December 2021 – January 2022 Q&A
Incident Prevention magazine still receives many questions about the different facets of equipotential bonding. In this installment of Q&A, we provide detailed answers to several of the most frequent questions in an effort to help the industry better understand and resolve these issues for the safety of their workers who use temporarily grounded systems.
Q: We have heard that we should be bonding baskets to the grounded transmission or distribution bus to equalize voltage differences between the basket and grounded phase. The idea of using a jumper bond from an insulating aerial lift to the grounded overhead conductor is new to everyone. How does this work, why do we need to do it, and how should we do it?
A: This issue is all about equalization of potentials. Potentials are the voltage states of different surfaces. When two surfaces are of different potentials, they will equalize their charges when connected together. An insulating basket is not a pure insulator. Current will flow across the boom, something that is clearly demonstrated in annual bucket testing, which is actually testing boom current leakage. Bonding an insulating basket to a grounded conductor has only one purpose and that is regarding a grounded circuit that becomes re-energized while workers in the basket are in contact with the grounded phase. Even if the phase is grounded, there is an instant that the circuit energizes before the circuit protection de-energizes it. The grounding circuit will have much lower voltage than normal because it is grounded and current still flows. The phase current in the grounded circuit and the boom leakage make them different potentials. A worker between basket and phase would equalize that charge across their body. This has happened in the past, and in the few cases we are familiar with, it was uncomfortable for the worker involved, but they did not sustain a serious injury. In this scenario, the worker does not receive a deadly shock because the current flow is limited by the resistance of the boom. We are not doctors, but a cardiologist we consulted explained to us that a worker with heart conditions could experience a medical emergency if they are exposed to an equalization of potentials across their body. That is enough for some utilities that have a zero-shock tolerance to require bonding fiberglass insulating buckets to the grounded circuit conductors they are working on. Historically, lineworkers realized you can get shocked between grounded phase and insulating bucket, but the risk was considered negligible.
It’s a different story when a worker in an insulating basket gets between a grounded phase and a pole or crossarm that is not bonded to the phase. In this case, the boom would not protect the worker from electrocution. A worker who gets between a grounded phase and path to ground through the pole is not protected by the insulating basket.
Utilities and contractors should be bonding conductive baskets to grounded conductors. The conductive basket and boom are a path to ground and that is why this is an OSHA rule.
There are no specifications for the equipment to use; it is up to the utility. It could be as small as #4 if you are simply bonding the basket. However, the size of the bond between basket and phase does depend on the resistance of the path through the truck to ground. If a worker is in a conductive aerial device, that boom is conductive. That low-resistance path increases high current across the basket in a fault. At low current for a short duration, that #4 will limit the current flow across the basket. However, if the fault current is high and lasts longer, the #4 bond would fuse or burn in two. That would isolate the basket but expose to the fault anyone who is in contact with the phase. In that case, the bond and ground should be the same size depending on the number of paths to ground created by the brackets provided for fault current division. It’s not documented, but we see basket bonds using 1/0 with a 40# to 80# magnetic separable connection at the lower end.
Q: In transmission rights-of-way, workers in fiberglass insulating buckets are constantly getting shocked. If the fiberglass bucket is insulating and the employee is inside the bucket, how does bonding the exterior of the bucket bring the employee and conductor to the same potential? A grounding mat inside the bucket would need to be utilized, correct?
A: All booms leak voltage, so any insulating boom will be at a different potential than a grounded conductor. Since the insulating basket is at a different potential, an unbonded worker inside the insulating basket will be at some potential relative to the basket and the worker’s contact with the basket. Contact between all three – worker and basket, worker and grounded wire, and basket and grounded wire – will cause any potentials to be equalized. It is especially noticeable in rights-of-way with energized 230-, 345- and 500-kV circuits parallel to the de-energized and grounded circuit. In very high induction, that difference can also be very high as well as painful. A bonding mat typically does not work in this exposure while the worker is wearing work boots that insulate them from the basket and conductor. Bonding an insulating bucket is meant to prevent the discomfort experienced in high-induction environments where workers will sense a shock with every contact with the grounded line. Those shocks can often be prevented with rubber gloves. Simply bonding the basket eliminates the difference in potential and usually eliminates the shock. When induction is very high, the worker’s body can receive a voltage charge separate from the bucket and boom because the worker is insulated from the basket by their boots. This can happen even if the basket is bonded to the grounded conductor. Conductive-soled shoes bonded to the worker’s leg or conductive boots and conductive socks will bond the worker to the floor of the basket and eliminate that shock. These are nuisance shocks and not a hazard of injury because the insulating boom prevents current flow and therefore electrocution risks. Still, we must state here that a cardiologist we consulted told us that a worker with an undiagnosed heart condition or workers with some types of diagnosed heart conditions should avoid even nuisance shocks.
Q: Please explain the idea that voltage differences can appear on a grounded circuit. Don’t we ground to prevent a circuit from becoming energized?
