Establishing an equipotential job site
The electric power industry has long recognized the need to protect workers aloft and on the ground from dangerous voltages that can occur during construction and maintenance operations on overhead transmission and distribution (T&D) lines. Current regulations, codes and standards require the establishment of an equipotential zone where there is the possibility a worker may be exposed to dangerous voltage at the work site. However, there is little direction regarding how to establish an equipotential zone. Methods used to protect workers on the ground vary throughout the industry. An equipotential job site can help mitigate the hazards faced by line workers on the ground.
The hazards facing line workers on a daily basis are increasing due to the changing T&D work environment. It is no longer enough to protect workers from accidental energization of the line they are working on or accidental contact with an energized line. Higher voltages and load currents (more congested right-of-ways, including multicircuit transmission corridors and underbuild on existing structures) and increasingly restrictive utility shutdown policies are resulting in dangerous induced voltages that must be addressed as well. Complicating this are variables such as the electrical characteristics of the system being worked on and the physical conditions at the work site. These variables can vary greatly from location to location even on the same line and have a significant impact on the dangers posed.
A worker receives an electric shock when two points on their body come in contact with a voltage difference, causing current to pass through the body between these two contact points. In essence, the worker’s body becomes the conductor, and the severity of the shock is determined by the magnitude of the resulting current as well as the duration of the current flow. In addition to the voltage difference magnitude, skin resistance—which can vary depending on conditions and clothing such as footwear and gloves that can introduce additional resistance at the contact point and lessen the shock—will determine the current flow through the body.
The time the worker is exposed to the voltage difference can be as important as current magnitude in determining the severity of the shock. For a contact fault where the worker or a piece of equipment comes in contact with an energized conductor, the voltage difference and associated shock potential will persist until the circuit breaker or fuse protecting the line opens to clear the fault. Overcurrent relays and fuses have inverse time characteristics, which means the further down the line from the power plant switchyard or substation the work site is, the greater the fault impedance and the longer it will take to clear the fault. Unlike contact faults where the voltage difference is removed when the fault is cleared, voltages induced by nearby energized lines in work site conductors, structures and equipment will persist as long as those nearby lines are energized.
It is very difficult to predict the current path through a worker’s body and the effect of a shock because of the many variables involved. Therefore, regulations and standards focus on three common shock modes:
- Hand-to-hand contact
- Hand-to-feet contact
- Foot-to-foot contact
Hand-to-hand contact occurs when an individual’s hands come in contact with two objects of different potential. This shock mode is very dangerous because the current path is directly through the worker’s upper body and can result in death due to ventricular fibrillation or respiratory tetanus. To prevent shock resulting from hand-to-hand contact, surfaces and equipment within the reach of workers that could become energized should be bonded together to prevent a dangerous voltage difference as required by OSHA Rule 1910.269(p)(4)(iii)(c)(2).
Hand-to-feet contact occurs when the worker’s hand and feet come in contact with two objects at different potential. Current entering the earth through a ground point, such as a structure foundation or temporary ground rod, tends to spread out and flow back to its grounded source, which could be the transformer supplying the transmission or distribution line. Since the earth is not a perfect conductor and has resistance, there will be a voltage drop between the grounded structure or equipment and where the worker’s feet contact the ground. This difference in potential is referred to as touch voltage. Touch voltage is illustrated in the diagram above and defined by the Institute of Electrical and Electronic Engineers’ (IEEE) Standard 1048 and other standards as the voltage difference between a grounded metallic structure or equipment and a point on the earth’s surface about arm’s length, or one meter (3.24 feet), away. Like hand-to-hand contact, hand-to-feet contact also is very dangerous because the current path is through the worker’s upper body and heart.
