Incident Prevention has been covering personal protective grounding (PPG) for many years. Most of the emphasis has been on overhead applications for transmission and distribution. Lately, however, iP and many consultants associated with the publication have been receiving more and more inquiries from utilities seeking to understand the issues related to PPG applications in underground.
Part of the issue with PPG is that, as I mentioned, most training and rules seem to coalesce around overhead applications. The majority of the written standards – both OSHA and consensus – are found in sections dealing with overhead scenarios. It’s anecdotal, but it seems that most of the injuries or accidents discussed in the industry are also related to overhead. Still, the OSHA standard has requirements for PPG that do not specify or exempt underground applications, such as 29 CFR 1910.269(n), “Grounding for the protection of employees.” Employers have recognized that the risks we are discovering related to current flowing in grounded systems exist in underground, too. As responsible employers, they are seeking information, and not all of the information out there is good.
Current Flow in Grounded Systems
The following may be old news to many readers, but because it is foundational information necessary to understand the issues, it warrants a review so that everyone is on the same page. In the electric utility world, all neutrals and grounds are interconnected. This is good as it provides multiple paths for bleeding off fault currents and redundant connections for ensuring stable voltages in typical wye-connected systems. The downside is that all grounds are current paths. Being current paths, connected to current-carrying system neutrals, those grounds also have current flowing through them. In reality, there is some level of current flowing in almost every ground in the systems you work on.
Regarding those current paths, readers must be clear on an important principle in order to truly understand the remainder of our conversation. The principle is this: In an electrical circuit, when there are multiple connected paths, current flows in every available path inversely proportional to the resistance of the path. This means that while we have been keenly aware that an open neutral poses a hazard to workers, we have paid little attention to the risk of pole bonds.
That leads us to two other important principles. One, it takes at least 50 volts to break the electrical resistance of human skin. In other words, skin is a low-level insulator. If electrical voltage cannot penetrate the electrical resistance, the body cannot be shocked, which brings us to the second principle. You may hear more precise numbers, but most authorities agree it takes about 50 milliamps of current to be a hazard to the human body. Charles Dalziel performed research from the 1940s through the 1960s that is the basis for industry-wide acceptance regarding electrical risks to the body. Based on Dalziel’s outcomes, the industry uses 50 mA as the threshold for risk. Fifty mA is a little higher than the pain and muscle contraction level of 23 mA but considerably lower than the heart ventricular fibrillation level of 1000 mA. From Dalziel we also learned that the average male body resistance is about 1000 ohms hand to foot. These numbers are simplified for the purposes of understanding and do not represent all of the variables that can factor into injury, such as body weight, electrical path across the body and duration of the contact. Still, this information is important to the understanding of the next part I am going to present.
Readers are cautioned here that the following is not offered as a calculus for risk tolerance but instead as an explanation of what has been happening in the workplace, often unbeknownst to the workers exposed. Even to those employers who adopt an absolute zero tolerance for risk, if they are operating with underground systems connected, there has been and always will be current potential present that does no harm because the voltage is undetectable or the current does not reach the level of risk to the worker. When you realize that there is current flowing in grounds, the underground system presents a problem. If you live in a world of absolute zero tolerance for risk, you run into issues trying to work risk-free in underground. Before I try to explain what might have read like a flippant statement to some, let’s look at the practical issues and solutions.
Single-Phase Transformer Change-Out
Following is a classic scenario that we as an industry have bumbled through for years. I know this because 40 years ago I was one of those doing the bumbling, oblivious that there was a risk. The scenario is a bad elbow on a single-phase transformer in a neighborhood underground residential distribution (URD) system. On arrival, the crew determines they have to change out the elbow. Conventional safety practices in line with OSHA standards require the crew to go to the other end of the cable with the bad elbow and ground the cable. The cable is now grounded and considered safe to touch. Back at the bad cable end, the crew installs a temporary ground, closes the cable on it to ensure it is dead and then begins disassembly of the old elbow.
As a trouble crew foreman, I don’t know how many times I did this, perhaps hundreds. What I didn’t realize, and what many companies do not understand, is that putting the far end of the cable on a grounding bushing only accomplishes removal of one of two potentially deadly hazards. Temporary grounding of the far end of the phase ensures the phase conductor is not energized at phase voltage. As far as protection of the worker, grounding only removes one of two hazards. Grounding does ensure that the phase voltage isn’t present, but that’s all. In fact, grounding actually creates a second hazard. Sketch out the neutral, phase and ground connections in the previous scenario and you will see that the phase conductor, grounded at the far end, is nothing more than a parallel conductor with the concentric neutral. That condition creates a potential difference between the pin on the elbow or the phase conductor and any ground in the padmount where workers are getting ready to bare-hand system-connected elbow components. So why has it gone unrecognized for all these years?