A: Yes, we do ground to prevent a circuit from becoming energized. More precisely, we ground to trip the circuit breaker should someone energize a grounded circuit. We also ground to collapse voltage that can appear on a conductor through induction. However, grounding only trips a circuit by shunting a high current through the ground path to ground. Grounding does collapse voltage, but it does not prevent current from flowing through the circuit. The current that flows, even if only a few cycles in duration, is what produces the voltage that becomes the hazard source. Here is how that happens.
Current flowing through any conductor produces a voltage drop across the conductor path. A grounded circuit is absent voltage, but if current flows through that grounded circuit, a voltage will appear across the length of that pathway. If we have several grounded pathways in a work environment, each pathway will be of different resistance, and each will have a different voltage appear across that path. If all the pathways are the same length, have relatively close to the same level of current flow, the same conductor resistance and are connected to the same grounding point, the differences in potential across the different pathways will be very low
This is a very important principle of resistance and current-creating potential that lineworkers need to understand because it is the source of potential on current-carrying grounded circuits that injures and kills lineworkers every year.
Let’s apply this principle to a grounded transmission or distribution bus. If we ground a bus with a 30-foot-long 2/0 ground and there is induction current on the bus, that current flows through the ground cable, creating a voltage across the cable. If we have another ground cable from a different phase to a different ground point, that cable will also have a voltage across it. However, because of variations in resistance between the two paths, the voltage that can be detected at any uninsulated point will be different. This is known as a difference in potential. The hazard arises when that difference in potential is high enough to break the natural electrical resistance of the worker’s skin. When voltage breaks the resistance of the skin, the worker becomes a parallel path. Since it only takes a current in the range of 0.005 amps to rise to the level of risk, the worker is exposed to a deadly hazard.
Short-circuiting the phases, as required by the OSHA standard, using short-length cables better equalizes the potential difference between them. The voltage between phases is higher when all three phases are independently connected to a ground source. This is especially true when all three phases are grounded at different points on the structure.
Q: What is a remote earth ground and why does it matter when grounding a circuit for worker protection?
A: Let’s start with this statement: There is only one true-zero voltage and that is the earth. Everything connected to true-zero earth is at some voltage above zero, depending on current and resistance flowing into earth. What the statement means is that relative to earth, if there is current flowing in a path to earth, that current flowing through the resistance into the earthing connection creates a voltage across that pathway. Depending on how well the pathway is insulated and where you access the conductor in that pathway, potentials (voltage) will vary. If those potential differences along or between pathways are high enough to penetrate the electrical resistance of the worker’s skin, current can flow and the worker can be injured. This is why we build an equipotential zone for the worker. All potentials within the zone are low enough that they cannot penetrate the worker’s skin, so current cannot flow and the worker cannot be electrically injured.
This principle is also why we keep the conductors in our bonding arrangement as short as practicable. The shorter the pathway, the lower the voltage across that pathway. We frequently receive questions regarding the voltage in the path across the worker that is measured in bonding testing. Yes, there will be a voltage across the worker. The objective is to keep that voltage low enough that it cannot penetrate the natural electrical resistance of the worker’s skin. The bonding jumper protects the worker by equalizing any difference in potential that occurs across the worker. The bonding jumper still has a resistance and a current flow that create a voltage across the length of that bonding jumper. The voltage that is measured in a test of bonding is the voltage that appears across the bonding jumper protecting the worker. That is the voltage that the worker will be exposed to. A short bonding jumper keeps the voltage low across the protected worker.
Q: How is a bonding jumper that is connected from phase to cluster on a pole different from a ground connected from phase to neutral? Aren’t they both parallel paths? Why does one protect and the other doesn’t?
A: The difference is the connecting points at the lower end of the two pathways and whether the neutral is bonded to a pole bond. A worker on a pole who is in contact with the grounded phase is exposed to two potentials. The phase is one potential, and the pole is a different potential because it, like the phase, is a conductor. The pole is not nearly as good a conductor as the phase, but it is a conductor, and worse, it is considered at ground potential because it is buried. If we connect a bond cable to the phase and to a cluster on the pole, we equalize any potential that can appear between the worker’s hands and feet because of inadvertent energizing or electrical current due to induction. The difference in the connection phase to the neutral is that the potentials between the worker’s hands and feet are not equalized if the neutral and pole are not at the same potential. Bonding the neutral to the pole bond can sometimes equalize the potentials depending on how effectively the pole bond is stapled to the pole. The value of using the portable grounding cluster has been demonstrated in several studies, but there are other methods of bonding to the pole used by utilities. You should be aware, though, that OSHA’s expectation is to employ a ground cluster as described in Appendix C to 29 CFR 1910.269. But even there, OSHA states that the cluster alone may not provide the level of protection necessary without augmenting the cluster with a strap and nails to improve the wood-to-cluster electrical connection. Workers are required to bond the pole they are working on to the phase they are going to contact to protect the worker. Whichever method you use, the employer must be able to justify that it protects the worker.
Do you have a question regarding best practices, work procedures or other utility safety-related topics? If so, please send your inquiries directly to email@example.com. Questions submitted are reviewed and answered by the iP editorial advisory board and other subject matter experts.
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