The wood poles commonly used to support overhead line conductors in Southern Brazil are produced from eucalyptus, an exotic tree species widely cultivated in the country. Unfortunately, wood is subject to deterioration, which can occur as a result of the action of physical, chemical and biological agents. Biological agents are the most important decay factor, and wood poles can be attacked by bacteria, insects, fungi and marine drills. In Southern Brazil, special attention is given to fungi, organisms whose forms and lifestyles vary from simple yeast to a mushroom.

The attack of wood-decaying fungi can be rapid, resulting in a dramatic loss of pole strength. Fungi mostly occurs about 0.5 m (1.6 ft) above to 0.5 m below ground line, where the presence of oxygen and moisture (greater than 20%) enables metabolic activity and growth of aerobic microorganisms. A large number of insecticides and fungicides are used to treat wood, but the efficiency, in terms of extended service life obtained from the application of these wood preservatives, varies greatly.

Wood Pole Population

In the electrical networks of Rio Grande do Sul, Brazil's southernmost region, more than 2 million wood poles are in service to support distribution and transmission lines. AES Sul supplies some 1.15 million consumers in 118 cities in an area of some 99,268 sq km (38,328 sq miles). The overhead networks are supported by 760,000 poles, of which approximately 90% are wood.

The common treatment for wood poles is a water-based chromated copper arsenate (CCA) preservative applied under pressure, as the use of pentachlorophenol and creosote are forbidden. In accordance with the CCA treatment, treated wood poles must have a lifetime of at least 15 years. Alternative preservatives have been proposed in Brazil as a substitute or to complement the CCA treatment. Special attention has been given to a boron-fluoride preservative, used in the retreatment of in-service poles, because of its high efficiency and lower toxicity for humans and the environment.

For the past three years, research has been conducted with the aim to inspect about 10,000 wood poles distributed over 23 cities in Southern Brazil. The main objective is to establish a practical, reliable and low-cost inspection procedure for in-service poles.

Materials and Methods

Approximately 10,000 in-service poles were selected at random from a population of 760,000 poles, located in the AES Sul area. From 2002-2004, inspectors visited 23 cities, representing AES Sul's five enterprise subregions. These cities were chosen because they present differences in soil and climatic conditions. In each city, several low-voltage lines were randomly sampled in urban and rural areas. The majority of poles (52%) were located in the largest urbanized metropolitan region, while poles located in rural areas were predominantly in the North and South Frontier regions.

The pole inspections involved three steps:

  • Visual assessment
  • Hammer test
  • Quantitative test of the decay.

The visual assessment of wood surface determined the extent of defects such as cracks, holes, and burned or rotten points. The hammer sound test was used to detect a hollow core caused by internal decay in the pole portion from the ground line up to 2 m (6.6 ft). A clear sound and hammer rebound confirms the internal condition of the wood is sound.

As the visual inspection and hammer test assessments are rather subjective, measurements of internal and external decay also were performed. The external pole inspection included digging out the critical region below ground line. As external decay can reduce the pole circumference, this parameter is measured in two different positions, 0.10 m (0.33 ft) above and 0.10 m below ground line. The difference in the pole circumferences is used to estimate external decay.

The internal pole decay is assessed by drilling a small hole parallel to the ground line, then inserting a probing rod with a hook into the hole to determine the thickness of the wood. The rod has measurements used to indicate the shell thickness and estimate the internal decay. All inspection drilling holes are treated with a boron-fluoride water-diffusible Polesaver Rod preservative from Preschem and plugged with a polyvinyl chloride dowel to prevent subsequent decay.

The inspection data registered in the database are pole location, the address and GPS coordinates (which are put in a map), decay evaluations, pole classification and recommended action.

Results and Classification

Among the 10,692 inspected poles distributed over five AES Sul regions, 90.7% were wood poles. The first important observation is the significant number of poles (58%) without identification tags. The metal tags may have been lost during transport or pole installation or, simply over the in-service pole life, affected by weathering or vandalism. The loss of identification tags reduced the information about in-service poles to only 4,075 of the inspected poles (42%).

Results indicated that 48% were regarded as Class 1 (in a good state of preservation), 24% were classified as Class 2 (partial decay but still serviceable; internal/external retreatment is recommended), 15% were rejected as Class 3 (rejected pole with advanced decay; rehabilitation — reinforcement/retreatment — should be made) and the remaining 13% were regarded as Class 4 (in need of immediate replacement).

The results confirmed a higher level of pole conservation in rural areas (63%) compared to urban areas (51%). In addition, the North Frontier region had three times fewer danger poles in rural (6%) than in urban (17%) areas. These results are probably due to the recent expansion of the rural network in Brazil, motivated by federal government support.

Preservative Type

From the inspected poles with an identification tag, it was only possible to identify the wood treatment used on poles treated with CCA or creosote. Pentachlorophenol was used in Brazil until 1970; the number of in-service poles with this preservative should not be significant, but the presence of creosote was expected because its use was only banned in 1995. For some poles without an identification tag, it was possible to assess the preservative type by visual inspection (CCA was a green color while creosote was a black color). However, this kind of identification has been taken with caution, because weathering can change a pole's color, making visual identification difficult and inaccurate.

The distribution of the poles preserved with CCA or creosote in the five regions was studied. The greater occurrence of poles preserved with CCA in the North Frontier was 93%, while the highest percentage of creosote-treated poles was in the Valley region (49%). These differences are significant compared to the global mean values (69% for CCA and 31% for creosote) and seem to be related to the differences in the replacement rates and pole distribution over rural and urban areas in each region.

