Poles are made from a variety of materials, with the most frequently used being of wood, concrete, and steel. None of these has a clear advantage in all situations; rather, the selection process should include the consideration of several criteria under site-specific conditions. These include availability, cost, weight and ease of handling, strength, and durability. Note also that in a single project, it might be advisable to use several types of poles. At the end of a span that has to be raised to provide sufficient access to vehicular traffic or that has to extend across a wide river or ravine, taller and stronger poles of concrete or steel construction might be more suitable. On the other hand, poles to support shorter service drops might be shorter wood or bamboo poles. At other places, if suitable live trees are found, these can be used.
Before proceeding with a review of pole options, one word of caution about poles in general should be noted. Because they can be the most expensive component of a distribution system, there is an incentive to minimize project costs by selecting the least expensive pole option. However, a less costly pole usually implies reduced strength and/or quality. This has several implications:
• Weaker, poorer quality poles and the conductors they support are more likely to fall under stress, resulting in a greater safety hazard to the population.
• Shorter life implies the need for additional investments when poles later will need to be replaced.
In addition to the cost of new replacement poles, the community will incur the additional cost of removing the old poles, resetting the new ones, and reconnecting the conductors, guys, etc., all of which contribute to additional cost and hassle. It is also likely that the manpower and expertise will no longer be on-site when poles need to be replaced.
Wood
Wood poles are widely used for electrification worldwide because they exhibit a variety of advantages:
• These are lighter than the equivalent concrete pole, the common alternative, and easier to handle in the field.
• Wood poles are not as susceptible to breakage during transport and handling.
• Wood poles can usually be field-drilled, permitting greater flexibility in the placement of mounting bolts and facilitating later modification.
• Wood poles are not adversely affected by airborne salt in coastal zones that can cause corrosion of the reinforcing steel in concrete poles.
• Local plantations permit self-sufficiency in the production of one of the costliest components of an RE program, creating employment, reducing the need for foreign exchange, and lowering the cost of RE.
• Larger, conventional wooden poles are easier to climb directly (with gaffs, sharp metal spurs affixed to the inside edge of a boot).
• Properly managed, wood is a renewable resource, requiring much less energy in the manufacture of poles and contributing no net carbon dioxide or other greenhouse gases, unlike those
associated with the production of cement or steel for poles.
• Numerous environmental benefits are associated with increasing forest cover for pole production in marginal areas—reduced erosion of land and sedimentation that leads to the destruction of riverine habitats, improved ground water quality and quantity, more abundant and diverse wildlife, and opportunities for increased employment opportunities from processing a range of forest products. Forests also serves as a sink for carbon dioxide, a gas increasingly recognized as contributing to global warming and its adverse implications.
• In a number of countries, rural households have little disposable income and the problem facing an RE program is the inability of these households to cover the cost of connection as well as the cost of energy. Growing trees for poles may be one option requiring few financial and labor inputs that can reduce the cost of electrification. Although growing suitable trees requires
perhaps a dozen years, it can eventually provide a regular income to rural households that, in part, can be used to cover the cost of their electric service.
Offsetting these advantages is the fact that untreated wood poles are susceptible to decay and insect damage. Tree species that are decay and insect resistant do exist but are not common. Local inhabitants should be able to identify resistant local species, but it needs to be verified whether this apparent
resistance is for wood under ground-contact conditions and exposed to the weather. The inspection of fence posts or building timbers of the allegedly resistant species should be able to verify this.
In Bolivia, for example, a tropical species called cuchi (Austronium Urundera) is stripped of its sapwood and widely used for posts, poles, and building timbers. The heartwood of this species is extremely resistant to decay and insect attack but is, unfortunately, crooked. Some old-timers see this as an advantage and call them "balcony poles" because they can be located beneath a balcony since the crook will still place the conductors at a safe distance from the front of the balcony. A certain species of palms with a very hard outer shell has also been used as poles on the Altiplano.
The alternative to finding resistant trees is to chemically treat wood poles. This is discussed below.
In countries where the electric utility uses wood poles, criteria have usually been developed to provide guidance as to what specific characteristics to look for when selecting suitable poles.* Generally, poles with the following characteristics are preferred:
• straight poles with little twist or spiral grain
• poles without large and/or numerous knots, as these weaken the pole
• adequate wood density as indicated by tree ring count (The width of the tree rings is an indication of the rate of growth of the tree, with wider spacing indicative of lower strength. In the U.S., with pine which is treated, rings spacing in the outer growth which average greater than about 4 mm indicates wood which has grown too quickly.)
In addition to the above, it is clear that poles should have sufficient girth to give them the required strength. This is further explained later in this chapter (p. 99).
