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Understanding Equipotential Work Zones

Feb. 14, 2018
Learn to think electrically, not mechanically.

According to data from the Bureau of Labor Statistics, electrical power line work is among the top 10 most dangerous jobs in the United States. One area within the electric utility and construction industries where fatal accidents and injuries continue to be seen involves line workers exposed to induced voltages on de-energized and grounded lines and equipment.

Everyone has seen the signs on substation fences that read: “If it’s not grounded, it’s not dead.” While this is true, it does not really tell the whole story. It must be grounded and bonded in such a way to ensure that the circuit opens in the fastest available clearing time — and that the potential differences between conductive objects in the employee’s work area are as low as possible.

Since 1994, OSHA has required grounding practices to protect employees. Its intent is clear: Temporary grounds and bonds shall be installed at the work site in such a manner that keeps the site at the same potential and prevents harm to workers, even if the line is accidentally re-energized or exposed to induced voltages.

To achieve as low a voltage as possible across any two conductive objects in the work area, you must bond all conductive objects in the area. Unfortunately, this important work practice somehow is still being overlooked.

In July 2013, a utility contractor crew foreman was fatally injured while preparing to remove a jumper from a sectionalizing disconnect switch on a 115kV transmission line. The plan of the day was to relocate two 115kV sectionalizing disconnect switches. There were two crews on the job working independently.

Crew 1 established an equipotential zone (EPZ) between the switch structure, the sectionalizing switch, the wood pole down ground, the driven ground rod, and the 115kV transmission line conductors. Crew 2 did not establish an EPZ because they did not bond the driven ground rod to the switch, thereby establishing two different ground potentials between the switch stand and the switch (see the Figure).

The foreman climbed up to the top of the switch stand to attach lift slings suspended from a crane. Once the lift slings were placed and pulled up snug, he positioned himself to assist in the removal of the blade end-sectionalizing jumper on B-phase. At this point, he made contact with a difference of potential across the blade end of the insulator. The accident board concluded that the root cause of the fatal event was the crew’s failure to establish an EPZ.

In my 40 years in the industry, I personally have investigated several similar accidents where workers had been working on de-energized “grounded” lines. One thing I have found in common with these accidents is that the crews involved had been trained multiple times (as OSHA requires) on equipotential grounding methods.

The dangers of induction are often underestimated; induction can kill. With more overhead lines being forced into corridors and operated at higher currents, induction sources must be considered and respected.

EPZ grounding is a reasonable and technically sound provision for protective grounding of lines and equipment; it is fundamental to the safety of line workers. I find it remarkable that this well-recognized concept of creating an EPZ is not better accepted and established.

The purpose of the EPZ is to minimize electric current flow across a worker’s body. It is simple and should be easily understood. Existing regulations, if followed properly, will protect a worker from hazardous differences of potential. If field workers understand and follow grounding rules, then they will be safe.

Grounds installed only on either side of the work location or bracketed grounds do not prevent potentially lethal current from reaching and flowing through a worker. There is a belief with grounding that somehow those bracketed grounds are going to stop the electric current from reaching and flowing through a worker, but this simply does not happen. The current takes every path.

Careful planning of work assures that the work is performed efficiently/safely. A hazard analysis is a critical part of work planning. A properly conducted pre-job briefing ensures the scope of work is understood, appropriate materials are available, all hazards (including potential for induced voltage) have been identified and protective measures have been established, and all affected employees understand what is expected of them. Knowing a hazard of induced voltage may be present, and not determining how to properly protect workers from it, is not enough. You must address how the induction hazard is to be neutralized and ensure everyone understands the grounding and bonding methods to be used.

I believe that too many power line workers do not fully understand the hazards of induction in their work. They see a set of grounds in the area, and believe they are protected. But they are thinking mechanically, not electrically. By this, I mean that they must be trained to consider themselves a component in an electrical circuit. All conductive objects in the work area that can be reached by the worker must be electrically bonded to eliminate differences in potential that the worker may be exposed to.  

Bahr, an associate of the Safety Institute (www.safetyinsitute.com), has been a safety professional in the electrical industry for more than 30 years. After being injured in an electrical accident in 1985, he has dedicated his career to the safety profession in the electric utility/construction field. He can be reached at [email protected].

About the Author

Mike Bahr | Safety Professional

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