Q: Is equipotential grounding now a personal protective grounding method required by OSHA?
A: The answer is yes, even though OSHA doesn’t specifically say so in terms we easily understand. The terminology isn't OSHA's fault. As an industry, we adopt certain familiar ways of describing or discussing things and simply don't recognize what OSHA is trying to communicate unless we do some diligent research. In 29 CFR 1910.269(n)(3), OSHA requires arrangement of grounds to protect employees without using the word “equipotential.” The title of the rule, however, is “Equipotential zone.”
The full text of 1910.269(n)(3) 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.” By definition, that is equipotential grounding.
Following the rule is a note that is pretty clear but largely missed by readers of the standard. It reads, “Appendix C to this section contains guidelines for establishing the equipotential zone required by this paragraph. The Occupational Safety and Health Administration will deem grounding practices meeting these guidelines as complying with paragraph (n)(3) of this section.”
There are three important things to recognize here. Rule (n)(3) is only about 30 words in a section on grounding that has hundreds of words, but those 30 words are probably some of the most significant in the standard when it comes to protecting employees. Pay particular attention to the part of the note to paragraph (n)(3) that states “equipotential zone required by this paragraph.” The second important thing here is a reminder that notes are part of the rules and can often clarify for us what a rule is about. Finally, in this case Appendix C is one of the best in the standard as far as communicating effective safe work procedures when it comes to grounding for personal protection.
Q: What are the safety requirements for grounding conduit racks in an environment exposed to induction?
A: Wherever there is induction or sources of current flow in a grounded system, there is potential for injury to persons who come in contact with those conductive surfaces. That could be the case with electrical conduits on a metal conduit rack. There are no standards we are aware of that include a blanket requirement for grounding of a pipe rack simply because it is near a power line. The rules for grounding and bonding of raceways and structures are found throughout Article 250 of the National Electrical Code.
The requirements for bonding and grounding are based on the likelihood that a conductive surface could become energized. Conduit, for example, is considered likely to become energized because it has electrical conductors within it that could become worn or shorted, and because the conduit is the fault current path from power equipment back to circuit protective devices in parallel with the circuit grounds. If the pipe rack was in a substation, the rack would probably be grounded to the station grid to minimize differences in potential between loosely connected racks and solidly grounded conduits.
Outside of a substation, a metal pipe rack should be grounded if the pipe rack is electrically isolated and subject to a source of induction or current from the grounded conduit. Based on the NEC rule, the rack might not be considered likely to become energized, but in the real world a conductive pipe rack holding a grounded conduit subject to a fault may, for the duration of the fault, become energized. Persons in contact with the rack may be shocked, depending on their path to ground and the magnitude of the fault current. This generally meets the criteria – may become energized and is subject to contact by persons or animals. We mention animals because NEC Article 547 has equipotential requirements for electrical installations in agriculture to prevent electrical disturbances, in particular with milking operations.
A conduit rack would be considered electrically isolated if it had nonconductive conduit pipe on it or if the conductive – and hopefully grounded – pipe on it was not clamped. The pipe, if conductive, clamped and properly grounded at its terminal points, will provide bonding of the rack. With electrical conduit, there are occasions that alternating current on poorly jointed conductive pipe will create noise that sometimes interferes with wireless communications. Solid bonding of the pipe to the rack is necessary to eliminate the noise.
So, the answer to your question is that it depends. You can simply choose to ground and bond if there is any chance at all of potential differences. If you want to experiment, you can also measure for potential differences using a low-scale setting on your VOM. If there is a significant voltage source in the grounded surfaces, a voltage can be measured across any ground path that has resistance in it, such as between racks and conduit.
Q: How do utilities treat the risks of and requirements for silica dust exposure? With respect to the OSHA guidelines, it would seem that we have little to worry about or do to be in compliance.
