Q: I read what was written about an air gap for worker protection in the June-July 2020 issue of Incident Prevention magazine, but one of our engineers who sits on a National Electrical Safety Code advisory committee brought something to my attention. NESC C2-2017 444.2 states, “Air gaps created (e.g., cut or open jumpers) for de-energizing equipment or lines for the purpose of protecting employees shall be tagged and meet minimum clearances as specified in Table 444-1 or separated by a properly rated insulator.” What are your thoughts on this matter?
A: Thanks for your question. Our thought is that your colleague is right regarding the table and we missed it.
To remind iP’s readers, in the June-July 2020 Q&A, we addressed what constitutes an air gap and stated that some utilities build their gap rules around minimum approach distance. We pointed out that MAD is a combination of minimum air insulation distance (MAID) and unexpected movement, which is 24 inches for distribution. We gave the example of a dropout switch that has an 8- or 10-inch-plus gap being acceptable where the MAID in a 15-kV distribution exposure is fewer than 2 inches for phase to ground. We could have worded it better, so we hope we didn’t give anyone the idea that MAID is all that’s necessary. In any case, we don’t want anyone to be misled by what we publish.
The part we were wrong about was stating that there are no rules or guidance about gaps. It turns out that there are. We don’t make many mistakes, so we went digging and found out that the rule was brand new in the 2017 revision of the NESC and we completely missed it. Section 444(D) didn’t exist until 2017, and Table 444-1 used to be a minimum approach table – not an air-gap table. Prior to that, the whole section was about minimum approach. It included the same example I used, with the open switch gap being sufficient and inadvertent movement not applying.
To clarify our guidance, and in consideration of the wording of the 2017 NESC, air gaps that are cut into a circuit or jumpers that are opened to create an air gap must meet the clearances specified in NESC Table 444-1 or be separated by a properly rated insulator. As we stated in the June-July 2020 Q&A, that would include but not be limited to dead-end bells, a manufactured/engineered gap tool or an open switch rated for the circuit voltage.
Q: I’m the field safety supervisor for a distribution contractor. There are lots of opinions out there on inspection and testing of mechanical jumpers. Where do I find information to share with our overhead line crews about the proper way to conduct a field inspection of mechanical jumpers?
A: We are not aware of any in-service electrical testing requirements of mechanical jumpers, sometimes referred to as “macs.” There are ASTM F2715 insulation proof tests for manufacturers of assembled macs that include a water immersion insulation proof test. Those tests are performed under lab conditions with controlled atmospheres and air-exchange efficiencies to prevent corona checking and insulation damage.
We rely on visual inspections of macs to provide a relatively effective assurance that the undamaged insulation will perform as determined in the initial proof testing performed by the manufacturer. A good visual inspection also will find strand failures and loose or distorted clamps, and it’s pretty reliable. If an employer assembles their own macs, the jumper cable purchased on reels has not been insulation tested. Employer-assembled assemblies are not required to be tested. Mac cable has a shield layer and an insulation layer. There also is jacket cable that has a more robust mechanical protective layer over the insulation. The insulation ratings are from 15 kV to 69 kV.
The inspection in the field should take place like testing line hose, blankets, sleeves and gloves. Check out the electrical integrity of the conductor, the condition of the cable insulation and the mechanical strength of the clamp. Inspect the conductor for signs of fraying, kinking and twisting, especially at the clamp ends. The insulation should be inspected just like a line hose: look for cuts, abrasions and chemical damage on the outer portion. The clamps should be wire brushed before attaching them and easily accessible with a hot stick. All of this depends on the load of the conductor being jumpered. The ampacity should never be exceeded.
By the way, as a good practice, the industry does not rely on the insulation of an installed mac. Any climber who has backed into one while climbing a pole will know why. We should avoid contact with energized macs and consider placing blankets between macs and the conductive surfaces they are resting on.
Q: We have a question for iP. The scenario is a lineman who is checking secondary voltage on the secondary bushings of a transformer. He is wearing the appropriately rated gloves and sleeves for the H1 primary bushing voltage. The question is, do you feel a line hose is needed on the neutral, or can we go without it? We also practice work positioning, keeping the worker’s body clear of equipment.
