It is currently Mon Aug 03, 2020 10:06 pm



Post new topic Reply to topic
Author Message
ekstra   ara
 Post subject: Does the 2 Second Rule Have a Voltage Requirement
PostPosted: Tue Jan 24, 2012 7:13 am 

Joined: Fri Oct 30, 2009 9:20 am
Posts: 47
Location: Texas
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?


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Jan 24, 2012 10:56 am 
Sparks Level

Joined: Tue Feb 24, 2009 7:05 am
Posts: 252
Look at NFPA 70E 2012 in Annex D.7.3.

Annex D.6 is from a Ralph Lee paper and is not the most widely used way to calculate the incident energy.

Annex D.7 is an excerpt of IEEE 1582-02, and is the most widely used way to calculate the incident energy (at least for non-utility work).


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Jan 24, 2012 7:31 pm 
Plasma Level
User avatar

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
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".


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed Jan 25, 2012 6:27 am 

Joined: Fri Oct 30, 2009 9:20 am
Posts: 47
Location: Texas
Vincent B. wrote:
Look at NFPA 70E 2012 in Annex D.7.3.


Thank you Vincent B. That solves the issue for me. It's a little embarrassing though. The text is even highlighted and I missed it.


Top
 Profile Send private message  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 4 posts ] 

All times are UTC - 7 hours


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
cron
© 2019 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883