A Close Look at Step and Touch Potentials
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The topic of step and touch potentials is controversial, which is precisely why we need to discuss it. In my role as a work methods auditor and consultant, I see more variations in how employers address step potential than in any other aspect of equipotential bonding. I know the reasons for this and will address them here. But first, I need to clearly state the following:
- The theoretical argument for hazardous step potential in electric utility work environments clearly exists.
- Every employer must assess the hazards of step potential in their work environments and adopt a plan to protect exposed workers.
- Every employer must train their employees to recognize step potential hazards and employ the procedures necessary to protect workers from step potential hazards.
Anyone who has read my past articles knows I approach each topic from the aspect of OSHA requirements, the consensus standards, best practices, and many years of research, training and experience. I have written about step potential before, most recently regarding horizontal boring operators being compelled to wear dielectric overshoes by manufacturer operator manuals. I received a lot of feedback from that article, some of it accusing me of ignoring the risks. That is not the case and will not be the case here. I know very well that step potential is possible. As a matter of record, my first incident forensics case was for the family of an 8-year-old boy who was fatally injured by step potential created by an improperly installed underground community baseball-field lighting system. Yes, I know when and how step potential becomes deadly.
As an employer or a worker, you must have the information necessary to perform a proper and effective analysis of the hazards associated with your work. An improper understanding can lead to a lack of preparation. On the other hand, a lack of information can also lead to overreactions that can include risks inherent to the work methods that are not necessary. Yes, if you don’t understand the nature of the risks and can’t decide which expert is right, all you have to do is equip everyone with dielectric boots and rubber gloves and you will prevent step and touch exposures. And yes, that is impracticable for a host of reasons, so read on and we will solve this the practicable way: through research.
What Does OSHA Have to Say?
First, let’s look at OSHA. As I understand it, OSHA has always respected the hazard of step and touch potentials occurring around energized equipment. And at least since 2014, OSHA literature and guidance have specifically included utility towers and ground rods as sources of step and touch hazards. The rules include the famous catchall, 29 CFR 1910.269(p)(4)(iii)(C). It’s a huge rule, so I will not repeat it here in its entirety. The rule generally requires that if the methods used to protect workers do not ensure protection, you must satisfy four specific requirements. Requirement three is the use of equipotential mats. Requirement four is to use insulating protective equipment or barricades to guard against any remaining hazardous potential differences. Equipotential mats are self-explanatory. Insulating protective equipment could be rubber or cover, but since the phrase includes “guard against any remaining,” the assumption is that the rule is referring to personal protective equipment such as protective footwear and rubber gloves. Throughout OSHA’s litany of PPE guides, the understanding is that employers use procedures, engineering barriers or guards to protect employees, with PPE being the last layer of protection. For that reason, we take rule 1910.269(p)(4)(iii)(C)(4) to include foot and hand protection through PPE, such as dielectric or insulating footwear and rubber gloves. In addition, a reading of Appendix C, “Protection From Hazardous Differences in Electric Potential,” finds references to use of PPE and distance as a means of protecting employees from step and touch potentials.
We can also take advantage of some OSHA guidance included in 1910.333, “Selection and use of work practices.” Some readers will say that this section does not apply; however, they are wrong. I have defended utility employers against this section, and the section was ruled as applicable because of the very precise wording of this note: “The work practices used by qualified persons installing insulating devices on overhead power transmission or distribution lines are covered by 1910.269 of this Part, not by 1910.332 through 1910.335 of this Part. Under paragraph (c)(2) of this section, unqualified persons are prohibited from performing this type of work.” Did you notice the precise wording of the phrase “installing insulating devices”? Utility operators of trucks are covered by this section, but that is good because of section 1910.333(c)(3)(iii)(C), which states, “If any vehicle or mechanical equipment capable of having parts of its structure elevated near energized overhead lines is intentionally grounded, employees working on the ground near the point of grounding may not stand at the grounding location whenever there is a possibility of overhead line contact. Additional precautions, such as the use of barricades or insulation, shall be taken to protect employees from hazardous ground potentials, depending on earth resistivity and fault currents, which can develop within the first few feet or more outward from the grounding point.” Here is a valuable reference for the employer in OSHA, noting that hazards develop “within the first few feet or more” from the source. This statement is not found in 1910.269.
