Australian Standard AS 2209 “Timber Poles for Overhead Lines”, is used to specify the requirements for wooden power poles. The standard applies to both hardwood poles with and without full-length preservatives and softwood poles with full-length preservatives.
The durability of differing wood species is categorised into Classes ranging from 1 to 4 (as per AS 5604). The most durable species are Class 1 and the least durable Class 4. Species such as grey box and ironbark tend to be high durability while messmate and radiata pine are lower in durability. Note that durability classification is not the same as strength rating.
The Australian Standard will only allow the more durable Class 1 and 2 species to be used for poles that are not full length preservative treated. Generally the current practice for new poles supplied untreated have the sapwood remaining with a primary preservative treatment. However, it is not uncommon that poles have been desaped and shaped, as old untreated poles with the sapwood intact tended to look unsightly as the sapwood started to strip off above ground. Note: The untreated sapwood of all classes of timber is very low in durability.
Currently only one type of primary pressure treatment are acceptable throughout Australia:
- CCA (Water based Copper Chrome Arsenic)
The Standard specifies minimum penetration depths and retention levels for both preservatives. The standard also details information required on an identification disc and acceptable tolerances for straightness of poles, sizes, splitting and size of knots.
Normally Authorities purchasing poles make direct reference to this standard and many insist on rigorous pre-delivery inspections.
The types of timber used for poles has varied over time and is usually influenced by local factors. In Australia the climate ranges from tropical in the north through to drier and milder in the south. Hardwood poles have been preferred in Australia because of their natural strength with approximately 14 different eucalypts have been utilised. The most common timber species used in in Australia for poles are treated spotted gum, blackbutt, messmate, untreated naturally durable species such as red/grey ironbark, bloodwood, tallowwood etc, and in Western Australia untreated jarrah. Small numbers of CCA treated pine poles have been installed on a trial basis in lightly loaded situations.
While traditionally many naturally durable poles have been sourced from old growth forests in NSW and Queensland, often local regional forests have provided poles of varying durability.
The emerging trend now is for plantation grown poles. Several Australian Utilities and now Heartwood Plantations, a company which is in the same business group as Preschem, have well established plantations for poles. The environmental benefits for this are obvious: plantation growth power poles are completely sustainable. Other countries such as South Africa and Brasil, have adopted this practice of plantation hardwood poles for many years now now.
Concrete poles are now considered relatively expensive and their use is limited to special situations such as termite infested areas etc.