Wood utility poles numbering approximately 150 million support the delivery of high-voltage electric transmission and distribution service throughout North America. After many decades of use, the wood pole remains a preferred material because of its durability, strength, availability in multiple lengths and classes, and low acquisition and life cycle costs. Wood poles are also a renewable resource.
Utility standards engineers and procurement personnel typically require their poles be manufactured according to American National Standards Institute (ANSI) and American Wood Protection Association (AWPA) guidelines to ensure the desired size, strength, material quality, original treatment loadings, and decay resistance properties are present. Utility pole owners would be well-served through a greater understanding of the deterioration modes for wood poles. Knowledge of wood as a material may lead pole owners to implement preventative measures that help retain pole strength, maintain pole condition, increase reliability, and reduce avoidable repair and replacement costs.
The Vulnerability of Pole Tops
Wood poles are susceptible to degradation by a variety of agents, both abiotic (non-living) and biotic (living). Abiotic deterioration can be caused by ultraviolet (UV) light, water, mechanical activities, chemicals, and temperature. UV sunlight is probably the most obvious abiotic agent damaging pole tops. It degrades both the wood and the original wood preservative, causing the wood to become weaker and more likely to be eroded by weathering. Additionally, the tops of poles originally treated with pentachlorophenol or creosote are challenged most from preservative depletion via the force of gravity over time. The constant exposure to wet-dry and hot-cold weathering cycles will negatively impact the longevity of the pole top if not protected.
As the preservative protection provided the pole top by the original treatment is weakened, cracks and checks develop near the pole top surface, exposing untreated heartwood or inner sapwood to moisture and wood-destroying fungi. Once these conditions are present, internal decay can advance rapidly in the pole top area. Because decay starts internally, it can go undetected until it has weakened the structure beyond repair.
The Risks and Costs of Pole Top Decay
Decayed pole tops can lead to split or weakened tops. These conditions eventually lead to problems with the integrity of the connection between the pole and the hardware it supports. Loose insulators and floating conductors can lead to outages and increased safety risks for the public and utility personnel. When a utility must deal with these problems on an unplanned basis, valuable crew hours are invested in repairs and, in many cases, revenue is lost. If the pole top cannot be repaired, there are additional costs for pole replacement.
Extending the Service Life of Pole Tops
Installing pole top protection devices extends wood pole service life by providing a “roof” for the vulnerable, exposed pole top. These protective “roofs” are designed to prevent UV degradation of both the preservative and exposed wood fibers. Pole top protection also significantly reduces moisture — a necessity for decay — limiting attack from wood-destroying fungi. High moisture content also encourages freezing and splitting of the pole top.
Without protection, pole tops are highly vulnerable to premature aging. The installation of a pole top protection device allows older, in-service pole tops to exhibit “like new” characteristics even though the pole may be decades old. In this way, pole top protection devices help to ensure that it is not the pole top that limits the life expectancy of wood poles.
Evidence that this strategy works has been validated through a study sponsored by the Oregon State University Utility Pole Research Cooperative (UPRC). This ongoing study demonstrates that installing pole caps can quickly reduce the moisture content of pole tops to help prevent conditions needed for decay. Additionally, the Rural Utilities Service (RUS) has accepted a pole top protection device for use on new pole construction.
Pole top protective products protect the pole top from UV damage and excess moisture accumulation. To this end, any material used should be able to withstand weather conditions like rain, wind, and sunlight without degradation. UV inhibitors are added to many products to enhance effectiveness. Materials, such as metals, that might cause condensation to form underneath when exposed to sunlight can be counterproductive in that they add heat and moisture to the pole top. In addition, it can be unsafe to add potentially conductive surfaces around the pole top.
A few devices exist that can provide decades of effective performance and meet a utility’s need. For example, the Osmose Pole Topper® is made of a butyl rubber, which is very similar to commercial roofing and can adhere without tools to almost any shape pole. Another example, the CoverCap®, is made of polyethylene plastic and is ideal for installation on new poles prior to setting. It takes only a few minutes to install either of these products and costs from $10 to $20 per pole depending upon the size of the pole top and the product used. A small investment at installation of a new pole, or before pole top decay has started on in-service poles, can save future replacement costs of a wood pole or more expensive maintenance due to pole top decay or splitting.
Justifying the Investment in Pole Top Protection
It is rare to find a simple, straight return on investment (ROI) for preventative maintenance actions. Most often, one can identify tangible financial and operational benefits to support a persuasive business case for these investments. In this case, the pole top protection helps to eliminate pole top degradation as a reason for pole replacement.
Over an extended number of years of service life, how many pole replacements can be avoided by managing the aging of pole tops? How many poles that otherwise may have been candidates for pole restoration will have to be replaced because they did not have pole top protection?
As an example, one utility (in an area prone to high decay) inspected 61,958 poles during a single year. Inspectors identified 18,026 poles with significant pole top decay (29 percent) and 3,712 poles (6 percent) with split tops.
For this utility, pole replacement costs range from approximately $2,000 to $5,000 per pole depending on the number of attachments. Conservatively, if 20 percent of these 21,738 decayed and split poles will become rejects far sooner than their counterparts without pole top damage, it’s reasonable to assume that this utility could have avoided or at least deferred 4,347 (20 percent) of these pole replacements. At an average replacement cost of $3,500, more than $15 million in replacement costs could have been avoided, or deferred, with a minimal investment in pole top protection.
Many utilities include the cost of pole top protection in capital budgets for materials on new pole installations (which are depreciated over time), creating future savings for both O&M and capital accounts. UP
The Author: David Skinner is director of business development – product sales at Osmose Utilities Services Inc. He has experience helping utilities inspect, maintain, and restore T&D infrastructure across the globe. He can be contacted at firstname.lastname@example.org.