We continue our discussion from the last post on performing arc-flash hazard calculations, IEEE 1584 Has Replaced the 125 kVA Exception: What Are the Implications For You? In December 2018, a comprehensive revision of IEEE(TM)1584:2018 IEEE Guide for Performing Arc-Flash Hazard Calculations was published. The 2002 edition of the standard excluded equipment <240V supplied by a transformer rated less than 125kVA. In the latest 2018 standard, that clause has been replaced by “Sustainable arcs are possible but less likely in three-phase systems operating at 240 V nominal or less with an available short-circuit current less than 2000 A.”
What “haz” been observed in the past year?There is no cookie-cutter rule that can be applied; transformer impedance, primary system impedance, cable specifications, primary protection schemes, etc., have all produced varying results. At one site all, 208V equipment fed from 45kVA transformers produced incident energies greater than 1.2cal/cm2, while 75kVA transformers at the same plant produced less than 1.2cal/cm2. Last week, I had 30kVA transformers produce greater than 1.2cal/cm2 in one area and less than 1.2cal/cm2 in another part of the plant. Time and arcing currents are a weird and wonderful phenomenon in arc flash hazard analysis, making it nearly impossible to merely look at a transformer nameplate or a breaker rating and decide the incident energy. A detailed evaluation using commercial software is the most reliable approach.
What options do employers have?The common question these days is whether companies can wait for the five-year recommended review cycle as required by NFPA(R) 70E: 2018 Article 130.5(G). The standard states that the study shall be updated either a) when changes in the electrical system can influence the results, or b) at intervals not to exceed five years. The standard does not mention changes to the engineering method used to determine the energy, but that intent is obviously implied! Also both OSHA (General Duty Clause) and NFPA(R) 70E:2018 ( Article 110.1(H)(3)) require the employer to identify hazards and protect workers when the hazard cannot be eliminated. So the following are some options that we have been recommending to employers:
- Consider typical systems in lieu of a comprehensive review. If companies are unable to allocate resources to a full system review, a “typical equipment” approach may be used to analyze 240V & 208V systems. This may be as simple as rerunning the old study using the 2018 equations or handpicking random equipment (with some engineering judgment, of course). Some manipulation may be required to fully comply with the IEEE(TM)1584:2018 standard, but this should be the least resource-intensive exercise.
- Perform a risk assessment using the likelihood of failure as per NFPA(R) 70E:2018 Articles 130.1(A)(4), 130.5.(B), and Table 130.5(C). Those who have attended our training classes or the webinar we presented with Hugh Hoagland, would appreciate our rather cautious approach to this method. Here is a nice write-up by Ken Sellars on the topic.
- Most crew in the workforce that are not considered electrically qualified (such as machine operators, mechanics, shift workers, operators, etc.) have their hands “on” 208V (240V) switches, disconnects or panelboards (the very equipment and locations impacted by the new requirement). Consider awareness training for these employees. Apart from this training being mandatory as per OSHA 1910.332, concepts and work practices discussed in class may preserve life in the event of an incident. Our online (or DVD) focused awareness class may be the most cost-effective solution for large groups. This video can be found at our online store.
- Replace melting / flammable clothing such as poly-cotton, polyester, acrylics, nylon, etc. with arc-rated daily workwear.
- Use engineering solutions (primary fusing, remote secondary mains, automation, etc.) to limit unqualified persons’ exposure to possible arc flash energies on 208V and 240V systems.
- Finally, if none of these solutions seem viable or applicable, then it may be time to perform a comprehensive review of the arc flash hazard calculation.
Concluding remarksThere are technical papers that discuss the sustainability of low voltage (<= 240V). The problem is that empirical data around these are limited, and it is near impossible for engineers without a R&D or academic background to provide an opinion on arc sustainability. A good peer-reviewed paper on the topic by Eblen and Short is worth reading. Even if the arc does not sustain the two seconds discussed in IEEE(TM)1584:2018, melting substrates (flammable clothing types) may ignite at lower energies. If this happens, the outcomes are usually an OSHA reportable offense (i.e., BAD for everyone involved!). Whatever you decide to do, please make sure that you take note of the impact of this change in IEEE(TM)1584:2018. Doing nothing is not an option; neither is indefinitely delaying a review. The risk will no doubt increase over time and increase with the number of person-equipment interactions. We look forward to your comments on this as well as temporary measures you may have adopted at your plant.
e-Hazard links used in the blog:
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