Arc Flash Precautions: A Review
David McPeak hosts the Incident Prevention Institute Forum (https://ip-institute.com/ipi-forum/) once a month. I often take part as a panelist, helping to answer questions posed by forum attendees. During a recent forum, topics ranged from fleet mechanics to arc flash exposures and required personal protective equipment. I decided I’d dedicate this installment of “Voice of Experience” to arc flash hazards by reviewing some of the minimum precautions that employers and employees should take as well as what the regulations require.
Rules and Regulations
Does NFPA 70E cover utility employees? Yes and no. Line crew employees are regulated by the National Electrical Safety Code, an industry consensus standard, and OSHA 29 CFR 1910.269. Facility and generation employees fall under OSHA Subpart S, “Electrical,” and 1910.147, “The control of hazardous energy (lockout/tagout),” for office buildings and power plants. Solar and wind employees’ tasks are specific as to what they are doing and the voltage present at the task. It is important to understand the regulations and follow your company’s policies.
NFPA 70E is a consensus standard that covers employees working on low-voltage systems (i.e., 51-600 volts). The question arises regularly, does NFPA 70E cover 1910.269 employees? The consensus standard is the safe work practices guideline for OSHA Subpart S low-voltage work. The NESC is the consensus standard for the 1910.269 high-voltage standard. How does the National Electrical Code fit into the picture? It is a consensus standard that contains certain construction and installation standards for low-voltage design, construction, and equipment used for identification and overcurrent protection. The NEC doesn’t deal so much with safe work practices on energized equipment. NFPA 70E contains requirements for the safety of employees from arc flash. An assessment must be made to determine the likelihood of arc flash; use the tables in 130.7(C)(15)(a) for AC and 130.7(C)(15)(b) for DC to determine the arc flash boundary and level of PPE. For electricians, the best way to avoid arc flash hazards in a low-voltage environment is to put energized equipment in an electrically safe work condition as found in the NFPA 70E standard, Article 120, using lockout/tagout found in OSHA’s 1910.333. As required in 70E, energized electrical work permits, using ground fault circuit interrupters when required, and identifying the correct level of PPE using insulating tools are other methods that can be used to limit arc flash.
There are no equipment-specific tables in the 1910.269 standard like there are for AC in the 2021 NFPA standard. But there are requirements to perform a task-specific assessment of the probability of an arc. So, there must be an engineering assessment for exposure, which was stated in the NESC in the 2007 cycle. In addition, according to 1910.269(l)(8)(i), “The employer shall assess the workplace to identify employees exposed to hazards from flames or from electric arcs.” After the required assessment is made, 1910.269(l)(8)(ii) requires the following: “For each employee exposed to hazards from electric arcs, the employer shall make a reasonable estimate of the incident heat energy to which the employee would be exposed.” These two regulations are further indications of the difficulty and differences in making the estimates. That is why the 1910.269 standard is found in Subpart R, “Special Industries.” There are arc flash tables for estimating incident heat energy exposure for utilities in Appendix E to 1910.269, tables 6 and 7. The tables were produced by OSHA using the single-phase estimating tool ARCPRO. The tables are limited to certain conditional exposures that are clearly explained in the table notes. For employers who cannot meet the OSHA requirements for estimating incident heat energy exposures using tables 6 and 7, there is guidance for estimating incident heat energy exposure in tables 1, 2 and 3. Table 6 (rubber glove work) and Table 7 (hot-stick work) include general exposures at a fixed distance in open air and don’t cover more difficult exposures like an arc-in-a-box. In most cases, low-voltage arc flashes occur in fixed closed equipment. Using the tables and/or software with all the values associated with the equipment will provide an accurate estimate of the amount of heat energy from arc flash incidents. A five-sided enclosure – like a self-contained meter base, for example – would be an arc-in-a-box event for a utility worker. Open-air arcs are less hazardous than enclosed equipment arcs, like pad-mount transformer enclosures, but they still register greater than 10,000 degrees Fahrenheit, and the test measurements of the arc are made at 18 inches, an arm’s length from the work.
A question often arises about what PPE to wear when performing a task. The level of arc-rated clothing must be based on the maximum amount of heat exposure. The amount of heat exposure is inversely proportionate to the distance. So, if at 18 inches you have an 8-cal exposure, at 36 inches the exposure in an open-air arc would be approximately half of the exposure. The industry has adopted a work practice of one level of maximum exposure at any point on the system for any task. That way, employees will always be protected from arcs whether they are performing the work or just near the work being performed. For instance, the maximum amount of arc flash exposure is at the substation low-side bus. The level of PPE to be worn anywhere on that circuit is determined at that point. There are no exceptions to the level of PPE when identified by an engineering analysis or the “reasonable effort” described.
