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 Post subject: Table 410-3 Working Distances?
PostPosted: Tue Feb 24, 2015 1:01 pm 
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Hi,
Can someone explain the "T" values given in footnote 1 of Table 410-3 in the 2012 NESC. I am trying to find out if they have an impact on the distance from the arc as one might infer from the language. "...calculated using the MAD... subtracting two times the assumed arc gap length and using the following T values...."

Thanks,
Craig


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 Post subject: Re: Table 410-3 Working Distances?
PostPosted: Wed Feb 25, 2015 5:52 am 
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Location: Rutland, VT
T is the maximum anticipated per unit overvoltage factor.

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Barry Donovan, P.E.
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 Post subject: Re: Table 410-3 Working Distances?
PostPosted: Wed Feb 25, 2015 6:29 am 
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Hi Barry,
Thank you for the response and information. I would assume that if we want determine that other transient voltage levels should be used then we would need to analyze the system for such rather than using the default values.

Craig


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 Post subject: Re: Table 410-3 Working Distances?
PostPosted: Wed Feb 25, 2015 9:37 am 
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The T values were used in calculating the MAD, see Table 441-1. I expect the NESC T values and MADS should be conservative enough. In any case, you can only increase the MAD. Also note that if you use a larger MAD to calculate the distance to the arc, the result will be less conservative than what's in Table 410-3.


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 Post subject: Re: Table 410-3 Working Distances?
PostPosted: Wed Feb 25, 2015 10:12 am 
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Hi Steven,
Thanks for the added reference. I can see that T values are TOV values. and I understand what you are saying with regards to less conservative results coming from increased MAD's but I think that when using in the NESC option for analysis in my software (CYME) the working distance is determined form me based on voltage level and the default T values given in the NESC.

Craig


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