Train the Trainer 101: Current in Grounds Can Kill
Over the past six months, three things have happened that I want to mention. First, I have answered numerous questions from clients and Incident Prevention readers regarding personal protective grounding (PPG). Second, the industry has experienced a rash of injuries and fatalities related to current in grounded circuits. The incidents most often have been associated with induction, but not always. And third, I have consulted with utilities and contractors, large and small, who are just now recognizing they have issues understanding PPG. It’s been hard to gauge the numbers – such as the frequency of incidents and especially comparing the seriousness of injuries – because there is no reliable clearinghouse for tracking incidents other than fatalities reported to the U.S. Department of Labor.
All of this is beside the real point, however, which is that there is no reason for any of these incidents to have occurred at all. Well, there is one: The utility industry is behind the curve in their understanding of the phenomenon of current in grounded conductors. There is an explanation for that, and it’s time to write about it again.
Let me be clear: The purpose of this article is to work toward solving the problem, not to find fault. To understand how we got to where we are, let’s first talk about industry awareness. Anyone who does research on the fundamentals of utility system grounding will notice that we have been struggling with PPG since as early as the 1950s. This has been documented in various papers from the IEEE archives of “Proceedings of the IEEE” – one of the first electrical industry journals, established around 1927 – and in “IEEE Transactions on Power Systems” since 1985.
As the IEEE 1048 standard, “IEEE Guide for Protective Grounding of Power Lines,” points out in the introduction to the 2003 edition, “Protective grounding methods have often not kept pace with their increasing importance in work safety as the available fault current magnitudes grow, sometimes to as high as 100 kA, and as right-of-ways become more crowded with heavily loaded circuits, leading to growing problems of electric or magnetic induction.” Did you notice the date of the standard? The 2003 edition is a revision of the 1990 standard on protective grounding. As I stated earlier, we’ve been struggling with PPG since as early as the 1950s. Over 60 years is a long time to still not have figured it out.
But as the IEEE 1048 introduction indicates, today’s fault current magnitudes and loading are part of the reason injuries on grounded systems have suddenly become a recognized issue. As you read industry journals and consensus standards, you may begin to understand the disconnect, which is how the standards are written and how the knowledge is applied in the field. As I often have written, my years as a lineman, even for one of the nation’s most respected utilities, lacked training that demonstrated an understanding by the utility of the hazard of current flow in grounded systems. In later years, my limited engineering training never addressed the hazards to workers created by multigrounded systems. Engineers, based on historic needs of utilities, essentially train toward efficient, quality power delivery, continuity of service to customers, and system protection to prevent damage to facilities and risks to the public. I need to say here that this is not an indictment on engineers or utilities. To do so would be inaccurate and unfounded.
Now, to utilities’ historical conundrum, let’s add OSHA’s contribution. OSHA knew there was an issue when preamble discussions debated “grounding” as a means of employee protection. The wording of the 29 CFR 1910.269 standard is part of the problem. In the 1994 version, the rule stated, “Temporary protective grounds shall be arranged to prevent each employee from being exposed to hazardous differences in electrical potential.” Most everyone knows that if you are in contact with a truck and standing on the ground, you are at risk of electrical injury whether the truck is grounded or not. Grounding is not a means of protection (see principle 2 below). Arranging of the grounds is protection. Arranging of the grounds to protect the employee is bonding. The grounding can be arranged to create bonding for employees, or more connections can be added to create bonding for the protection of employees. Many employers and contractors did not read that rule regarding arrangement as a requirement for some form of equipotential protection, even though that was the intent. In 2014, OSHA revised the rule to make the intent clearer by adding “Equipotential zone” as the title to rule 1910.269(n)(3), which states: “Equipotential zone. Temporary protective grounds shall 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.” The rule applies to fault current in particular, but it is exactly the same for grounded systems where induction is present.
This is significant because equalizing potentials, next to total isolation of the circuit, is the most effective method of protecting workers from electrical hazards of current flow in grounded circuits. Where you can’t equalize by bonding, insulation or isolation is required.
Five Important Principles
Equipotential is the solution, but effective education is what’s been lacking. When I train on PPG, I begin with a review of basic electricity. It doesn’t matter if the audience members are engineers, lineworkers or civil constructors. Here are the five most important principles of basic electricity, followed by examples, that will give any worker an understanding of the issues, the hazard of current in grounded circuits, the risks associated with the hazard, how to see the hazard and how to remediate the hazard.
- Current flows in every available path, inversely proportional to the resistance of the path.
- The purpose of grounding is immediate operation of a circuit protective device. The purpose of bonding is protection of workers.
- In the industry, we accept that 50 volts is the level of voltage necessary to become a risk to workers. We accept that if voltage can penetrate the resistance of the worker’s skin, current can flow.
- If current flow across the worker is greater than 50 milliamps, there is a shock risk to the worker.
- Any resistance introduced on a circuit, including a grounded circuit, results in a voltage drop that can be dangerous to the worker.