Self-Contained Meter Bases
NESC testing in 2005 and 2006, and EPRI testing prior to the 1910.269 updates in 2014, illustrated that self-contained meter base exposures are very similar to that of low-voltage switchgear. The arc-in-a-box type with five-sided contained equipment with an open front causes a frontal ejection of the arc. That is the worst-case scenario and can increase the effects of the arc immensely more than an open-air arc. The data from the testing reported that on a 240/277/480 self-contained meter base, the heat value from an arc could be as high as 18 to 20 cal/cm2. The bus bar and lug spacing are very similar, and most single-phase arcs will certainly turn to three-phase arcs in either switchgear or self-contained meter bases. The available fault current at that location will determine the effect of the arc blast. Arc blast and arc flash are two different issues that must be dealt with. An arc flash may occur with low incident energy, but an arc blast can be significantly worse depending on the available fault current.
UD Transformer Switching
Switching elbows in a dead-front transformer on the end of a switch stick is another topic I’m often asked about. One of the greatest dangers is the ejected arc from a failed load-break bushing. But the distance must be considered. The level of PPE is determined by how much exposure is at the 18-inch working distancing. There is far greater exposure on a 6-foot stick than on an 8-foot or a 10-foot stick because of the distance from the arc. While some may still use a 4-foot stick, I highly recommend against it. If the employee must use a shorter hot stick, wearing rubber gloves with leather protectors is a valuable addition to limit arc flash exposure. Rubber gloves have no recognized arc thermal protective value. Arc flash testing has proven that Class 2 rubber gloves with cowskin or goatskin leather protectors will not break down up to 40 to 100-plus calories. Leather protectors can be rated up to 28 calories. The gloves add more protection from the heat effects of the arc. Sticks for primary protection are for electrical shock, and rubber gloves provide additional protection from heat exposure. In any case, the gloves will lessen the exposure to arc flash.
Many times, live-front transformers have no additional barrier between the employee and the exposed live parts/bushings and UD terminations once the door is opened. I advise my clients to unlock the door, remove the bolt and stay behind the door as they’re opening it to avoid potentially violating the minimum approach distance of the energized parts. Also, you never know what is behind that door. An inspection of all terminations and environmental conditions may indicate additional hazards even when the equipment is in working condition. Employees should wear all PPE necessary for protection against unanticipated equipment failures and arcs even when there is no sign of past failure.
Every company I work with has a different policy on entering a substation. Some companies require a full set of arc-rated clothing just to unlock the gate and enter the yard. This is a policy, not a regulation – unless the exposure of an arc at the gate is 2 cal/cm2, you are entering a minimum approach distance zone of energized conductors greater than 600 volts, or an arc could ignite your clothing or material in the immediate vicinity. Following are the regulations associated with exposure found in 1910.269(l)(8)(iv): “The employer shall ensure that the outer layer of clothing worn by an employee, except for clothing not required to be arc rated under paragraphs (l)(8)(v)(A) through (l)(8)(v)(E) of this section, is flame resistant under any of the following conditions:
- “1910.269(l)(8)(iv)(A): The employee is exposed to contact with energized circuit parts operating at more than 600 volts,
- “1910.269(l)(8)(iv)(B): An electric arc could ignite flammable material in the work area that, in turn, could ignite the employee’s clothing,
- “1910.269(l)(8)(iv)(C): Molten metal or electric arcs from faulted conductors in the work area could ignite the employee’s clothing, or
- “1910.269(l)(8)(iv)(D): The incident heat energy estimated under paragraph (l)(8)(ii) of this section exceeds 2.0 cal/cm2.”
Some substations are very small, and sometimes they are very large with great distances between the gate and equipment. So, there are a few things to consider. Where is the 2-cal exposure? How far away from the equipment is that point if there were a catastrophic failure? If I’m not wearing arc-rated clothing and I’m not in a zone that requires it, what would happen if I were asked to perform a different task? There could be an exposure. In this scenario, take the safest, most conservative approach.
All employees need to remember that the intent of arc-rated clothing is to meet the PPE standard, which requires that the clothing an employee is wearing does not further increase injury after the initial flash. This PPE will not keep you from getting burned in an accidental arc flash – the arc-rated clothing will keep you from being burned worse. This type of clothing has the mechanical strength to keep from breaking open during an arc flash. There is some thermal protection from the clothing, but it is not unusual for a victim to receive a second-degree burn. If the clothing is wet with sweat or water, a second-degree burn is almost a certainty from the vaporization of the steam. Appendix E to 1910.269 contains guidelines for the selection of appropriate arc-rated clothing as well as other PPE, such as face shields, hard hats, safety glasses, and hand and foot protection.
Lastly, remember, the OSHA regulations and consensus standards discussed in this article are just the minimum. Employers may add more measures for the protection of employees.
About the Author: Danny Raines, CUSP, is an author, an OSHA-authorized trainer, and a transmission and distribution safety consultant who retired from Georgia Power after 40 years of service and operated Raines Utility Safety Solutions LLC for nearly 15 years.
- Opening a Can of Worms
- Testing and Test Facilities
- Arc Flash Precautions: A Review
- The Art of Safety: C5 Leadership
- Gauges: How Do You Measure Safety?
- 3 Sustainability Considerations When Evaluating FR/AR Apparel
- Industry Trends and Solutions to Improve Worker Readiness
- Matted Surfaces: Safety Considerations and Controls