A: Your question comes to us at an opportune time. In the August 2015 issue, Incident Prevention published an article by Jarred O’Dell, CSP, CUSP – “N95 Filtering Face Pieces: Where Does Your Organization Stand?” (see http://incident-prevention.com/ip-articles/n95-filtering-face-pieces-where-does-your-organization-stand) – regarding silica dust and low-level exposure compliance issues. The OSHA standard as well as the consensus standards related to silica exposure were written assuming the exposures were in a workplace where dust was common and that the exposures were frequent and long-term. In these cases, it is fairly easy for a worker to wear an air-sampling pump and produce an accurate, measurable exposure rating.
Most utility-related silica exposures occur very infrequently, and thus it is not possible to measure them. We may infrequently saw-cut pavement or a sidewalk to set a pole and even then we often use contractors. We do drill concrete poles, and that is likely the most frequent source of silica exposure and a circumstance in which positioning is hard to choose relative to dust. We are aware of a case where a health technologist followed crews for four months, measuring dust created by a pole-setting crew, and he couldn't make a case for requiring protection. The problem is that may not be the case for 40 years of exposure. A worker may not be exposed while setting poles during the testing period. He may then spend a month cutting sidewalks for pole sets or two years cutting roadways once every two to three weeks to convert underground conduit systems, all the while inhaling low amounts of suspended silica crystals that don't meet the exposure risk action level. Other exposure risks include those to a contract lineman, for example, who may have 50 employers over the course of his career with no consistent silica policy in place or monitoring of his workplace exposures.
The standards do not recognize or discuss compounded risk over a 40-year career. They focus on exposures like those faced by workers who cut concrete every day. As such, even if we have no regulatory standard obligation, we do have a moral obligation to provide education on long-term exposure and risks as well as a robust voluntary use program. Consider annual refresher training on the cumulative risks of silica and voluntary dust mask use. And do read Jarred O’Dell’s article in the August 2015 issue of iP as you formulate your silica risk prevention program.
Q: What are circulating currents in overhead systems? I have always thought circulating current had something to do with ground rods.
A: When we refer to circulating current, in particular regarding hazards, we are usually referring to grounded systems. Sometimes it's the case of dissimilar metals, like ground rods or screw anchors and gas pipes in damp earth in a galvanic cell relationship, creating currents that flow through the respective components like a battery. That's a problem for engineers. For overhead lineworkers, there has been a rise in fatal contacts in recent years associated with currents flowing or circulating in grounds, sometimes installed for their protection.
As an industry, we have been good at increasing the use of grounds for safety, but not so good at training on the limitations and hazards created by grounding. When we are told it's not safe to touch unless it's grounded, we are only hearing part of the story. Grounding does not make all conductors safe, especially if you create a gap in a grounded circuit and then bridge the gap with your body, or if you get between two grounds that are at different potentials because of resistance in the respective circuit paths. We could write about this for pages and pages, but let's stick to your question with this scenario.
Consider a wooden H-structure with three 500-kV phases suspended under the crossarm. The two pole tops have a 3-strand, 7-galvanized static on one pole top and a strand-shielded fiber-optic cable (OPGW) on the other pole top. The pole tops are tied across with a 3-strand bonding them together, and that strand is connected to the pole bond at each end. All of this is very typical. Our pole is in a 60-mile transmission corridor with two other 500-kV circuits. On our circuit, the static and OPGW are bonded at every pole so there is very little induction current on the statics or pole bonds.
Today we take an outage on the circuit to do some work. With the circuit de-energized, we ground phases one and two to the right-hand pole bond wire and phase three to the left-hand pole bond wire. In doing so, we have now coupled induction on the circuit to our two pole bonds. In the configuration described, part of the current will go down to earth. However, we have also closed a fairly low-resistance path that loops between the phases, to the pole bonds by way of the grounds and up the pole across the pole-top bonding, creating a closed loop for current to flow. This is a grounded circuit intimately in contact with the pole. A voltmeter will not detect voltage despite the circulating alternating current. It appears safe to touch, but it isn't. If you open the loop at any point, a very high voltage will appear, often measured in thousands of volts. Of course, it's dependent on the voltage of the nearby lines, length of the circuits and so on. But it is the case that when there has been opportunity to measure these circulating currents, often after accidents, currents measured are frequently reported in the 100-amp range. Wherever you close a loop in a grounded system, even if it's through earth, there is the possibility of current flow.
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