Utility companies need to monitor the condition of wooden poles regularly and predict their future condition accurately to operate their distribution system continuously and safely. This paper covers the current methods and techniques used in wooden pole maintenance across the globe and presents an analysis of the same points, considering the need for automation in wooden pole testing to enable standardized outcomes.
The current market for utility poles is driven primarily by wood. Wooden poles are popular and widely used to carry electric power lines and telephone lines all over the world due to their high strength per unit of weight, low installation and maintenance costs, and excellent durability when they are properly treated with wood preservatives. Increased energy demand and the extension of current power networks are the main drivers of this market's growth opportunities. In all, the energy transmission and distribution segment is the largest consumer of utility poles.
The tree species most commonly used for poles is the Scots pine (Pinus sylvestris), but other species such as jack pine, western red cedar, and Douglas fir are also used. Wood is a natural biological material and, unfortunately, is susceptible to fungal and insect attacks. Decay in wood usually occurs when its moisture content exceeds its fiber saturation point. The outer surface of a pole, although usually treated with CCA (chromated copper arsenate), creosote, or with other wood preservatives to protect it from fungi and insects, can still be attacked by fungi. The internal parts of the pole can be attacked by basidiomycetes fungi that enter the wood during the drying process through deep checks and splits or through the knots that form due to branches. Ultimately, failure of wooden poles is inevitable once they lose their structural strength due to decay .
The inspection and treatment of wooden utility poles is necessary for determining the strength and serviceability of a pole. It’s extremely important to determine the deterioration and degradation of utility poles and predict the next failure in order to prevent it or reduce its effect through maintenance or a contingency plan.
Replacing a utility pole can be expensive, which is why it’s necessary to determine which power poles are healthy, which have some deterioration but can last for some time (this needs to be predicted), and which ones need to be replaced immediately. In other words, an effective pole inspection program strikes a balance between accurately identifying poles that put both system reliability and human life at risk while minimizing the number of still serviceable poles being replaced.However, due to a complex combination of variables (i.e. wood species, preservation methods and material, soil and climate conditions, insect and mechanical damage, waste management, inspection methodology, and human error) involved, no fail-proof inspection method exists that can guarantee the condition of a standing wood pole with 100 percent accuracy .
Current methods and techniques
The three predominant methods used to inspect wooden poles are visual, sound, and bore-based inspections .
Visual inspection is suitable for identifying gross defects visible above the ground level. Many utility pole inspections for measuring rot or strength are currently being performed with the naked eye, simply by looking closely. Is there visible rot or decay in the wood? Are there cracks, holes, burn marks, or other imperfections in the structure? These factors can significantly impact a utility pole's ability to handle stress.
Inspection based on sound is another method that’s been used for generations: skilled pole inspectors tap the wooden utility poles with a hammer at certain intervals all along the pole as well as along the circumference, recording their own perspective on whether the point at which it is tapped is good or decayed, based on their hearing ability and experience. Sturdy and solid wood will produce a clear and resonating sound. At places where there might be rot or decay, the wood will produce a dull thud. However, this method is limited by the rather limited frequency range perceptible by the human ear, the upper limit of which is about 20 kHz.
If any points of probable decay are found, the inspector bores into them with a drill. This allows them to measure the level of decay within the pole. Inspection of the sawdust coming from the bore hole will convey if decay is present or if there is soft, spongy, or punky wood present in the pole. If the bit goes in rapidly without resistance, it means there is a decay pocket in the hole or a void .
These and other similar methods of testing poles for decay may not give accurate results. Research showed that a large number of poles are replaced unnecessarily, and a significant number of poles continue to fail unexpectedly while still in service, causing damage to assets as well as human lives. Consequently, several semi- and/or nondestructive methods are also used to determine the internal defects of utility poles. Numerous attempts have been made to detect internal decay within wooden utility poles in a minimally invasive manner. You’ll find popular techniques below.
Driven by a drill motor, a long and thin needle is inserted into the wooden pole to determine its density, often with the intent of identifying areas of substandard structural integrity such as decay. Variations in resistance to torque are recorded by a stylus on graph paper. Newer models may record information digitally. Inspection by resistograph for each pole is conducted at breast height and below ground using a 45-degree adapter (e.g., The IML-RESI F-300 instrument). Variations in resistance result in increases and decreases in the amount of torque applied to the drill shaft (Figure (1)). The poles aren’t harmed because the drilling hole closes itself due to a special drilling angle that was customized for the drill bit. Additionally, with a 45-degree adapter, it’s possible to examine a utility pole below ground level (Figure (2)).