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 Post subject: DC systems; at what values does one stop worrying?
PostPosted: Sat Feb 21, 2015 7:16 am 
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Are there values for DC systems that by imperical analysis or by experience have determined that below those thresholds, one does not need to worry about arc flash? In this forum and others, there seems to be a consensus that at 120 VAC, there is no need to worry about injury from arc flash. In NFPA 70E-2015 Table 130.7(C)(15)(B), it seems at less than <100 VDC one does not need PPE.


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 Post subject: Re: DC systems; at what values does one stop worrying?
PostPosted: Sun Feb 22, 2015 12:37 pm 
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Look through the Battcon archives which is the battery conference. Duke Power commissioned resrs by Kinetrics that show that at 130 VDC at 20 kA or less with fairly narrow spacing (1/4“), if you normalize the data to the maximum sustained arc at 0.8 seconds and 18“ working distance instead of the screwy 12“ and stretching the arc data to an impossible 2 seconds, it is right at 1.3 cal/cm2. Thiss covers typical 125 VDC substation battery arrays. The data also shows that Ammerman's theoretical model is off by a factor of 2-4 on the high side so for other cases, you can use that as an upper bound. Ammerman's model is also mentioned in 70E, Annex D.8. Also by using Hertha Ayrton's model or similar ones it becomes clear that around 20-30 VDC, an arc cannot exist irrespective of the current. To date we don't have good models of arc/air cooling so predicting whether an arc is self sustaining which is critical to predicting low voltage arc flash can't be done directly.


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