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 Post subject: 10 cycle max clearing time for 208V systems?
PostPosted: Tue May 10, 2022 6:56 am 
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Joined: Thu Apr 18, 2019 11:42 am
Posts: 80
As many of us know, the NESC uses a max clearing time of 10 cycles for 208V systems, based on some real world testing. As far as I know, OSHAs interpretation of table 410-1 is acceptable as a means of ascertaining the arc flash hazard for this case (I do understand that OSHA does not accept some of the assumptions in the table for higher voltage systems).
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How many of you use a 10 cycle max clearing time for your client's AF studies? I am in a disagreement with a contractor who is doing some maintenance for my organization. I maintain we should use the 10 cycle clearing time basis, while they say that "208V poses as much of a risk as any other voltage, so we use either the OCPDs clearing time or the 2 second rule."

Thoughts?

I appreciate it,
Mike


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 Post subject: Re: 10 cycle max clearing time for 208V systems?
PostPosted: Tue May 10, 2022 7:26 am 
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 584
Is your organization in the scope of NESC? If so, it would be appropriate to use the tabulated IE values.

The 10 cycle limit isn't really a part of the code, just an explanation for a table entry. The EPRI study referenced is publicly available at https://www.epri.com/research/products/000000000001022218.

Looks like they needed 40kA at a half inch to get it to sustain past 9 cycles.

The maintenance contractor should have the final say regarding his own safety if more conservative.


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 Post subject: Re: 10 cycle max clearing time for 208V systems?
PostPosted: Wed May 11, 2022 11:55 am 
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Joined: Thu Apr 18, 2019 11:42 am
Posts: 80
stevenal,

We fall into both 70E and NESC. I have read a number of papers (including the EPRI paper) which is moving me to think this is a reasonable approach to 208V AF calc's.

The contractor will absolutely have final say regarding the safety of their own employees. My main goal from the question was to gauge how others are seeing the data and if others are using this as a basis. I am strongly considering making this part of our company policy/practice for our own calculations.

Mike


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 Post subject: Re: 10 cycle max clearing time for 208V systems?
PostPosted: Mon Jun 20, 2022 8:51 am 
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Joined: Sat Nov 21, 2009 10:23 pm
Posts: 120
Location: Ohio
If it was common knowledge on how little testing was done on <= 240V, more engineers would be using the NESC tables. I should have said successful testing, if you look at the IEEE testing on <= 240V, there is a common theme, a large number of the tests are incomplete due to the arc self extinguishing.

If you can find the following paper done by a senior 1584 member (in the past), "Calculating Arc Flash Energies and PPE for Systems < 250V", it is a good read. You will also see that the recommendations (.5sec clearing) line up very close to the NESC 4 cal/sqcm findings.


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 Post subject: Re: 10 cycle max clearing time for 208V systems?
PostPosted: Sun Jun 26, 2022 8:52 am 
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Joined: Mon Sep 17, 2007 5:00 pm
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Location: Scottsdale, Arizona
This continues to be a subject that requires more work. The IEEE 1584 project team has been looking more closely at this however due to NDA, I can't provide any details (yet). As pointed out in a few of the posts, there are several observations.

Duration is important (sustainability) but this isn't all. Fault current is important. A small fault current that sustains for 5 or 10 cycles is likely to be more of a "POP" A larger current that sustains for a few cycles is another matter.

I am familiar with the paper (and author) that uses 0.5 seconds. It seems reasonable but is not supported by any standard so it's use is up to someone else's decision.

As many know, the 2018 Edition of IEEE 1584 cut the 240V fault current down to 2000A at which point with the help of lots of disclaimer language, someone may choose to exclude those locations. One reason this is such a low value is legal asked: can we guarantee the arc will not sustain below _____amps?

From 2018 IEEE 1584:
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.


I had an inquiry from a utility around 2004-05 that had an electrical worker on the 208V secondary side of a padmounted transformer sustain a large second degree arc flash burn on their upper arm. So: Never say never!


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