries vertical load better than horizontal load.