WOODEN TRANSMISSION LINE POLES BASICS AND TUTORIALS

Wood poles are considerably cheaper than steel for many types of construction. The lower cost is due, in part, to the more conservative basis of design normally adopted for steel.

Generally, steel structures are designed to support safely one or more broken conductors, whereas wood structures are often not so designed. It is logical that the reasons for choosing the more expensive steel construction should require conservative design throughout and that conditions justifying the cheaper and shorter-lived wood structures would warrant accepting some of the more theoretical hazards.

For voltages of 69 kV and lower, wood is quite generally used. Wood-pole construction for many years has been used for all voltages up to and including 345 kV. H frames with various modifications have been designed, the most popular using the main crossarm as the bottom member of a truss.

Butt-treated cedar and full-treated pine are used almost exclusively in transmission-line construction; the use of untreated poles has been practically abandoned as uneconomical since the supply of chestnut and northern cedar poles has been exhausted. Treated fir has also been supplied in some quantity from the Northwest.

Cedar poles resist decay, but satisfactory life is not secured unless the butt is treated. The pole is usually treated from the butt to about 2 ft above the ground line.

The balance of the pole is not treated. Pine and fir require complete treatment of practically all the sapwood. This treatment is applied under pressure.

No universally effective protection has been devised against woodpecker damage. Some localities are often subject to serious epidemics of woodpecker trouble.

Preservative Treatment. 
Pole decay is due to a fungus which requires air, moisture, warmth, and food for its subsistence; the wood o the pole constitutes its food. The conditions most favorable to the growth of the fungus are found at the ground line.

The preservative has toxic or antiseptic properties which make the wood either poisonous or unfit food for the fungus. Preservatives and preserving methods conforming to the standards of the American Wood Preservers Association (AWPA)85 should be used in the treatment of poles.

There are many wood preservatives, including those using poisonous salts such as copper, mercury, zinc, and arsenic compounds.

However, only two are included in AWPA recommendations for poles, Standard C-4-74-C:
1. Coal-tar creosote, AWPA Standard P1-65
2. A 5% solution of pentachlorophenol in a petroleum distillate, AWPA Standard P8 (commonly called “penta”)

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