George wrote:
In doing our site's Arc Flash Analysis I chose to use the 2 second rule where necessary if there was physical space available for a worker to back away from a potential arc flash event. My basis for using this was IEEE 1584 Annex B.1.2. I now see the 2 second rule discussed in Annex D.6 of the 2012 edition of NFPA 70E. This discussion in 70E all takes place under the heading of "....Greater Than 600V...." Are we to take this to the exclusion of using the two second rule at the lower voltages? I didn't believe that was the intent; however, I have others in my company that interpret this differently than I do. What is your interpretation? Are there any 70E, Annex D authors out there?
The 2 second rule is not just "escape". The other consideration is whether or not the faut clears (burns out). And it's not to say that you actually "walk away". There's a good chance at higher incident energies that the individual is literally blown away from the area. These various ideas are mentioned in 1584, but the 2 second cutoff is not fully justified under just a single scenario.
This is one of those areas where there's not much in the way of justification. I've seen the "wall" argument before. But if there is a wall in the way, then what? There's just no way to go from there.
It's similar to the world above 15 kV. Everyone know's that the Lee calculation gets progressively more and more "conservative" (overkill) and that the method that IEEE C2 (NESC) uses is probably low balling it (Arc Pro) but we don't have the actual "answer".
