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DC Arc Flash Calculations
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Author:  WBurtt [ Wed Mar 16, 2011 1:55 am ]
Post subject:  DC Arc Flash Calculations
Hello Folks,

While reading through Dan Doan's "Arc Flash Calculations for Exposures to DC Systems" and Ravel Ammerman, etal "DC Arc Models and Incident Energy Calculations" I noticed that neither one mentions calculating a 2nd Incident Energy using 0.85 of the arcing current as IEEE 1584 and NFPA 70E recommend.

My questions:
1) Is this just an oversight since DC Arc Flash is a work in progress and will be included sometime in the future?
2) Is there a reason why the 2nd calculation at 0.85 of arc current is not performed?
3) Are these papers just providing a way to calculate DC Incident Energy to insert in the appropriate step in the procedure upon which you then proceed by doing the 0.85 calculation?

Thanks in advance

WBurtt
Author:  Jim Phillips (brainfiller) [ Wed Mar 16, 2011 6:16 am ]
Post subject: 
Good Question. Dan Doan's (very highly respected in the arc flash community) paper is more theoretical in nature. Ravel, Ammerman, P.K. Sen, etc. paper focused more on a review of existing research that has already been conducted. It's conclusion was that most of the works out there (some dating back many decades) all point toward the same general direction/conclusion. The 0.85 factor was not considered in years past. It is used to cover the "what if" scenario in case the actual arcing current falls below a devices instantaneous setting.

Most use methods to calculate the DC arc resistance rather than the existing IEEE method of directly calculating the arcing current without knowing the arc impedance.

As far as the 0.85 multiplier, that kind of issue, or some variation, will likely be addressed when this all becomes part of an official standard. Until then, the user can certainly adopt their own multiplier to account for "unknowns" in the calculation process.
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