Wood pole production
An obstacle facing the widespread use of wood poles is that, in a growing number of countries, forests are disappearing or do not have suitable trees. It is possible to plant trees specifically for pole production, but adequate lead-time is required until newly planted trees can be harvested for this purpose. Tropical pines can produce a 9-m pole in about 15 years but have limited strength. Faster growing soft wood species exist but these tend to be weaker. More commonly found hardwood species such as eucalyptus, are another option, but these do not get very good preservative penetration and retention. However, because wood poles will continue to be in demand for expanding rural electrification as well as for replacing existing damaged poles, the need for poles will continue decades into the future, well after any tree plantation starts yielding trees of adequate dimensions.
On the national level, the advantages of wood poles and their production should be sufficient incentive for a national commitment to the creation of local tree plantations, possibly in collaboration with other gov-ernment departments, non-govgov-ernmental organizations, or private entrepreneurs.
* An example of pole specifications are those utilized by the rural cooperatives in the United States. These can be found in the section “Electric program regulations and bulletins” located on the Web at
<http://www.usda.gov/rus/regs.shtml>. This is the document “Specification for Wood Poles, Stubs and Anchor Logs” referred to as Bulletin 1728F-700 (formerly Bulletin 50-24).
For example, in the Philippines, the National Electrification Administration (NEA) recognized the numerous advantages of using wood poles in rural areas. It also realized the dwindling source of forest resources in its own country and the high cost in importing poles from overseas. Consequently, the Power Use Development Division of the Cooperative Services Department of the NEA initiated a tree-planting program in 1993. Nearly half of the 119 rural electric cooperatives in the country are now involved in this program.
These rural electric cooperatives raise seedlings that they donate to their consumers (either individuals or users groups) or sell under contract to large landowners. A condition for membership in some coopera-tives is planting a couple of trees on the member's own land. The largest single area under cultivation presently is 400 ha. Upon maturity, the co-op agrees to purchase these trees for their eventual chemical treatment and use as wood poles.
Specifically for the Philippines, the NEA recommends planting Gmelina Arborea, Eucalyptus Deglupta, and Acacia Mangium which all can adapt to the varied climatic regimes in the country.6 It is expected that a 35-foot (10.5 m) pole with a diameter of 8 inches (0.20 m) would be available after about 8 years following the planting of the seedling.* The planting density is at least 500 trees per hectare. It is expected that the co-op will save roughly 50 % over the current price of imported poles. At an estimated development cost of roughly $1,000 per hectare, NEA projects a 50-fold return on investment after 10 years.
Wood pole treatment
One of the most characteristic features of wood species used for poles is the presence of two distinctly different types of wood within each stem: sapwood and heartwood. Sapwood, normally much lighter in color than heartwood, forms the outer periphery of poles, a layer which can range from a couple of centimeters to more than 10 cm in thickness, depending on the species. In living trees, the outer sapwood zone is where nutrient transport and storage occurs. Heartwood is found in the center of the stem. It is composed of wood cells that have ceased any active function and have gradually been filled with organic substances known as extractives. These extractives tend to darken the wood in this portion of the stem.
Heartwood is generally more durable than sapwood due to the presence of these extractives, many of which are toxic, to some degree, to the organisms which cause wood to deteriorate. Sapwood, in the absence of these extractives, is readily degraded by any number of wood deteriorating organisms,
including fungi, molds, stains, and insects such as termites and certain beetle species. For this reason, it is essential to the longevity of wood poles that the outer , susceptible sapwood layer is protected from these organisms by the addition of preservative chemicals that make the sapwood unavailable as a food source.
Proper application of these chemicals in the sapwood will enable the treated pole to last for an extended time in service.
Before poles can be treated, they must be properly dried. Green trees have a very high moisture content, often well above 100%. After felling and peeling, they gradually dry until their moisture content comes into equilibrium with the environment (at which time their moisture content is usually down to less than 30 %). This drying process is called seasoning. As the pole dries during seasoning, the wood shrinks and develops longitudinal “checks” on its surface. Depending on the character of the species, such checking can be very limited or quite extensive.
* Although they are of useful size, what is left unclear is the strength of these poles after only 8 years of growth.
It is very important that drying be done properly, so that normal checking takes place before treatment.
During treatment, all wood surfaces exposed in open checks are well treated. Subsequent drying of the treated pole in storage may open the original checks, but will not expose untreated wood. In a dry climate, poles can be adequately seasoned by natural air circulation, but care must be taken to avoid the onset of incipient decay or insect attack during the air drying process. Significant strength loss can occur with little visible outward sign of decay in such material. Poles can also be seasoned by artificial means, including kiln drying or steam conditioning, both of which, when done properly, sterilize the wood and kill any decay fungi present.
Three basic groups of wood preservatives are used to treat wood poles: oil-borne preservatives, and water-borne preservatives, and creosote. The major oil-borne preservative is pentachlorophenol, commonly referred to as “penta”. The major water-borne preservatives are chromated copper arsenate, commonly referred to as CCA-C, and ammoniacal copper zinc arsenate, also known as ACZA.*
Creosote, a constituent of coal tar and a by-product of producing coke from the destructive distillation of coal for the steel-making industry, is normally used to treat poles through a controlled pressure/vacuum process. However, depending on the species being treated and the amount of sapwood present, some poles can be creosote-treated with an extended hot/cold soak.