Now remember, “Train the Trainer 101” is a column that provides practical explanations, so again, we are explaining the conditions and results of the conditions. Unlike a theory-calculated circuit, there are any number of interconnected systems that make the risks at the elbow change-out widely variable. The hazard in the previously referenced scenario remained unrecognized because there were very few – if any – injuries associated with the grounded phase conductor, which now is nothing more than an open parallel neutral. Just because few people you know – if anyone – have ever been injured with this scenario does not mean there is no risk, nor does it mean you can ignore the risk. Again, however, if you sketch it out, you will also see why the risk has been low. You should also be able to decipher that if conditions are right, that phase conductor can represent a real hazard.
Why Haven’t There Been Injuries?
The most probable reason there haven’t been injuries is because the open that creates the risk between phase conductor and any neutral or ground in the transformer, is actually bridged across by many other interconnections of the system. Remember that the phase conductor is grounded at the other end. The cable concentric you are working with is connected at the other end, too. At your end, if the concentric remains connected in the transformer, the cable phase conductor being worked on is bridged by the concentric connection. So, what if you have to disconnect the cable concentric you are working on? If the phase you are working with is part of a three-phase URD system, the other two cable concentrics are theoretically bridging the open at your location.
Yes, you say, that bridge two streets over is a long way away, but here is the practical explanation. The concentric is a grounded system, meaning there is no measurable voltage on it, except at points where there are differences in potential resulting from resistances in the respective parallel circuits. Remember the principle? Current flows in every available path inversely proportional to the resistance of the path. That means that as long as the parallel phases of the three-phase system are in relatively good shape, they are maintaining a jumper around the open we create when we open the concentrics or put a phase conductor in parallel with the neutral and then open one end.
That’s the explanation of why, but it doesn’t address the risks. In the explanation I presented earlier, the assumption is that the parallel systems’ connections are good and make up a low-resistance path around the open at your work area. The low-resistance path keeps the potential difference between the two conductors in your work area low enough – below the 50-volt threshold – so as not to pose a risk of electrical shock. If there is no shock, there is no current flow. But what would happen if one of the connections in one of the other phases were bad? After all, some of the systems we work on are more than 50 years old. Don’t forget that you are at that transformer to repair a cable failure. If those jumpers are defective or become defective while you are between the conductors, there could be a sudden rise of potential and you would be at risk. An open parallel circuit is not the only risk. Recall again, you are working on a cable terminal that failed. The system you are working on has a common system neutral. The system neutral is the return current conductor that also carries fault current dissipating into the ground in a fault. The circuits on that three-phase system are still humming along and your neutral is part of it. It is not impossible that a cable or system failure on any part of the system could create a fault-related voltage rise on the concentric or the phase in parallel with the concentric that you are working on. These same risks exist in any connected system exposure, including cable repairs in midspans between transformers.
About 10 years ago I wrote an article for iP dealing with this subject and suggested some means and methods of reducing risks, one of which was isolation or grounding the cable to ensure it is de-energized and then isolating it on a parking bushing to keep the phase conductor safe to touch. Many readers wrote in, concerned that I was making recommendations that violated OSHA requirements to ground the cable. But actually, OSHA recognizes the risks as previously described and permits not grounding the cable if grounding at the remote end creates risk. Specifically, in 1910.269(n)(7), OSHA states the following: “The employer shall ensure that, when an employee performs work on a cable at a location remote from the cable terminal, the cable is not grounded at the cable terminal if there is a possibility of hazardous transfer of potential should a fault occur.”
At about that same time, IEEE published an excellent paper entitled “Worker Protection While Working De-Energized Underground Distribution Systems.” The paper discusses in great detail the phenomena associated with current in underground neutrals as well as risks and procedures for grounding for protection of personnel, including isolation as a possible means of mitigating risks.
One comment you will find in the IEEE paper is that isolation is not without risks. My favorite method of isolation is to ground the cable on a grounding bushing and then – having assured it is de-energized – remove the grounding bushing connection from the transformer ground and connect it to the cable concentric. After the ground bushing is connected back to the cable’s concentric, disconnect the concentric from the transformer, thereby completely isolating both the cable and concentric from the system. At the same time, that connection puts the concentric and phase in series, eliminating the possibility of capacitance on the cable. The ground bushing must not be seated in the holder on the pad, and it is a good idea to wrap it in a blanket to ensure isolation. There is risk that if other system connections are poor or missing, the cable concentric you disconnect could be a critical system-neutral connection. Use of a clamp-on amp meter is one method of helping to determine the safety of disconnecting a concentric. A lack of current helps with decision-making concerning risks.
In summary, there are risks associated with current in URD systems. As the employer, you must analyze the risks and take steps to mitigate them. Education of the workforce is a good start.
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