It is a common misunderstanding that there are lower voltage levels inside a substation fence than outside of it, when in reality, substation workers face just as serious risk of electrocution. The statistics show that many serious injuries and even fatalities happen while conducting electrical substation maintenance, repairs and expansions. In five out of every seven of these cases, which is more than 70% of the time, incidents are attributed to the task of installing and removing personal protective grounds and bonds.
Substations are one of the most congested work locations in the utility industry, requiring workers to get around obstructions in tight spots, often at height. Except when required by state regulations, many utilities choose to use uninsulated mobile elevating work platforms, such as boom lifts, rather than bucket trucks, for the maneuverability that they provide.
Switching back to using an insulated truck-mounted aerial device was problematic because it could not access all areas where the work was located. Even if procedures to protect the worker in an uninsulated boom lift had been permitted, the process is laborious and subject to human error.
For many utilities, however, no such restrictions apply. Best practices for using uninsulated boom lifts in these applications have evolved over many years. The most common solution for protecting workers in an uninsulated boom lift is to bond the basket to the de-energized conductors prior to work being performed so that an equipotential zone (EPZ) is created for the employee.
Risk to Workers
As system fault currents have increased, worker exposure voltage in substations has increased. In the United States, most substation work is performed by de-energizing a circuit, testing for absence of nominal voltage, and applying personal protective grounds (PPGs) and bonds in attempt to establish a safe work zone. Dangerous worker voltage exposure can still occur when one or more existing safe work practices are missed, there are unforeseen equipment failures, or when detrimental environmental factors are present.
One source of worker exposure to voltage is caused by inadvertent energization events. This type of shock hazard occurs when a de-energized piece of equipment becomes energized while a worker is present. There are many ways this can occur, but three examples are energization during switching activities, mechanical failure of vertical switches, and energization by conductor movement.
Another source of worker exposure is induced voltage caused by nearby energized conductors. If a standard steel boom lift is being used on non-energized equipment, there is still a risk of worker exposure voltage caused by capacitive or magnetic coupling and induction. This happens when the boom is being used near energized conductors or equipment such as transmission lines or sections of the substation that are still energized. In these situations, the nearby energized circuits can induce current and voltage to the mobile elevated work platform. Research has shown that low magnitude induced current can still exceed the let-go threshold and cause significant injury.
These risks are sometimes recognized, and a common mitigation method is to ground the lift’s drive chassis to the grid. However, because of resistance in the boom from painted surfaces, plated pins, lubricants and wear pads, a difference in potential can still exist between the platform and the structure the worker is in contact with. This may be addressed with breakaway bonding cables attached to the steel platform. This work practice requires knowledge of the risk, proper selection of clamps and cables, and correct installation. Additionally, bonding the bucket can inhibit movement which decreases productivity.
A third potential shock hazard in substations is caused by the improper selection, installation or removal of grounding assemblies.Selecting the proper clamps and cables is an important but complex part of protecting from maximum available fault current and creating an EPZ. The large number of variables involved in selecting the proper equipment increases the risk that the wrong materials will be selected. In a case where undersized cables or excessive lengths are selected, and the voltage rises, the employee may think they are protected, but they are not.
Even with the proper equipment selected, there is still risk associated with the proper installation and removal of personal protective bonds. Several variables such as contamination, loose hardware, etc., create risk. Clamps commonly loosen due to wind, vibration or simply not being installed correctly.
Certain events such as inadvertent energization, back feed, or induction present shock hazards are very difficult to avoid and are not easily understood by the industry.
Safer Substation Worksites
Transformer maintenance, circuit breaker testing, and inspection of batteries, bus connections, vacuum breakers, and relays are all recommended at monthly intervals. When you consider the frequency with which substation maintenance occurs and how many steps are involved to bond the basket to the de-energized conductors, it is worth exploring alternatives.
Using an insulated substation boom lift manufactured by Genie and upfitted by Terex Utilities helped Avista Utilities protect the operator from shock hazards resulting from accidental energization, induced voltages and current, improperly installed grounds, and violation of minimum approach distance.
Avista has one crew dedicated to substation maintenance of more than 150 substations. They have an additional three crews that are dedicated to substation construction and three crews working in generation facilities. To comply with state regulations, the utility had been using either insulated truck-mounted aerial lifts or customized insulated telescopic booms.
While both options protected workers from electric shock, they hindered productivity. The bucket truck required parking, setting outriggers, and then re-setting it if it was not in a position to reach all of the work. The custom insulated telescopic boom was often able to fit in the work zone, but rotation was restricted due to tail swing and longer stowed boom length of telescopic boom lifts.
As the utility already had some experience working with Terex Utilities and the Genie brand, Avista saw a concept for an insulated articulated boom lift from Terex Utilities at a trade show. Launched in 2021, the Z-45 Substation Utility Boom (SUB) is based on the Genie Z-45 XC with the same handling and
performance characteristics and features of a fiberglass jib section and fiberglass bucket. The units carry dielectric protection in compliance with ANSI A92.2 standard for Category E insulation rating of 20 kV. In addition, the Z-45 SUB can work in the confined spaces because of its zero-tail swing and dual parallelogram boom structure.
Avista substation maintenance crew members have found installation of bus or air switches to be much easier, faster, and safer with the Terex Z-45 Substation Utility Boom (SUB). Previously a lot of time was spent trying to reach the work with an insulated bucket truck, seeing if it would work, and if not, trying another setup option. Even prior to the WAC regulation change, the Avista crews found it less efficient to use an uninsulated boom lift using the bonding method — which took a couple of minutes each time the machine was moved. In addition, if it was a rental unit, the crew would have to grind, drill, and tap a 1/2-in. hole on the frame to provide a grounding point or have the fleet services department weld a grounding stud.
For repetitive tasks like crimping bus bar pipes, the Z-45 SUB saves a significant amount of time by eliminating repeated equipment setup and bonding procedures. Avista has also maximized its capabilities by using a battery-powered crimper that has the necessary power to get the job done without negating the insulation qualities of the boom lift. Ultimately, Avista crews find that they can do the same work they were used to doing in an uninsulated articulated boom lift but with more protection for the operator.
Side cutouts in the insulated bucket provide line of sight visibility to workers on the ground, and a swing gate makes it easier for workers to get in and out of the bucket.
Avista currently owns two Z-45 SUBs and would likely rent additional units for short-term projects. The Z-45 SUB within its voltage range limitations does not reduce the need for proper PPE and work practices but removes workers from a path to ground, insulates the operator, reduces human error factors associated with grounding and bonding, and protects workers that violate minimum approach distances.
Acknowledgments
Thanks to Matthew Potter, a fleet specialist working in the Fleet Services department for Avista, Spokane, Wash., for contributing to this article. Avista Utilities is involved in the production, transmission, and distribution of energy. Providing energy services and electricity to 411,000 customers in a service territory that covers 30,000 square miles in Washington, Idaho and Oregon.
Rick Kennerly is a utilities safety expert based in Florida, who has worked with dozens of Investor-Owned Utilities to review equipment, work methods, and in service testing and maintenance procedures for personal protective grounding and permanent structure grounding. He has served on multiple ASTM standards committees,
including F855, F2249, F712, F1796, F2939 and an IEEE task group member. He is currently vice chair of IEEE 1048.