Pole Lifetime

The aging profiles of the inspected poles for each region were studied and the in-service time divided into four time periods: less than 5 years; 6 to 10 years; 11 to 15 years; more than 15 years.

These periods were used to simplify the data analysis and to take into account the Brazilian standard that establishes 15 years as the minimum in-service lifetime for a treated pole.

The majority of poles (60%) had an in-service time of less than 10 years, with the Central region having the highest level (75%) of young poles (less than 5 years), and the Metropolitan and North Frontier regions having the oldest pole networks (mean age of 11 to 15 years). This difference is probably associated with the more intense replacement rate and construction of new networks, especially in rural areas, in specific regions during the last decade.

The pole-aging profiles observed in this study differ from other countries. In Europe and North America, the average pole age generally ranges from 25 to 50 years, but the wood species and treatment used differ from that in Brazil. In Australia, where the poles are mostly eucalyptus timber treated with CCA, the durability for these structures ranges from 35 to 45 years.

The durability of the in-service wood pole is related to several factors:

  • The quality of new wood poles going into service

  • The environmental factors

  • The effectiveness of the inspection and maintenance programs.

All these factors are very different in Brazil compared to other countries, and the use of fast-growing eucalyptus species is probably one of the most important aspects for the shorter in-service lifetime observed.

The influence of preservative type has a marked impact on pole decay as shown by the results. CCA-treated poles deteriorated at a relatively steady rate during the four lifetime periods, whereas poles treated with creosote were noted to exhibit an improved level of conservation. The results indicate a fast and significant decrease in the life of poles, probably because of the quality of white wood and the CCA treatment process. The results further suggest that creosote offers better protection against fungi decay in Brazilian conditions, probably due to its higher toxicity and a more efficient treatment process.

Quality-Control Standards

Although the study results are preliminary, it will be possible to use them to establish a more realistic replacement pole rate. In addition to introducing quality-control standards for the white wood preservative impregnation process, the implementation of a periodic and systematic inspection program of all wood pole networks is recommended.

Acknowledgement

The authors thank AES Sul for funding this work through its research and development program, Preschem Pty for the preservative donation and John Hellier for his support on the field tests.

Adriano Gabiatti (adriano.gabiatti@aes.com) received bachelor's degree in mechatronics and a master's degree in energy systems from Pontifical Catholic University of Rio Grande do Sul. Currently, Gabiatti is working with management of the maintenance program for distribution lines on AES Sul Distribuidora Gaúcha de Energia S.A.

Pedro Daniel Bach Montani (pedro.montani@aes.com) received his BSEE degree from Pontifical Catholic University of Rio Grande do Sul. Since 2006, Montani has been working with management of the research program for AES Sul.

Marçal Pires (mpires@pucrs.br) is professor of the program of engineering materials at the Pontifical Catholic University of Rio Grande do Sul in Brazil. He received his Ph.D in environmental chemistry from Federal Institute of Technology of Lausanne, Switzerland, in 1995. Pires is active in research projects about wood pole inspection, retreatment, and timber waste analysis and disposal.

Berenice Anina Dedavid (berenice@pucrs.br) holds a bachelor's degree in physics and a Ph.D. in materials science from Universidade Federal do Rio Grande do Sul in Brazil. She is a professor of the program of engineering materials at the Pontifical Catholic University of Rio Grande do Sul in Brazil and is active in research projects on wood mechanical proprieties.

Flavio L.R. Vidor (flavio.singepla@terra.com.br) received a bachelor's degree in biology from the University of Vale do Sinos in Brazil and a master's degree in materials engineering from the program of engineering materials at Pontifical Catholic University of Rio Grande do Sul, where he is pursuing a Ph.D. Vidor is responsible for research on topics that include inspection programs and mycological pole decay.

Inspected In-Service Poles in Five AES Sul Regions

Region Number of cities Inspection
Number of poles in each region Percent inspected
Central 2 976 9.1
North Frontier (extended rural areas) 2 859 8.0
South Frontier (extended rural areas) 3 1,583 14.8
Metropolitan 11 5,540 51.8
Valley 5 1,734 16.2
Total 23 10,692 100

In-Service Classification Based on Decays and Procedures

Inspection Class Procedure
Internal External Evaluation Action
Healthy wood Rotten wood
> 0.10 m
(> 0.328 ft)
Without 1 Good None
0.07 m to 0.10 m
(0.23 ft to 0.328 ft)
maximum 0.01 m
(maximum 0.03 ft)
2 Initial decay Retreat internal/external
0.03 m to 0.70 m
(0.01 ft to 0.23 ft)
maximum 0.02 m
(maximum 0.06 ft)
3 Advanced decay Retreat internal
< 0.03 m
(< 0.01 ft)
Total 4 Failure Replace

Inspection Results

Region Number of poles ID tag (%) Decay classification (%)
1 2 3 4
Central 909 39 63 12 13 12
North Frontier 827 51 54 19 14 13
South Frontier 1,501 38 42 33 16 9
Metropolitan 4,654 45 46 25 16 12
Valley 1,355 30 46 19 16 18
Total 9,246 42 48 24 15 13

Companies mentioned:

AES Sul www.aessul.com.br

Preschem http://preschem.com