Foot-to-foot contact occurs when the worker’s feet come in contact with two objects at different potential. This difference in potential is referred to as step voltage and occurs as a result of current flowing through the earth where the worker is standing. IEEE Standard 1048 and other standards define step voltage as the potential difference between two points on the ground separated by one meter, which is about the length of a worker’s stride. Unlike hand-to-hand and hand-to-feet contact, current passes through the worker’s legs and does not involve the upper body or heart. As a result, foot-to-foot contact requires much more step current through the worker’s legs to induce ventricular fibrillation or respiratory tetanus than those shock modes involving the hands. However, step currents tend to knock workers down, and once in a prone position, current passing through the heart electrocutes them.
The Occupational Safety and Health Administration (OSHA) Standard Number 1910.269 addresses electric power generation, transmission and distribution. Paragraph 1910.269(n)(3) is titled “Equipotential Zone” and requires temporary protective grounds be placed at such locations and arranged in such a manner that will prevent each employee from being exposed to hazardous differences in potential. The term “equipotential zone” is not defined in OSHA 1910.269, but it is defined in OSHA’s Electric Power eTool Glossary as a zone of equal potential used to protect workers from hazardous step and touch shocks.
The IEEE defines the term “equipotential work zone” in IEEE Standard 524a, Subclause 2.2, as a work zone where all equipment is interconnected by jumpers, ground rods and/or grids that will provide acceptable potential differences between all parts of the zone under worst-case conditions of energization. Therefore, an equipotential zone is an area where workers are protected from dangerous step and touch potentials under worst-case conditions.
The temporary grounding of conductors, equipment and structures during construction and maintenance operations does not necessarily result in an equipotential zone as defined by OSHA and the IEEE. Referring back to the simple figure used to illustrate touch and step potential, current entering the ground through a single ground point can result in touch and step voltages that could be hazardous for workers on the ground depending on fault and site conditions. Therefore, grounding may be only part of the overall shock protection strategy used at the job site.
Grounding of conductors, structures and equipment certainly can be an important part of any protection strategy. An effective ground can provide an effective low-impedance path for fault and induced currents to flow helping to minimize touch voltage and facilitate the operation of overcurrent protection. Isolation also can be used to protect workers from dangerous voltages that could be encountered around temporary ground rods and grounded equipment. Barricades can be used to keep workers and the public a safe distance away.
Bonding almost always is required to establish an equipotential job site. Bonding involves electrically interconnecting conductors, structures, equipment and grounds at the job site to minimize differences in potential between them. Bonding typically is accomplished by using properly sized temporary grounding-jumper assemblies. In addition to grounding, step and touch voltages can be reduced around equipment and at work areas by the use of ground mats. Ground mats extend equipotential areas by providing a conductive surface for workers to work on and all but eliminate touch and step potential if properly used.
Insulation also can be used either as a primary protection strategy on distribution work or as a supplemental strategy in conjunction with grounding or isolation on either transmission or distribution work. Insulation can involve the use of insulating blankets or personal protective equipment such as insulating boots and gloves.
Thorough job preplanning is the key to establishing an equipotential job site. Preplanning for an equipotential job site includes knowing the layout of the job site; the location of needed equipment, tools and materials; and where linemen, groundmen and equipment operators not only need to be and move. Preplanning also should take into consideration the job site soil conditions and terrain, expected weather conditions, available fault current and characteristics, and any nearby energized lines. Using this information, a strategy should be developed to provide a safe work site using grounding, insulation, isolation or a combination of these methods. This strategy should comply with industry regulations and codes as well as be based on industry standards such as the aforementioned. In addition, any safety equipment employed should be thoroughly inspected and tested prior to use and should only be used in accordance with industry standards and manufacturer instructions. EC
This article is the result of a research project investigating shock protection at transmission and distribution work sites that is being sponsored by ELECTRI International Inc. The author would like to thank EI for its support.
GLAVINICH is an associate professor in the Department of Civil, Environmental and Architectural Engineering at The University of Kansas and is a frequent instructor for NECA’s Management Education Institute. He can be reached at 785.864.3435 or firstname.lastname@example.org.