To comply with the OSHA standards, we must clearly interpret a somewhat confusing paragraph in Appendix C. Let me warn you from experience: I have had to defend employers against this paragraph before the Occupational Safety and Health Review Commission, an independent agency created by the U.S. Congress to adjudicate contested OSHA citations at the federal level.
Paragraph (III)(D)(2) to Appendix C, “Acceptable methods of grounding for employers that do not perform an engineering determination,” is written in very precise legal terms designed to be easily interpreted in the event that an employer contests an OSHA violation based on Appendix C guidance. Of value to employers is that this paragraph clearly explains the primary rule requiring equipotential grounding in the workplace – if you can figure it out.
The paragraph explains that rule 1910.269(n)(3), “Equipotential zone,” was written to satisfy two principles: tripping the circuit, and to ensure that potentials in the work area are as low as possible (bonding). However, the paragraph also explains that 1910.269(n)(3) does not specifically require that the employer’s grounding methods “meet the criteria embodied in these principles” (tripping and bonding). In its specific wording, 1910.269(n)(3) requires that the protective grounds be “placed at such locations and arranged in such a manner that the employer can demonstrate will prevent each employee from being exposed to hazardous differences in electric potential.” In essence, this guidance from OSHA says the intent of the rule is tripping the circuit and bonding the worker, but the requirement of the rule is demonstration that the intent is satisfied.
Then we get to the real hook. In every appendix throughout the OSHA standards, there is text stating that the appendix does not create mandatory requirements for the employer, explaining that the procedures in the appendix can assist the employer in meeting the requirements of the standard. In some cases, such as in Appendix C, we read that the “Occupational Safety and Health Administration will consider employers that comply with the criteria in this appendix [Appendix C] as meeting § 1910.269(n)(3).” That means the employer must perform an engineering analysis of risks and protection methods if they don’t follow OSHA’s procedures generalized in Appendix C.
My first question is, who did the engineering analysis for the work methods called out by OSHA in Appendix C? The answer is, not OSHA. OSHA is relying on the research of others to compel employers to follow OSHA procedures or do their own engineering analysis. The methods in the appendix are based on the electrical physics of charge, transfer and current flow, as well as accepted theory. OSHA knows these steps will work and is using the threat of citation over an engineering study as leverage to get employers to take seriously the risk of step and touch potentials. And we should do that. The reality is that many employers don’t take any precautions to protect workers from step and touch potentials, and that is what the appendix is trying to change.
By the way, OSHA does not specify what the engineering determination consists of nor does the agency require any calculations or protocols to be used to meet the requirement of a determination. If the employer turns the task over to a qualified engineer, the assessment may only need to be an assessment of system protection compared to work methods, PPE and barricades. It’s up to the engineer to decide.
The bottom line is that OSHA expects the employer to either perform an engineering analysis of the employer’s grounding methods to ensure the worker is protected, or you can simply follow the guidance in Appendix C. So now, let’s look at the issues.
The first one – touch potential – is very easy. Solution one is, if touching the equipment will get you electrocuted, don’t touch the equipment. Yes, it’s that simple. There are two conditions to consider. One is direct contact with the equipment with an energized bus. That could mean the boom contacts the bus or the bus falls on the boom. If either of those things occur, anyone in contact with the equipment who is standing on the ground or who is not at equipotential would be at risk. The other is a grounded truck. If a truck is grounded to a system neutral or a pole bond that is connected to a system neutral, the truck is a parallel path to ground through any worker standing on the ground touching the truck. In either case, not touching the truck mitigates the risk. Of course, this is only part of the problem and doesn’t solve step potential risks.
I’m sure some readers are now wondering about grounding the truck for tripping the circuit. Well, here is the deal: Tripping the circuit removes any continuing threat of electrical hazard to workers. This may sound like a joke, but it’s truly a response to the OSHA issue. If an ungrounded truck does not trip the circuit quickly or does not trip the circuit at all, and if no worker is put at risk by that condition, would OSHA cite that employer? The answer is no. OSHA does not care if you burn the truck to the ground as long as no one is at risk. So, whether the truck is grounded or not, if touch potential is the issue, there is no risk to the worker if they are not in contact with the truck. Again, this only solves the touch potential issue, not the hazard of step potential.