Penta, a man-made chemical, is dissolved in a mixture of petroleum solvents and then impregnated in the pole by either a pressure treating process or in some cases, an extended hot/cold soak.
CCA-C and ACZA are comprised of several different water-soluble chemicals that are combined and then forced into the sapwood layer of poles during a pressure-treating process. The preservative is then
chemically bound to the wood fibers, and once fixed, it cannot leach out into the ground. Due to the chemical nature of the water-borne preservatives, pressure treating is the only method than can be used with these chemicals to properly treat poles.
Without chemical treatment, many poles may not last beyond one year, especially in the warmer, moist climates. Their frequent replacement is costly and places an additional burden on those operating and maintaining a mini-grid. Furthermore, system reliability is reduced. However, with the proper chemical treatment and with careful quality control, poles can last for decades, even in wet environments. With a ground-line treatment procedure incorporated in a line inspection and maintenance program, this can be increased considerably.
The following paragraphs described the most common methods for treating poles.
Pressure cylinders
Conventionally, wood poles are treated in large, centrally located treatment plants. They are first properly dried and then, when the moisture content has decreased sufficiently, they are treated in a pressure
cylinder. Several procedures are possible:
• Empty cell method: In the pressure method, the flooded cylinder is placed under considerable pressure to force the preservative into the wood. This provides deeper and more uniform penetration of the preservative, higher absorption of the preservative, and more effective protection than obtained with other methods. After penetration, a vacuum can also be drawn to
* These variants of the arsenate preservative are preferred because they exhibit less conductivity when the pole gets wet.
recover some of the preservative. This still leaves the cell walls coated, but the cells only partially filled.
• Full cell method: In the double-vacuum method, the timber to be treated is placed in a sealed cylinder and a vacuum is drawn. The cylinder is then flooded. As the vacuum is released and the pressure within the cylinder increases, usually to atmospheric, preservation is sucked up in the wood. After a period of soaking, the preservative is withdrawn, and a final vacuum is drawn to recover some of the preservative that had been absorbed by the timber.
Although typically large, small units have also been built.7 Being smaller in size, they might be built at scattered points in the country where rural electrification projects are being implemented. For
electrification in more remote areas, the advantage of growing trees locally is largely defeated if these then have to be transported long distances to these centrally located plants. Other options are required.
Hot/cold soak
One method that is probably the more readily available in less-developed countries is the hot/cold soak approach (Fig. 33).8 Rather than applying pressure to force preservative into the poles or drawing a vacuum to draw the preservative into the wood, it relies on a partial vacuum within the wood induced by varying the temperature of the preservative in an open tank.
However, this method must be used with caution as one is dealing with hot, toxic preservatives and subject to exposure to large volumes of vapor from these heated
preservatives. Pollution of the local environment is also possible if care
is not taken in handling the preservative, the treatment process, and the treated poles. Because of the nature of the process and the inability to carefully monitor all the variables, the results are inconsistent.
Seasoned wood contains minute air spaces that usually amount to slightly more than half the volume of the wood. When wood is placed in a preservative that is then heated, the expansion of the air that accompanies its increased temperature forces some of it to be expelled. Then on cooling, the remaining air contracts and the preservative is drawn into the wood. Typically, the preservative is heated to 85 ° to 95 °C, maintained for about an hour and then let to cool before the poles are withdrawn.
The amount of preservative absorbed depends on the species and size of wood being treated and is controlled by the difference in these temperatures used for the treatment. As with the other methods described above, after penetration, excessive preservative can be recovered by removing the poles before the preservative has completed cooled. Or heat can be applied a second time and then removing the poles 1 to 3 hours after it has been in the hot preservative.
Fig. 33. The hot/cold soak method for treating poles.
restraining
flue to draw fire along tank
supporting blockwork
fire
This process can be used with any preservative that will remain stable when heated, with creosote-type preservatives being the most commonly used. The arsenate water-borne preservatives cannot be used with this method because the salts precipitate out under high temperatures. For other preservatives which contain inflammable solvents or are liable to decompose on heating, a variation of this method is used whereby the heating of the wood and the absorption of the preservative are performed separately. First the wood is heated for 1 to 2 hours using hot air, steam, or water. The wood is then quickly transferred to a tank of cold preservative where it is absorbed while cooling. Arsenate preservatives would again not work in this situation, because the salts would precipitate on touching the hot exterior of the wood, preventing the absorption of the preservative.
The plant required for this treatment can be easily made locally. Oil drums are cut longitudinally and welded to form a long trough as shown in the figure. These tanks must be suitably stiffened and supported to prevent bulging under the weight of the preservative and poles and the action of the heat.
Some bars must also be used to ensure that the poles remain submerged during the entire process. Two
Some bars must also be used to ensure that the poles remain submerged during the entire process. Two