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 Post subject: Does OSHA 1910.269 allow referencing 2012 NESC Table 410-1
PostPosted: Fri Nov 21, 2014 1:52 pm 

Joined: Wed Jun 24, 2009 4:45 pm
Posts: 41
Location: WA State
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.


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 Post subject: Re: Does OSHA 1910.269 allow referencing 2012 NESC Table 410
PostPosted: Tue Nov 25, 2014 6:29 am 

Joined: Thu Mar 13, 2008 4:23 am
Posts: 25
Location: Canton, OH
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.


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 Post subject: Re: Does OSHA 1910.269 allow referencing 2012 NESC Table 410
PostPosted: Tue Nov 25, 2014 10:49 am 
Arc Level

Joined: Tue Jan 13, 2009 5:00 pm
Posts: 566
Quote:
Note 1 to paragraph (l)(8)(ii): Appendix E to this section provides guidance on estimating available heat energy. The Occupational Safety and Health Administration will deem employers following the guidance in Appendix E to this section to be in compliance with paragraph (l)(8)(ii) of this section. An employer may choose a method of calculating incident heat energy not included in Appendix E to this section if the chosen method reasonably predicts the incident energy to which the employee would be exposed.


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.


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 Post subject: Re: Does OSHA 1910.269 allow referencing 2012 NESC Table 410
PostPosted: Tue Nov 25, 2014 11:01 pm 

Joined: Wed Jun 24, 2009 4:45 pm
Posts: 41
Location: WA State
stevenal wrote:
Quote:
An employer may choose a method of calculating incident heat energy not included in Appendix E to this section if the chosen method reasonably predicts the incident energy to which the employee would be exposed.


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.


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 Post subject: Re: Does OSHA 1910.269 allow referencing 2012 NESC Table 410
PostPosted: Sat Nov 29, 2014 6:16 am 
Plasma Level
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Location: North Carolina
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.


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 Post subject: Re: Does OSHA 1910.269 allow referencing 2012 NESC Table 410
PostPosted: Fri Dec 12, 2014 2:10 pm 

Joined: Thu Mar 13, 2008 4:23 am
Posts: 25
Location: Canton, OH
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?


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 Post subject: Re: Does OSHA 1910.269 allow referencing 2012 NESC Table 410
PostPosted: Sat Dec 13, 2014 10:41 am 
Plasma Level
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Location: North Carolina
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.


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 Post subject: Re: Does OSHA 1910.269 allow referencing 2012 NESC Table 410
PostPosted: Sun Dec 14, 2014 2:19 pm 

Joined: Wed Jun 24, 2009 4:45 pm
Posts: 41
Location: WA State
GPD_Steve wrote:
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?


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.


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 Post subject: Re: Does OSHA 1910.269 allow referencing 2012 NESC Table 410
PostPosted: Tue Dec 16, 2014 12:59 pm 
Plasma Level
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Joined: Wed May 07, 2008 5:00 pm
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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

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 Post subject: Re: Does OSHA 1910.269 allow referencing 2012 NESC Table 410
PostPosted: Thu Jan 15, 2015 1:25 pm 

Joined: Tue Apr 08, 2014 12:03 pm
Posts: 8
PaulEngr wrote:
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 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.


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 Post subject: Re: Does OSHA 1910.269 allow referencing 2012 NESC Table 410
PostPosted: Fri Jan 16, 2015 7:53 am 

Joined: Tue Apr 08, 2014 12:03 pm
Posts: 8
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


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 Post subject: Re: Does OSHA 1910.269 allow referencing 2012 NESC Table 410
PostPosted: Wed Aug 05, 2015 9:40 am 
Arc Level

Joined: Tue Jan 13, 2009 5:00 pm
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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


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