Gradients, or step potential, are a little more difficult. First, let’s clarify that the risk is not the level of voltage in the ground; it’s the level of voltage between the worker’s feet that creates the hazard. The voltage between your feet is created by the voltage drop that occurs across the resistance of the earth between your feet. The circuit voltage into the earth may come from a 13.8-kV circuit or a 345-kV circuit. The earth will substantially reduce that voltage, but it’s still the voltage drop at your feet that creates risk. The question we should be asking is, how hazardous is step potential?
Earlier, we discussed issues created by grounding trucks associated with touch potential. The fact remains that if you don’t get the boom in the bus or drop wire on it, nobody is at risk, so why ground it? But if you can’t assure a truck will not become energized, there is no doubt that effectively grounding a truck will lower step potential by limiting voltage and current leaving the truck into the earth. Effective grounding collapses voltage and divides current between the low-resistance ground path and the path through the high-resistance earth. The theoretical basis is clear that gradients will occur. The employer must decide both where they will occur and how dangerous they may become. Not every space around a truck, structure or ground rod will be at the same level of risk. A means of protection includes defining the spaces that are risky and keeping those areas barricaded. I want to mention here that a barricade is a line of cones or maybe even barricade tape. A barrier is a physical separation, such as a construction fence. When we train our workers to respect the invisible barricade, defined by a frame of cones, and when we remind workers during the tailboard how to respect that invisible barricade, we meet the requirement of barricading a prohibited space.
Now, back to the question: How hazardous is step potential? That question brings us to current flow versus potential in the analysis of the step potential hazard. Voltage absent current is not a deadly hazard. That is partly why the industry has mixed feelings regarding step potential. I find that we talk about it, but in the field, most workers don’t worry about it. You would have a hard time finding a lineworker who knows of someone who was killed by step potential in a utility workplace. For that reason, few workers are concerned that step potential might be a threat.
What about the research? Any aware reader would see that in research on step potential, current is not an aspect of the data but an assumption of testing. I would be interested in speaking with any researcher who has measured current flow across the ground from an electrode. Every test published can measure current through the electrode from the source, but I have yet to see any tabulated data that measures current 10 feet away from the electrode. That is pretty easy to explain, and it also explains why the consensus standards have considered step potential a negligible risk up until the most recent revisions. Step potential is a theoretical concept that can be measured with the right instruments. Current flow is also a theoretical concept that has not been demonstrated in testing with surface-bearing electrodes. Herein lies the problem. We control or equalize potential because if a worker is not exposed to a potential that can penetrate their body insulation, current cannot flow and the worker cannot be injured. Even if there is a highly unlikely possibility of current flow across the soil, a potential that can penetrate the worker’s skin would allow current, if it were present, to penetrate the worker and cause a fatal injury. For the most part, current flow is also a theoretical concept that can’t be demonstrated in testing with surface-bearing electrodes. Since voltage without current represents a negligible risk, very little attention has been paid to it.
Here is the difference between voltage and current and dirt as a conductor. Voltage is developed across the earth simply by electrical transfer of the charge between conductive elements in the soil. Current must have a path to earth to flow horizontally across the soil. That is almost impossible without an extremely high voltage to push it, and that voltage is hard to develop in a grounded circuit. Current cannot flow where there is no conductive path to ground. However, as noted in IEEE 80, “IEEE Guide for Safety in AC Substation Grounding,” if there were a high-resistance layer beneath a lower-resistance layer of conductive soil, and if the soil were conductive enough to support current flow, and if the current flow were a path across the insulating layer into a path to ground, then current could flow with the voltage. For this reason, we cannot assume there is no current risk. Now, what do we do about it?