Principle 1 is the preeminent principle that needs to be understood. The best scenario is a system neutral. Everyone understands the neutral is the circuit return path and has current on it. Not everyone realizes that anything connected to the system neutral becomes a path from the neutral. Pole bonds are number one. Typically a higher resistance than the neutral, the pole bond is nonetheless a path to ground, and it will have current on it. How much current depends on the resistance of the available paths and how much current is on the neutral.
Just like a pole bond – and this will disturb some readers – a truck connected to the system neutral for the purpose of grounding the truck also becomes a path to ground. I know of many utilities that prohibit bare-hand contact with a system neutral but do not prohibit touching a truck connected to the system neutral. Yes, OSHA says in 1910.269(p)(4)(iii)(C)(1) to use the best available ground to minimize the time the lines remain energized. Yes, the system neutral likely is the best available ground path, just like most agree that for transmission, the static is the best available ground. That doesn’t mean connecting to the neutral is not without risks. OSHA didn’t say that, and in fact, OSHA has not used language anywhere that says to use the neutral as a ground. There are several standards that list the neutral as the best ground, but not many with a discussion of the risks of doing so. It is the employer that must decide what the risk is and then take steps to protect the worker. I have always assumed that is why, following the 1910.269(p)(4)(iii)(C)(1) rule, you find the requirements to provide ground mats to extend areas of equipotential ((p)(4)(iii)(C)(3)) and to employ insulating protective equipment or barricades to guard against any remaining hazardous potential differences (((p)(4)(iii)(C)(4)). There are other ways to accommodate the risk of current flow from a truck connected to a system neutral, but those two OSHA rules will eliminate the risk.
I expect readers to ask, why haven’t we been getting shocked using the neutral as a truck ground? Well, the answer is that we have. Over the years, I have investigated several worker shock incidents that involved trucks grounded to a system neutral or a pole bond connected to a system neutral. It is simply coincidental, depending on the level of current in the system neutral at the time contact is made. Remember principles 3 and 4: If the path – in particular, the gap (principle 5) between truck and earth – does not result in a voltage drop high enough to be a hazard, no current will flow. This coincidental condition is exactly the same when dealing with downed or open neutrals. No overhead lineworker would ever bare-hand across an open neutral or handle an open neutral bare-handed while standing on the ground. But for decades, we have been sitting in ditches, reconnecting concentric neutrals bare-handed without a second thought. Talk about latent hidden risk. The concentric neutral is no different from the overhead neutral, but we have thought about them differently forever.
There is still another risk associated with parallel paths that I have identified as the mechanism of injury in several investigations. The risk is when the current involved is induced. This is more prominent with transmission, but it also occurs in distribution wherever there are energized circuits parallel to de-energized conductors. When you ground phases to a tower or structure of any type, if there is induction on the phases, grounding will couple that induction current to the structure. I have personally measured as high as 2,600 volts AC, and up to 140 amps on towers. I know of colleagues who have reported even higher voltages. What would happen if you grounded a truck to a structure with 2,600 volts on it? Whether you can measure the voltages on the tower or not, understanding the possibility that they are there and why they occur are the first steps to protection of workers. The solution is arrangements of cables to bond around workers, equipotential mats, or insulation and barricades placed to protect against risks identified by a competent review of the electrical energy sources.
The last principle we have room to address is No. 5: Any resistance introduced on a circuit, including a grounded circuit, results in a voltage drop that can be dangerous to the worker. To explain this, I use the image of a gap. However, this does not just apply to open points in a circuit. A gap can be a point of resistance on a circuit or a point of resistance in a parallel path with a current-carrying circuit. The last example is the step-and-touch idea. Referring back to our energized pole bond, here is a detailed explanation of how touch can be deadly. A pole bond can have high enough current on it, particularly in fault conditions, to develop a voltage drop. It does not appear energized because it is grounded, but it does have current in its path from the top of the pole to the butt wrap in the ground. Since the pole bond circuit is grounded, in many safety standards it is considered safe to touch. However, a worker is at risk if the individual has sufficiently low body resistance and is standing on sufficiently conductive earth, and where the current on the pole bond is high enough. There is a gap between the pole bond and earth. If the worker touches the pole bond, he is a resistance introduced in the gap completing a circuit to ground, parallel with the pole bond. The same condition applies to a worker at the foot of a tower or back of a truck, making up materials.
Being able to identify hazards is part of the skill set necessary for workers to protect themselves. Workers must be able to understand the sources of current flow, the hazard of current in grounded circuits, the risks associated with the hazard, how to identify the hazard and how to protect themselves from it.
About the Author: After 25 years as a transmission-distribution lineman and foreman, Jim Vaughn, CUSP, has devoted the last 20 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 [email protected].
Editor’s Note: “Train the Trainer 101” is a regular feature designed to assist trainers by making complex technical issues deliverable in a nontechnical format. If you have comments about this article or a topic idea for a future issue, please contact Kate Wade at [email protected].