A: This is – necessarily – one of those answers that relies on conditions and interpretations as well as popular best practices. In the photo you sent us, the lineman is on the pole and wearing Class II rubber gloves and sleeves. The neutral is on the opposite side of the pole. The gloves and sleeves, as stated in your question, are required based on the proximity of the high-voltage bushings on the transformer just above the worker. Gloves and sleeves are considered the lineman’s principal means of protection, so technically he is isolated from the primary voltage, and there is no risk of electrical contact even with the nearby neutral as long as everything goes well. However, the best practice across the industry requires cover in all secondary points of contact at different potential. This practice comes from the IEEE 516 consensus standard, “IEEE Guide for Maintenance Methods on Energized Power Lines,” 22.214.171.124, General Rules, paragraph (d), which states, “Energized or neutral conductors, ground wires, messengers, guy wires, etc., in the proximity of the work to be performed should be covered with approved protective equipment.” The rule does not include any particular scenario and is intended as a general practice. The reasoning is that the neutral is a current-carrying conductor associated with the distribution primary system.
This also is the explanation for OSHA’s general rule for prevention of electrical contact, 29 CFR 1910.269(l)(3)(iii)(B), which states, “The employer shall ensure that no employee approaches or takes any conductive object closer to exposed energized parts than the employer's established minimum approach distance, unless the energized part is insulated from the employee and from any other conductive object at a different potential.” This rule often is interpreted as insulation of the system from the employee and requires covering of both the energized parts and any part at a different potential, which certainly includes neutrals.
In addition to system neutral current, there are additional potential hazards from the transformer neutral being bonded to the tank and the tank being bonded to the pole, as well as the arrestor that unloads into the pole bond and neutral. At the same time, those connections and the neutral bonded to the pole prevent an arrestor leak from putting the pole at a different potential than the neutral. That seems to make the environment relatively safe for the climber, and it does, but wherever current flows in the grounded system, there is always a risk associated with voltage rise between those pathways if they are poorly bonded or not bonded, or if they get cut or damaged.
The last issue might be, what if the lineman cuts out? He is in personal portable anchorage fall protection, so your work rules must take other possible conditions into account. The fall arrest would not keep him off the secondaries. Could he be electrically exposed if he fell with his upper body across the secondaries and neutral? OSHA addresses that possibility in 1910.269(l)(5)(i), which states, “The employer shall ensure that each employee, to the extent that other safety-related conditions at the worksite permit, works in a position from which a slip or shock will not bring the employee's body into contact with exposed, uninsulated parts energized at a potential different from the employee's.” The phrase “uninsulated parts energized at a potential” is universally interpreted as including system neutrals.
What you have read above describes a practical, rules-related hazard analysis, something that all employers – usually through their safety personnel – are supposed to do. You could argue that with all of the above considerations, the risk of not covering the neutral is low, but you also have to recognize that the risks are not eliminated. Yes, there are a lot of opinions, and in our experience about half the employers out there don’t require covering the neutral – or pole bonds for that matter.
So, you may have thought this would be a simple answer. It can be if you don’t want to do the hazard analysis yourself. You can listen to our experts. Our advisers all agree that the answer is yes, the neutral should be covered. After all, it only takes a few minutes to cover it, and that’s a lot shorter than forever if something goes wrong.
Q: Working on de-energized lines where induction is present can be mitigated by bonding the equipment to the conductor. However, an insulated bucket provides isolation for protection of the worker. Is there a transmission voltage that is high enough to warrant the bonding of an insulated bucket to eliminate the induction hazard? Are there any rules, regulations or literature that address this issue?
A: First, we must clarify the difference between the hazard of current across a worker and the annoyance of voltage across a worker. We say “annoyance” because it would be difficult to create enough current across an insulating platform to injure the worker, but the shock can be very annoying. When we put a bucket in the air, it will be at a different potential than the de-energized phase, whether the phase is grounded or not. When the bucket and phase come in contact, there will be an equalizing of potentials, and most likely that equalizing will be charging of the lower potential platform, bringing it up to the potential of the de-energized phase. By design, the insulated boom supporting the platform limits the current through the boom to less than 800 microamperes, well below the risk threshold of 20 to 50 milliamperes. That, of course, depends on how clean the boom is and the integrity of the insulation system. The problem is that a worker between phase and platform will be subjected to that transfer of electrical charge, and while that can be uncomfortable, it is rarely deadly, if ever. The higher the voltage, the greater the discharge. It is particularly painful above 230 kV where longer parallel sections are influenced by mutual induction. We say “rarely deadly” because the boom limits the current flow across the platform, and the mass of the platform itself is not large enough to pull a high-charging current from the movement of the electrical charge into the platform. The shock encountered at contact with the phase is easily avoided by bonding the basket to the phase before you touch it. Metal platforms and fiberglass baskets on an insulating boom can lead to a shock if the induction load is high enough. It’s the same if a fault occurs, such as unexpected re-energizing of a grounded circuit. The worker will definitely feel an electrical shock, but again, the boom limits the current to well below the threshold of injury.
© 2004 - 2020 Incident Prevention™. All Rights Reserved.