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Tuesday, 1 October 2019

Timber and Wood Insulating Materials

Timber Insulation:

This is a natural forest product and is quite abundantly available. It is cheap compared to many other materials. It is not durable as steel. Some useful varieties of timbers are teak, mahogany, rose wood. Well seasoned and impregnated with materials like paraffin wax, creosote varnish and paint, wood is used for engineering purposes. They are used for rural electrification to minimize the cost and for transmitting small amount of power at low voltage i.e., 220 / 440 volts. They can be erected and replaced easily and does not require skilled labour. Their maintenance is easy and they take transverse loads. The span is kept upto 85 metres in case of single poles and upto 160 metres in case of A and H type wood poles. They are made of seasoned wood of salon chair. The wooden poles are impregnated with creosote oil upto a height of two feet from the bottom. Such poles can be used upto a line voltages of 11 kV. The advantages of wooden poles are their cheapness, its insulation properties, less weight and easiness in their erection. Their disadvantages are short life (about 20 to 25 years) bad appearance, need for regular inspection and limitation of their use upto 11 kV line voltages. They are embedded in the ground to a depth of about 1/5 of the total height. Those poles are covered under IS 876 of 1961. The permissible stresses for pine and hard wood are given in Table.

Permissible Stresses of Pine and Hard Wood

Permissible stress
For pine
Hard wood
Tension or bending
145 kgf / cm2
190 kgf / cm2
Pressure in the direction of Fibre
110 kgf / cm2
120 kgf / cm2
Pressure at right angle to the director of fibre
35 kgf / cm2
50 kgf cm2
Shearing in the direction of Fibre
18 kgf / cm2
20 kgf / cm2
Shearing at right angle to the direction of Fibre
30 kgf / cm2
40 kgf / cm2
Round tension flexure strength
500 kgf / cm2
800 kgf / cm2

Wood Insulation: 
It is frequently used for low voltage installations. It is light in weight having density 0.5 to 1.0 gm / cm2. Depending on the type of wood, its tensile strength varies from 700 to 1300 kg / cm along the grain. However, across the grain the tensile strength is low. Wood is very hygroscopic and after absorbing moisture tends to loose its mechanical properties. Temperature to which it can be worked is limited. Because of the above limitations wood is rarely used. But if impregnated with oil, it can be used as an insulating material for making terminal boxes, boards, casing and capping, batteries and round blocks, electrical poles, slot wedges in motors and generators, instrument and equipment covers, separators in accumulators, sealing of H.T. and L.T. winding in transformers etc. Wood is cheap and can easily be fabricated.

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