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Grounding systems are designed so they provide the necessary safety functions. Understanding different grounding methods is critical for utility workers. Incident Preventions relies upon industry experts to author these much needed articles. For better insight on grounding methods used in the field you may want to attend iP Safety Conference and hear their in-depth presentations.
While working "at home," line workers have a consistent environment of personnel, safety rules, work procedures and construction standards. Protecting themselves from unexpected hazards is part of the everyday job. The hazards are there, but the people they work with, as well as the work procedures they use, are safe and familiar. However, the mutual assistance agreements developed in recent years by utility companies are bringing help from other utility companies after a major event, introducing a mixture of safety cultures and work procedures into a concentrated area. The hazards increase exponentially due to unfamiliar locations, construction standards and crews. The unknowns are difficult to predict. Protecting your crews in this environment is now more important than ever.
American Electric Power (AEP) is very familiar with the hazards associated with mutual assistance work. Our safety culture focuses on protecting our line workers while working at home or on another company's property. One aspect of that effort is to require AEP line mechanics to use the equipotential grounding method. For many years, the most commonly used method to develop an equipotential zone is to use a pole band or cluster bar. However, testing has shown that cluster bars do not consistently achieve a sufficiently low resistance contact with the wood to provide an effective equipotential zone for the line mechanic working on the pole. In addition, variables such as wood type, treatment age and moisture content greatly affect the pole's ability to accept an electrical current.
QUESTIONING THE THEORY
AEP employees, while understanding the theory of equipotential grounding, questioned the effectiveness of the cluster bar to establish an equipotential zone on a wooden pole under all conditions. In 2005, management responded with formal testing at the AEP Dolan Lab facility outside of Columbus, Ohio.
Under the direction of Larry Dickerman, director, Distribution Engineering Services and several key stakeholders from the individual operating companies of AEP, criteria for testing was set with the engineers at the Dolan Lab. The goal was to confirm the cluster bar's effectiveness in establishing an equipotential zone on a wood pole. When the test results began to waver from the expected outcomes, Dr. John Schneider, strategic technical consultant, provided investigative technical support.
The testing, it turned out, exposed a problem. In short, if the grounded line accidentally becomes energized, the pole band or cluster bar energizes the surface of the pole. The problem is the line worker's gaff penetrates the surface of the pole. This may not sound like much, but testing proved there is enough resistance between the surface and inner layers of wood to prevent the pole band or cluster bar from effectively energizing the wood in contact with the line mechanic's gaffs. This problem created dangerous levels of potential across the simulated line worker in the tests.
With the problem identified, testing began to explore the question, "How do we consistently establish an effective equipotential zone on a wood pole?" In order to provide the most protection, a worse case scenario was identified. Tests revealed that working from an aged wet penta pole provided the highest amounts of amperage. The voltages used for these tests were from a 13.2 kV system, and the results were extrapolated to 12.47 and 34.5 kV supply voltages.
To effectively energize the pole requires a strong direct connection with the wood. Pole construction hardware does contact the heartwood, but the best contact, a bolt lying on the bottom of the bolt hole, was not strong enough to consistently energize the wood. However, the testing did discover that the pole ground in conjunction with staples, or nails with clips that are used to hold the ground wire on the pole does produce a strong and effective connection with the wood. This is the key to the two new methods that AEP uses to develop an equipotential zone.
Today, AEP line workers can create equipotential zones by utilizing one of the two following methods:
The first method utilizes a full length pole ground. To be effective, workers must ensure the ground wire is intact down the pole, and strike the staples or nails with a hammer to ensure a strong connection is established with the wood. If necessary, additional staples or nails are installed.
The second method requires the installation of a temporary pole ground in the absence of a full pole ground. The minimum length of the temporary pole ground down from the neutral is ten feet. No less than six staples or nails with clips are used to secure the ground to the pole.
The equipotential zone with either of these methods extends from midpoint of the ground wire to the top of the pole. Workers are allowed to work on distribution circuits in leather gloves once they have positioned themselves inside this equipotential zone. AEP has submitted information to OSHA about these two new methods that we now use to protect our line workers from hazardous differences in potential. These methods are supported by test results included with the presentation, and hopefully, will be taken into account as new rules for grounding are developed for the utility industry.
The theory of equipotential grounding is sound. Test for yourself the methods you use to protect your line workers. ip
Ned Smith is a Training Specialist at American Electric Power.
The procedures and methods presented herein are for informational purposed only and do not represent an endorsement by American Electric Power of any particular grounding practices.
American Electric Power Company and its affiliates expressly disclaim all liability, both direct and indirect, arising from the use or application of any of the information, methods, or procedures found in this presentation.
Additionally, American Electric Power Company and its affiliates disclaim any and all warranties with respect to the accuracy or use of the information, methods, or procedures found in this presentation, whether expressed or implied, including the implied warranties of merchantability and fitness for a particular purpose.