For decades with contractors, we have been constructing equipotential grids during stringing, pulling and splicing operations. We have also been using them around the bases of structures. You will find these site-constructed grids mentioned in both Appendix C to 1910.269 and in the IEEE consensus standards. The grids are typically constructed using 16-foot-by-50-inch cattle panels made of #4 welded galvanized wire or concrete-reinforcing panels made of uncoated, welded #4 wire. Today there are commercially available grid sections that can be assembled to create the same effect. These eliminate step potential around any equipment staged over them.
Where a grid is not used, the most recent study I have read – and found great value in – is the Electric Power Research Institute’s “Vehicle Grounding and Personal Utility Distribution Guide Technical Report 2019.” This is a copyright-protected document and the property of the utilities that sponsored the research. For those reasons, I cannot reproduce its tables or citations. I can share with you that many of the findings support what we are discussing here. Anyone can purchase a copy of the report from EPRI (www.epri.com) at the sponsor’s fee. What I found most valuable is the measuring of actual step potential voltages, not just the voltage measured in the ground. There is also a valuable section on the efficacy of insulating, dielectric and electrical hazard-rated boots. Further, there are recommendations for protective actions based on the research. In the summary of the EPRI findings, as I have been recommending for years, there is not one solution but a system of steps used together that help to ensure worker protection against step and touch potentials.
In summary, to meet the requirements of 1910.269(n)(3), the requirements of 1910.269(p)(4)(iii)(C) and the guidance of Appendix C:
- An engineering determination must be made to quantify the hazards of step potential and mitigation strategies for the workforce.
- Those who don’t use an engineering analysis must use a combination of methods as described in Appendix C.
- Training for the workforce must be delivered regarding the hazards determined and the strategies to be employed to protect the worker.
Regarding the second bullet above, based on all the discussion in this article, here are some recommendations for combinations of work procedures that can limit risk to workers as part of a system of protection.
For all system voltages:
- Observers for aerial booms to prevent contact.
- Good and effective cover to prevent contact.
- Work plans that eliminate trips to the truck.
- Position vehicles out from under conductors when practical.
- Consider not grounding to limit voltages being coupled onto vehicles through the neutral (if no workers are endangered as specified in 1910.269(p)(4)(iii)(C)).
- Ground vehicles that are an electrocution hazard to the best available ground.
Where vehicles are grounded to the system or subject to induction-coupled magnetic current:
- Prohibit contact with vehicles that are grounded to the system.
- Bond conductive-boom man baskets to the grounded bus to protect workers aloft.
- Establish barricades for non-entry spaces around equipment.
- Maintain distances from equipment away from hazardous gradients.
- Employ dielectric, insulating or electrical hazard-rated boots where hazardous gradients cannot be eliminated.
- Use equipotential mats with insulating approaches where touch potential or gradients are a risk.
- Lay a bonded grid to create a plane of equipotential around all trucks, structures and ground rods.
- In the absence of any combination of work methods, use dielectric overshoes and rubber gloves rated for the system voltage or expected voltage differences.
Whatever your motivation – whether it’s just to check a box or to seriously protect workers – don’t wait until it’s too late to take the steps. Have a defensible, compliant plan in place.
About the Author: After 25 years as a transmission-distribution lineman and foreman, Jim Vaughn, CUSP, has devoted the last 24 years to safety and training. A noted author, trainer and lecturer, he is a senior consultant for the Institute for Safety in Powerline Construction. He can be reached at firstname.lastname@example.org.
Paragraph (III)(D)(2) of Appendix C to OSHA 29 CFR 1910.269
“The grounding methods presented in this section of this appendix ensure that differences in electric potential are as low as possible and, therefore, meet § 1910.269(n)(3) without an engineering determination of the potential differences. These methods follow two principles: (i) The grounding method must ensure that the circuit opens in the fastest available clearing time, and (ii) the grounding method must ensure that the potential differences between conductive objects in the employee’s work area are as low as possible.
“Paragraph (n)(3) of § 1910.269 does not require grounding methods to meet the criteria embodied in these principles. Instead, the paragraph requires that protective grounds be ‘placed at such locations and arranged in such a manner that the employer can demonstrate will prevent exposure of each employee to hazardous differences in electric potential.’ However, when the employer’s grounding practices do not follow these two principles, the employer will need to perform an engineering analysis to make the demonstration required by § 1910.269(n)(3).”