OSHA indicates within Table 3 that IEEE Std 1584b-2011 is preferred for 600V and less, three phase arc in an enclosure. The enclosures I'm focused on for my electric utility are pad-mounted distribution transformers.

Back in 2011 I performed these IEEE 1584 calculations and for 480Y/277 transformers the incident energy was up to 12 cal for 300 kVA and smaller, between 20-32 cal for 500-750 kVA and >32 for 1000+ kVA. Since then, the 2012 NESC Table 410-1 came out and has indicated that based on testing of pad-mounted transformers, exposures do not exceed 4 cal. I've always thought that this was more recent information and testing that was superior to my previous calculations.

So now with the new OSHA requirements, can I base my reasonable estimate of incident energy, not on IEEE 1584, but on the NESC Table 410-1?

I've not been close to arc flash since 2011, so please forgive me if the answer is already on the forum, but I was not able to find any recent similar discussion.

This is a great question and one I hope others comment on. I am presently working on an arc flash assessment for a utility who has adopted the NESC as a standard and was pondering the same question. For all the arc flash studies my company has performed over the years almost always the secondary side of the transformer is 25 cal or above. I have a hard time with the NESC table 410-1 which indicates exposures do not exceed 4 cal. To date I have not used Table 410-1 as an assessment tool for types of equipment.

Have others used this Table as an assessment tool for their Utilities? Has anyone not used the table for transformer secondaries but applied it for the other types of equipment and voltage ranges?

I appreciate any comments.

I would say that Table 410-1 is entirely reasonable, with industry testing to back it up. Note that other than Table 3, Appendix E is very NESC. A few rows and columns were added to the tables and they rounded the values more, but otherwise they went with the NESC.

Thank you stevenal for highlighting this statement from OSHA. I've obtained a copy of the Eblen and Short 480V testing document referenced by NESC table. Assuming my review of this confirms that the tesing reflects typical utility equipment, my intent is to use this OSHA statement to favor using NESC Table 410 to determine the indident energy for these types of equipment rather than the IEEE 1584 calcs.

IEEE 1584 is based on arcs in a small (some would say medium) size box, or open air. It is an empirical estimate. NESC data is based on equipment-specific tests. Thus you can use 1584 for more generic situations but NESC is better when the conditions match. The entries in NESC are often dramatically lower than 1584 results. Most of the test work is documented by EPRI.

I am working on a Utility arc flash study where the owner has some outdoor 23kV switchgear within their substation. They need arc flash labels on the equipment doors. Which calculation method do you use? Looking at OSHA 1910.269 Table 3 it looks like ARCPRO is the only suitable calculation method for determining incident energy levels. Any thoughts?

Same problem here. The overhead equipment is easy...NESC Table 410-2 if nothing else. But enclosed/indoor the only choices are Lee or Arcpro with multipliers.

I'm also dealing with this same problem for a 34.5 kV wind farm overhead and underground collector system. Like you, I've concluded that ARCPRO will need to be used. My problem is what to do with the underground, enclosure calculations. The most straight-forward would be to assume phase-to-ground fault in a box and use ARCPRO's 1.5 adjustment factor. Problem is that OSHA Table 3 seems to imply that for an arc in an enclosure, a three-phase arc should be assumed since their is not a column in the table for single-phase arc in an enlosure. Given the deadfront nature of the 34.5 kV jacketed cable system, single-phase seems like a reasonable assumption to me. I'm curious what others think.

If I'm to stay with OSHA's presumed intention of using only three-phase for an arc in an enclosure, then I'm presented with the problem of being given a range of adjustment factors (3.7 - 6.5) to utilize. I have yet to find any recommondations that would help me determine what a reasonable adjustment factor would be. I read on this forum that ARCPRO has since released updated factors with a reduced range, but have not seen anyone post what that new range is.

When using ArcPro, the result is for single phase, open air faults. A multiplier must be used to convert to any other configuration. The prevailing presumption is that a arc in an enclosure will go to 3 phase arc.

I know there is not much guidance from ArcPro on multipliers and some can have quite a range. When in doubt, I would go conservative.

There is a forum thread that discusses the arcpro multipliers: viewtopic.php?f=15&t=3275

I think that the key is "when the conditions match". The NESC tables are defined by a maximum clearing time which doesn't apply for my analysis and so I think that I will be relegated to using the results from the Lee equations.

Scratch that last comment. I was reading my response time in ms not cycles as defined in the NESC tables.

On another side not I am wondering if anyone knows why the NESC tables and OSHA tables while very similar are not 100% identical.

Thanks

An answer from OSHA at http://files.engineering.com/download.aspx?folder=0fff1569-201e-4501-97fa-c61776c0ecc8&file=Titus_Diamond_20150716184438_-_OSHA_LoR_on_410-1.pdf