T&D Poles are an Electric Utility's Greatest Single Infrastructure Investment. They represent one of the utility's biggest risks, as pole failure can seriously impact public safety and reliability. There has been no proven technique to provide an accepted empirical measure of the remaining strength of in-service poles — that is, until recently. After learning more about the benefits of mechanical pole testing (MPT), Georgia Power (Atlanta, Georgia, U.S.), a Southern Company, put this new type of inspection method to the test.

POLES AND INSPECTIONS

The distribution poles at Georgia Power are primarily of the Southern Pine species and are subjected to very hot and moist weather conditions. The utility's older poles — mostly pressure-treated creosote — normally begin to deteriorate below ground at about 20 to 25 years into their service life.

Georgia Power has had a robust inspection and treatment program in place since the late 1980s. Prior to 1987, the utility primarily used the hammer-sounding test as the initial means of identifying suspect poles — there was no remedial treatment program in place. All of Georgia Power's purchased poles have been supplier treated with chromated copper arsenate (CCA) preservative since the late 1980s. To date, the utility has seen no deterioration of properly manufactured and treated CCA poles.

In addition to the decay damage done to Georgia Power's creosote poles prior to 1987, many attachments have been added to the poles for telecommunications, Internet and cable TV equipment. This all adds to the horizontal and vertical loading of the poles. The additional loading must be accounted for and compared to the pole strength for in-service poles.

It is imperative that unserviceable poles be removed from the system or properly reinforced. However, it is just as important not to remove serviceable poles prematurely. The cost of pole replacements vary from US$400 to $10,000, depending on the complexity of the attachments and the equipment on the pole.

In recent years, Georgia Power was finding that pole inspection vendors were becoming increasingly conservative in their evaluation of poles to reduce their risk and that of the utility. Georgia Power pole replacement crews expressed to management that they were being asked to replace more poles that appeared to be sound than in previous years.

MECHANICAL POLE TESTING

Georgia Power's Distribution Design and Performance group, which handles the asset management guidelines for the distribution side of the business, recently decided to pilot and evaluate a new type of inspection method: the MPT 40. This process was developed by Deuar Pty Ltd. (Burpengary, Queensland, Australia). It was quite different than any of the traditional pole inspection methods used by most electric utilities in the United States.

Georgia Power began discussions with Dr. Kris Deuar in early 2006 to better understand the technology, safety issues and costs. The utility was initially concerned about the safety of these partial load tests, because it only would be testing weakened poles occasionally. It became convinced of the safety of the tests, as the weaker poles would be found with either a good visual and sounding inspection, or with only a minimal amount of force applied by the MPT device.

The MPT 40 approach made sense to Georgia Power. It gave a "direct" indication of the pole's strength, taking into account the differences inherent in the wood species used to produce the pole, the orientation of the defects and so forth. The theory is that by applying a known bending force, and then measuring very accurately how the pole geometry changes, the bending strength of the pole can be calculated. MPT had been used extensively in Australia, New Zealand and China with good reported success. Furthermore, the Forest Service Research Institute of New Zealand recommended it as the best method available for determining in-service pole strength.

The method uses digital protractors, attached to a pole, which measure the tilt (bending back) of the pole as the small pressure against the pole (always much less than the residual pole strength) is first applied and then released. Each pole is audio-visually inspected and subjected to a small initial load of 200 lb to 300 lb (91 kg to 136 kg) and then analyzed for safety before a final target load of 2000 lb to 3000 lb (907 kg to 1360 kg) is applied.