Train the Trainer 101: Solving PPG – Without Electrical Math
This installation of “Train the Trainer 101” may have an odd title, but it was inspired by some recent conversations I’ve had. I’ve learned a lot about personal protective grounding (PPG) in the past 20 years, and I continually learn even more as others share their research and experiences. Some time ago I learned that much of the fundamental electrical math upon which electrical circuit theory is based does not adequately explain the risk from high currents imposed on grounded systems. That does not mean there are not theoretical explanations for all of the results in high-current fault testing. But the simple circuit math of Ohm’s law cannot explain the complex electrical physics that occur in a high-current fault, and that is partly what confuses the issue concerning EPZ.
What is simple is this fundamental of worker protection: It takes 50 volts to break the electrical resistance of a worker’s skin. If you can break the electrical resistance of the skin, current can flow, and the worker can be injured. However, if voltage cannot penetrate the skin, current cannot flow. You cannot eliminate system current by grounding; you can only divide it (i.e., send most of it through a different path) and hope for the best. But you can eliminate voltage in the worker exposure. You eliminate voltage potential by bonding. Once you’ve eliminated the voltage potential hazard, current no longer matters and thereby the risk is altogether eliminated.
If you are still grounding for worker protection, you are content to estimate the exposure a worker will face in a parallel path between phase and ground, and you are content to live with the risk. If you are still grounding for worker protection, that means you also are willing to ignore the OSHA rule on bonding found at 29 CFR 1910.269(n)(3). The rule was renamed in 2014 to make its intent clear. Its new title is “Equipotential zone,” and it states, “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.” Why is the rule important? It’s important because equalizing potentials is a sure way of protecting workers as opposed to hoping for a favorable imbalance of hazardous current across parallel paths. Why else is the rule important? Because lineworkers are still bracket grounding, assuming the low-resistance grounds will pass more current than what will cross their body in a fault. It’s a gamble. Mathematically, I could estimate current across a lineworker in a parallel path with a ground. I could do that if I knew how much current would be available, and if I knew the impedance value of both the grounds and the lineworker. I could be pretty accurate if I knew how well the conductor was brushed, and the electrical integrity of the ground connection in parallel with the lineworker. Speaking of maximum current, the calculation of current that will be seen on a meter is not near what will be in the asymmetrical fault during the first 15/60th of the fault duration. I’m not going to explain asymmetrical faults in this article, but I recently saw a utility’s presentation at their quarterly contractor safety meeting that measured some five times the asymmetrical current exposure than was RMS measured in fault. That’s important to you, the lineworker, because it means that the cables likely will survive the short-term magnitude, but if you are part of the circuit … well, you can figure that out.
So, can you altogether dispense with the math, the calculations and the estimates for current division across the worker in your PPG? Yes, as far as the bonding of your workspace is concerned. You still need to know what size ground cables are needed to manage the fault current for the time required to clear the breaker, but that’s not about protecting you, the worker. You are protected by bonding, and fault-current calculations are about cables and circuit tripping. If you are bonded in, the only thing that matters is that bond connection integrity. That’s the protection without the math that I was talking about. It’s that simple. It doesn’t matter how much current is available in the system. If you are bonded in, there is no potential difference in the workspace. If there is no potential difference, then current, no matter how much is available, can’t flow and the lineworker can’t be hurt. There does need to be practical cable size as well as arrangement. Your cables for fault clearing do have to be sized for the fault. It’s how you arrange those grounding cables that creates bonding of your workspace, or the connections you add that bond your workspace.
Bracket Grounding is Not the Best Protection
Here is some more news: Bracket grounding is not the best protection. In fact, in most cases it is certain to fail to provide protection. Why? The circuits we work on have impedance. That impedance limits current flow. When we install grounds, we are creating a low-resistance path to ground meant to cause immediate operation of the protective device. With grounding installed, the circuit impedance is lowered so that high current can flow, tripping the circuit protective device. So, grounding, installed to increase current flow, also becomes the limiting factor for the total current that can flow, particularly for the first few cycles. The thought has been that the low-impedance path to ground will carry the majority of the current, and very little current will cross the lineworker. It sounds good. In fact, I heard this logic as late as just a few weeks ago: If one set of grounds is good, then two must be better because they will carry more current and the current division will be even lower across the lineworker. But that’s not what happens; what happens is that the second set of grounds further lowers the circuit impedance and is likely to increase the current and the voltage in the path across the lineworker. Yes, brackets may increase the risk. Here is the scenario: A lineworker is on a wood pole in contact with a grounded primary phase. The grounds are installed between phase and system neutral. There is a parallel path – one through the grounds, one through the worker. Here is the explanation:
- Current follows every available path. With a single set of grounds as described above, there is one path through the grounds between the phase and ground, and a second path through the worker between phase and ground.
- Current flowing through parallel paths divides according to the resistance in the path. In a reasonable estimate, the electrical path through the worker has an electrical resistance of no less than 1,000 ohms. The electrical path through the ground set will have a resistance of less than 2 ohms. Much more current will flow through the ground path than the worker.
- The grounds increase the amount of current that can flow in the fault, causing a quick operation of the protective device. However, the impedance of the ground also creates a limit to the total current flow. Doubling the grounds increases the total amount of current that can flow in the ground path, raising the current available that will divide between the paths, including the lineworker.
The hope is that the current through the worker will be low enough to prevent a serious injury.
There is no doubt that there have been occasions when the gamble worked – the current divided in the parallel paths and the worker was not seriously injured. But we don’t have to gamble if we bond.
Placement of Grounds
So, here is even more news. We get the question all the time about – and see procedural specifications all the time for – placement of grounds. The majority of those specifications require the grounds to be on the source side of the structure where the lineworker is working. Well, it doesn’t matter which side of the pole the grounds are on. If you are engaging in equipotential grounding, one side is not better than the other, and it doesn’t matter which side of the pole the station protective device is on. The reason is that grounds do not protect the worker. Bonding protects the worker.
It is true that in fault testing of grounds in parallel, the ground closest to the source carried more current. It also is true that when parallel grounds are separated by progressive 1-inch increments, the current between the two paths divides in a progressively more imbalanced fashion. All of that knowledge applies to parallel grounds. It has absolutely nothing to do with protecting the worker who is at equal potential. We ground to cause operation of the protective device. We bond to protect the worker by controlling and eliminating the unequal potentials in the workspace. Once we have eliminated the unequal potentials in the workspace, the electrical hazard to the worker is controlled.
I am concerned about lineworkers when we ignore bonding principles, putting lineworkers at risk. I know it can be confusing, especially when so many error-laden opinions and theories have been passed around for the last 30 years. I hear from well-meaning parties who have gotten grounding and bonding principles confused, and I see people confusing the law of parallel paths for current with EPZ and the task of equalizing potentials. I see simple division used to estimate current across a lineworker in a parallel path with a grounded circuit as justification for bracketing. Worst of all, I see bracketing – that is, hanging grounds on both sides of the workspace – promoted as a best practice, when incontrovertible evidence based on theory and actual measured testing proves that bracketing is not twice as good as a single set of grounds. In fact, it actually doubles the voltage across the lineworker compared to a single set of grounds protecting the same workspace.
The Bottom Line
The bottom line is this: Grounding is used to increase current to trip circuit breakers. It will not protect workers from an electrical shock risk. Bonding controls voltage by eliminating electrical potential differences in the workspace. Eliminating potential differences eradicates electrical shock risk and prevents injury by preventing current flow across the worker. And, when the worker is bonded in, it does not matter what the circuit current is.
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].