I suspect that in the past 20 years the utility industry has grounded more circuits for the protection of employees than were ever grounded in the first 115 years of utility operations. Judging from the number of serious incidents and hazardous conditions created by temporarily grounding systems, it seems that we may not have understood all of the issues. It's almost intuitive; grounding makes the work safer, but for whom?
I once investigated a case in which an apprentice standing in a muddy right-of-way was grounding a truck to a pole bond and received a severe shock. A system fault many spans away – completely unrelated to the crew’s work – raised current and voltage on the system neutral connected to the pole bond at precisely the time the apprentice came in contact with it. This raised an issue for me: We had a policy about gloving system neutrals, but we didn't have a policy about gloving trucks connected to system neutrals. We were grounding trucks because we thought the rules required it and we were told that the pole bond, connected to a ground rod and the neutral, was the best place to connect. As studies have shown, the system neutral is a low-resistance path and the best ground source for temporary grounding. However, this incident demonstrates that there are risks.
Monitoring and Measuring Risk
Installing grounds to protect one group of employees raises the risk for others. This point became even clearer to me when I recently used the SNT-02, a ground potential monitor developed by Bonneville Power Administration and manufactured by Delta Computer Systems (www.stepandtouch.com). I was offered the opportunity to accompany an Arizona utility as they performed maintenance work on a 500 line to put the SNT-02 through a field test. The 500 line to be worked was de-energized, but parallel to an energized 500 line. While we knew there would be risks, we also knew the SNT-02 would allow us to measure and monitor them. As is their practice, the crew installed equipotential mats at each leg, including 15 feet of flexible insulating walkway material as an approach to the mat. In the pre-task review, they went over the electrical hazards that would be created at the tower legs once phase grounding was in place. Before they ascended the structure, we tested for current on the structure grounds using a clamp-on amp meter before we set up the SNT-02. This line had an isolated static so the clamp meter showed no current on the structure ground and the SNT-02 showed no voltage.
We knew that induction current would be coupled to the tower when the crew aloft grounded the phases to the tower, but we were surprised when the SNT-02 immediately went into alarm mode, warning of hazardous ground potential. The hazard? The SNT-02 showed a ground potential of 326 volts at the foot of the tower. The amp meter on the structure ground showed 49 amps. Consider that there was no current recorded prior to grounding. Consider also that each of the grounded structure legs, in parallel, split the current on the structure four ways. While grounding the phases made the crew aloft safer, the crew on the ground was definitely at higher risk.
In the end, as the other phases were bonded together, the current on the structure went down to about 15 amps, but the final voltage reading displayed by the SNT-02 was almost 600 volts in ground potential. Not only was the crew at risk, but it was evident by the meters that the lower 125 feet of the tower had hazardous voltage and current running through it. We noticed loose structural member bolts so we took current readings on individual structural members and found no two members were alike. They all showed current flow anywhere from 0.02 to several amps. Here is a good place to note that for the purpose of grounding phases, the static on a transmission circuit is low-resistance and a very effective path to ground for tripping circuits. When a static is isolated, it has the added benefit of not conducting current or induced voltage onto the tower.
The Best Way to Understand Ground Hazard Issues
If we install grounds by the book, have we arranged them to their most effective use? The best way to understand the risk of currents in grounds is to think of grounds as what they are – current-carrying circuits. Remember that in a circuit, current flows in every available path proportional to the resistance in each path. That means that every ground connection we make becomes a path for current to flow. Knowing this, we can also use resistance in a circuit to limit risks. For instance, if we categorize grounds by their functions, we can use resistance to control, to some extent, what happens if we have conductor sag into a feeder while stringing.
For the purposes of this discussion, let’s classify protective grounds according to their function. Recall that the standard states that the purpose of a ground is to cause immediate operation of a circuit protective device (1910.269(n)(4)(ii)) and also to protect each worker from the hazards or differences in potential (1910.269(n)(3)). Tripping grounds are the bracket grounds on a de-energized buss, at a splicing stub or grounded travelers in a stringing operation. Their purpose is to trip the device feeding the source of unexpected contact. In a fault, these fault-rated grounds shunt current to earth or phase to phase, quickly tripping the circuit feeding the contact. Personal protective grounds serve a different purpose when used in equipotential arrangements. As an equipotential ground, they operate more as a bond when used with tripping grounds and function as both a bond and a tripping ground when used alone.
Bonds have a different role. They are electrical connections to conductive surfaces designed to bring each conductor to the same electrical potential. Now, consider the traveling ground placed at a tensioner. Rule 1910.269(p)(4)(iii)(C) requires that “measures used shall ensure that employees will not be exposed to hazardous differences in potential.” What is the traveling ground’s function or, more to the point, what could be its better function? The operator on a grounded tensioner is in one precarious workplace. If the conductor in a pull should become energized, current will arc through the conductor, tensioner wheels, reel, hubs, brakes, mandrel and mounts onto the frame on its way to earth through the equipment. The operator, stationed on the tensioner frame, is close to the conductor and conductive mechanical parts. He is at risk, either from differences in potential or from an arc blast caused by current flashing across the resistances in the mechanical interfaces of the equipment. Understanding those risks, isn't bonding conductor to equipment the best function of the traveling ground? So where should the traveling ground connection be? A temporary rod in the ground 20 feet away or the tensioner frame? Bonding the conductor to the tensioner frame helps to create an area of equipotential for the operator, reducing his risks considerably.
One Step Further
Let’s take our understanding a step further. I once was asked whether a 6-foot driven ground or a screw-in ground would better protect a tensioner. I answered with this question: If current takes all paths proportional to resistance, and if tripping grounds are proportionally sharing the fault current, why would you want to ground the tensioner, drawing high fault right to the place where most workers are at higher risk? Would it be better to allow those tripping grounds to handle the fault while the tensioner on a bonded mat remains a floating area of equipotential? And to the reader who is saying you have to ground the tensioner, well, that's not exactly what the rules say, but that's a whole different article.
Ultimately, no one at Incident Prevention is telling you how to arrange your grounds; we are presenting recognized principles important for understanding both the need for and limits of grounds used for personal protection. We could spend hours reviewing Kirchhoff's second law, Maxwell, Faraday, Gauss and Ohm, but it all boils down to understanding that current flows in grounded circuits proportional to their resistance. When planning for personal protection from electrical hazards, you have to know the sources of energy, the hazards presented, the controls to be applied, and the limitations of those controls to eliminate or reduce risks.
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