Q: With all the talk about grounding, cover-up, EPZ and minimum approach distances, we have been debating the best practice for setting steel poles in energized 138 kV. A big question is, what class gloves should ground personnel wear while handling the pole? How can Class 3 or 4 gloves protect against 138 kV?
A: The short answer is that a Class 4 glove won’t protect against 138 kV. However, if you do it right, there is a very good chance you won’t be exposed to 138 kV even if you do get the pole in the 138. Here is how and why. At transmission voltages, we rely on planning, equipment setup, and precise/predictable control of the equipment and airspace to prevent contacts. We then take additional equipotential bonding actions to protect against a worst-case scenario like loss of control and pole contact with a circuit.
Here are some recommendations for those additional actions. Grade the work area. Grading the area flat around the pole hole gives the crew space for equipotential mats or grids. In best-case planning, it is ideal to stand the pole up with little hands-on contact until you get to the grabbers. If you are using portable mats, the prime location is at the stand-up/grabber location. During handling, the crew members on the pole butt will be in an EPZ. The pole then gets swung to the hole without crew contact. At the hole, mats are used to line the hole for crew who will handle the pole setting. Many crews are now using cattle panels as grids to create equipotential mats in the pole-setting areas. The panels are available at feed stores, constructed of welded #4 steel wire and bonded to the ground rod to create a large walking area around the pole-handling area that will be at equal potential with the pole.
A single ground rod installed at a relatively equal distance between the EPZ mat locations and the route to the hole keeps cables short. A bonding bracket on the pole above the setting depth with a short-as-practical grounding cable to the same ground rod should be made. In this arrangement, any worker in contact with the pole and mat will be in the EPZ and thus not exposed to voltage in the case of a pole contact with phases.
The worst-case scenario is when mats are not in the right spot. Workers on the ground in contact with the grounded pole will be exposed to a potential difference between earth and pole. We are not recommending this as a work method. We do recognize that there are pole sets being handled this way, and the risk is much greater than in an EPZ matted or grid area. Without mats or grids, the level of voltage will be determined by several conditions. The closer the ground rod is to the work area where the workers are standing – and the length of the ground cable from the ground rod to the pole – will determine the voltage drop exposure between the pole and earth. It is not easy to estimate the voltage drop, only that the voltage will be considerably less than system voltage the closer the spacings are, if the pole is effectively grounded to earth in the work area. This means the impedance of the rod and quality of connections matter. Workers in this exposure want to be in overshoes and rubber gloves.
Q: We just learned that 72-kV plastics for 69 kV are available, and we are considering whether we can use 72-kV plastics for 115 kV to ground for setting steel poles. There is very little information available for such an application and we are seeking recommendations. Can you help?
A: We are aware that there are some utilities and contractors extending the use of 72-kV plastics for phase-to-ground application. We urge you not to do it. The simple reason is the application established by the manufacturer, which is described as 72-kV “guarded phase to guarded phase.” This means plastics installed on each energized phase with a system exposure limit of 72 kV, which, for all practical purposes, is 69 kV.
We hear suggestions for using 72-kV plastic in a phase-to-ground exposure at 115 kV, believing that a little protection must have some positive effect. That is not the case. We spoke with Dr. George Gela of Berkshire Electric Transmission Consulting, a recognized expert on phenomena associated with high-voltage transmission circuits. Gela has tested plastics on transmission circuits and assures us of one thing – that plastics used on transmission, including 115 kV, in phase-to-ground exposures will fail quickly. The tests showed that the plastics do not just puncture, but flash over along the inside of the length of the plastic to the contact point outside the plastic. Linking plastics together to create a longer insulating length fared no better. They just flashed over through the joints. There are phenomena associated with transmission voltages that likely contribute to these failures normally associated with insulation and magnetic fields. These phenomena are well understood when it comes to coils, conductors and insulation, but exactly what happens when we cascade insulation between high-voltage fields has not been tested sufficiently to understand what other hazards may inadvertently be created by such an application. We don’t have room here to completely explain the concerns, but not understanding the limits of use does not make them any less of a risk. Two of the possible phenomena are dielectric constant and partial discharge. These conditions normally are associated with capacitance and coils but may have some negative effect on the air-as-insulation between inadequately insulated phases, which become sort of a capacitor when conditions are right.
