When OSHA updated 29 CFR 1910.269 and merged almost all of its requirements with 1926 Subpart V, the requirement to protect employees from step potential was enhanced. In the months following the publication of the final rule, this change was rarely mentioned in the major webinars conducted by several prominent utility industry groups, so I want to take this opportunity to cover what you need to know.
First, let’s talk a bit about the basic fundamentals of Ohm’s law and Kirchoff’s law of current division in order to ensure you understand the seriousness of step potential hazards. Ohm’s law states that electricity will take any and all conductive paths, and Kirchoff’s law of current division states that the amount of current flow is dependent on the resistance and impedance in the current path.
As I travel around and conduct training, I find that many electric utility employees – much like me in the 1970s – do not understand these and other basic laws of physics that determine the number of hazards we face. The human body is not much more than a 1,000-ohm resistor when put into an electrical circuit. If a human body is placed in an electrical path/circuit, the amount of electricity that enters the body is about 50 volts AC. During this type of occurrence, the soles of normal work boots and shoes will provide an employee a small amount of protection, but if the employee were to kneel down and touch a vehicle grounded to a system neutral, or place a hand on a grounded object, the amount of protection would be significantly reduced.
Further, when contact is made with ground or a grounded surface, a point of entry has been established, and now all that’s needed is a path to exit the body. If a current path enters a person’s foot and exits through the other foot, or a knee or hand, we are down to about a 500-ohm resistor path for current to flow. With electricity traveling at about 186,000 miles per second, it doesn’t take long for the path to be decided. Current flow of as few as 30 to 50 milliamps crossing the heart can interrupt rhythm, create atrial fibrillation and electrocute a person.
Many injuries and fatalities have occurred as a result of step potential accidents because appropriate protective measures were not in place. A variety of factors determine the number of potential differences on the surface of the earth, with soil composition being the largest variable. Sandy soil will conduct less electricity across its surface, but my friends at the copper and gold mine in Morenci, Ariz., for example, face a huge step potential challenge.
There are two OSHA standards utility employers and employees should be familiar with when it comes to step potential protection.
The first is 1910.136(a), which was updated in the 2014 final rule and states, “The employer shall ensure that each affected employee uses protective footwear when working in areas where there is a danger of foot injuries due to falling or rolling objects, or objects piercing the sole, or when the use of protective footwear will protect the affected employee from an electrical hazard, such as a static-discharge or electric-shock hazard, that remains after the employer takes other necessary protective measures.”
Why did this rule require an update? The more I travel and teach, the more I understand the variables of how employers protect their employees. In the past, many employers, especially smaller companies, were not as familiar with step potential hazards as they should have been. OSHA wants an equipotential zone implemented by employers for employees to work in without being exposed to hazardous differences of electrical potential, as stated in 1910.269(n) and 1910.269(p). Following the OSHA standard requirements decreases risk of step potential.
In 1910.269(p)(4)(iii)(C), OSHA states that each employee “shall be protected from hazards that could arise from mechanical equipment contact with energized lines or equipment. The measures used shall ensure that employees will not be exposed to hazardous differences in electric potential. Unless the employer can demonstrate that the methods in use protect each employee from the hazards that could arise if the mechanical equipment contacts the energized line or equipment, the measures used shall include all of the following techniques: using the best available ground to minimize the time the lines or electric equipment remain energized; bonding mechanical equipment together to minimize potential differences; providing ground mats to extend areas of equipotential; and employing insulating protective equipment or barricades to guard against any remaining hazardous electrical potential differences.”
Essentially, the updated standard now requires the employer to take other necessary protective measures in addition to grounding of equipment. Step potential protection is the result of this change.
Grounding Mats, Overshoes and Other Considerations
When equipment is not grounded, a greater resistance exists in the path from the equipment to the earth. In the event of an electrical contact, that resistance creates a higher potential difference between the equipment and the earth, increasing the hazard of step potential for exposed employees. Areas of equipotential can be extended via grounding mats attached to equipment.
Now the question is, how do we safely approach the grounding mat? How close can we be to equipment that may be energized and not get electrocuted by step potential? The type of soil at a work location can increase or decrease the distance at which a hazardous difference of potential may be present. Safety equipment will offer protection from hazardous step potential.
Additionally, many companies ground nondielectric equipment to the system neutral when available; others rely on temporary driven grounds at the equipment site. Both of these work practices increase the possibility of step potential in the work area, and OSHA now requires additional protection.
I started wearing super dielectric overshoes in the mid-1970s. That type of overshoe is still the same today: rated at 20 kV for step potential protection. Forty years ago, I didn’t really understand why that was the case, and no one ever told me much more than it was a safety procedure that had to be followed during certain work tasks, including switching on transmission and distribution systems, opening and closing breakers, opening doors on padmount transformers and thumping UD cables. I never had a problem or even knew of an incident that involved an employee wearing super dielectric overshoes. In fact, I was switching in a substation on a hot summer evening when a thunderstorm came up; lightning struck the station as I was attempting to close a breaker, and I never felt a thing.
The more we understand why and how work practices and regulations protect us, the more likely it is that employees will follow the rules and regulations for their own protection. I have heard many workers say things such as “We don’t need this because nothing has ever happened in the past.” The fact is, environmental and soil conditions are different at every work location across the U.S., while the regulations are written to cover a large majority. Site-specific hazards should be identified in each on-site job briefing (see 1910.269(c)) so that it can be determined if additional precautions are needed.
About the Author: Danny Raines, CUSP, safety consultant, distribution and transmission, retired from Georgia Power after 40 years of service and opened Raines Utility Safety Solutions LLC, providing compliance training, risk assessments and safety observation programs. He is also an affiliate instructor at Georgia Tech Research Center OSHA Outreach in Atlanta. For more information, visit www.electricutilitysafety.com.
Danny, great article and thank you for the insight into this subject. You are right, there hasn't been a lot of talk about this issue or the update in OSHA regs. You mention that you used to wear overshoes while performing certain task, now with this new update on step potential, are you suggesting that employees and contractors (or anyone for that fact) working in substations should where dielectric overshoes at all times? Also what is your stance on the grounding mats and overshoe pairing to protect against step potential? One last question, what is the highest reading you have ever encountered in regards to step potential, obviously this will vary for different voltages, environmental factors and other variables,? Again, thank you for your time and insight into this subject.
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