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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: 77
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: 582
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: 77
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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