It is by far not a certainty, but the question is, would the plastics replace the dielectric constant of the air in the gap, allowing a voltage buildup in that airspace? It’s not clear if it would, but it is theoretically possible. In addition, there is another phenomenon known as partial discharge, which normally is associated with disturbances over the surface of conductors where there are two or more conductors in parallel creating an electrical stress. Corona is a type of partial discharge that can occur on the outside of plastics under electrical stress. Where corona is occurring, the air insulation around the energized space is stripped of electrons that etch insulation, resulting in failure. Of course, we haven’t even considered the additional hazards associated with overvoltage concerns, common with transmission.
When it comes down to it, we don’t have to worry about the what-ifs if we stick with what we know works. We have relied on precise, predictable control of facilities, poles and airspace for decades. This is both the best and the accepted practice. We don’t recommend pushing the limits of plastic insulation.
Q: We saw that there was an extension of the certified operator deadline for cranes. Does that change the exception for digger derrick operators?
A: No, the extension is the final rule and only affects the relief OSHA provided for employers who had trouble getting needed certifications from third-party licensing agencies as required by the crane standard. Under the federal rules, digger derricks used for digging holes and setting poles – along with the exception added for setting padmount transformers – are still exempt from certification under the crane operator licensing requirements. State rules that may require third-party certification of digger derrick operators are not affected by this final rule.
This final rule ends the extensions for the deadline to certify operators that were due to the lack of third-party resources for the number and type of operators who needed licensing. OSHA originally required operator certification for capacity and type of cranes. Many employers only needed hydraulic certification at a capacity below 60 to 100 tons, but most training and certification was for complex operation of lattice-type cranes used in heavy industry. OSHA extended the deadlines, hoping the certifying agencies would be able to catch up with the demand. The last extension was to be this past November. On November 9, 2018, OSHA issued the final rule, making the final date for certifying of operators December 10. This last one-month extension was because OSHA also revised the wording for certification of operators from strictly “capacity and type,” adding a certification based on type of crane only. OSHA also extended the requirement to assure that operators who are not currently certified are competent to safely operate. As explained in the final rule, employers are required to train operators as needed to perform assigned crane activities, evaluate them and document successful completion of the evaluations. Employers who have evaluated operators prior to December 9, 2018, will not have to conduct those evaluations again; they only have to document when those evaluations were completed. You can read and download the final rule at www.govinfo.gov/content/pkg/FR-2018-11-09/pdf/2018-24481.pdf.
Q: We are trying to figure out if there is a line of demarcation when lockout/tagout (LOTO) or clearances are required at generating plants. Our electricians are trained on LOTO and operate under our LOTO rules. They get a task assignment, then use one-lines to locate and open clearance points, and then install locks and tags. At the metal-clad bus and the yard where the plant transmission bus goes overhead, line crews come in and get clearances from operations with step-by-step switching orders. Once switches are open and tags are hung, clearance is granted. Is LOTO required in the substation yard since the yard is part of the generation plant?
A: We don’t think it has anything to do with where the yard is located but who the qualified operators are that need clearances and the nature of the equipment being opened for worker safety. Your plant likely is operating under OSHA 29 CFR 1910.269(d), which is an LOTO section just slightly different from the general industry LOTO section found in 1910.147. Standard 1910.147 does not cover generation plants and clearly states as much in 1910.147(a)(1)(ii)(C). The generation, transmission and distribution standard 1910.269(d) does cover LOTO for generation plants. That rule also differentiates and exempts transmission and distribution from the generation LOTO rule.
T&D crews are trained to operate under the clearance requirements of 1910.269(m), “Deenergizing lines and equipment for employee protection,” and the adopted rules of part 42 of the National Electrical Safety Code. These rules were specifically created to reflect typical operation of the T&D systems. Both of these programs serve a particular need, and both result in the same level of safety in the respective workplaces. It is our belief that plant operators who may need to enter the plant substation yard to open a switch to ensure isolation from outside the plant should continue to follow their LOTO rules as developed in compliance with 1910.269(d). T&D crews who get an order to open and tag in the same station should operate under their exception to the LOTO rules and follow the rules for their workplace in 1910.269(m). On most substation gang switches, a hasp is available and line crews also will lock and tag. In the end, the result is the same: No matter who opens the switch, locks and tags are installed. Even without a lock, no one will close a switch with a filled-out, dated tag affixed to it.
KNOWLEDGE, INSIGHT & STRATEGY FOR UTILITY SAFETY & OPS PROFESSIONALS
Incident Prevention is produced by Utility Business Media, Inc.
© 2004 - 2018 Incident Prevention™. All Rights Reserved.