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Do you use the "2 second cut off rule"?
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 Post subject: Do you use the "2 Second Cut Off Rule"?
PostPosted: Mon Aug 23, 2010 4:09 am 
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Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1637
Location: Scottsdale, Arizona
When a time current curve indicates the arc flash duration is considered long, do you use the 2 second cut off as discussed in IEEE 1584 Annex B.1.2? (provided there is sufficient room for escape?)

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PostPosted: Wed Oct 06, 2010 9:57 pm 
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Joined: Thu Oct 18, 2007 7:10 pm
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Location: NW USA
Upon initial calculations we had some 480V transformers with unbelievable approach distances; using 2 second cutoff made the results more credible.


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PostPosted: Wed Nov 10, 2010 8:00 pm 
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Joined: Mon Nov 05, 2007 10:10 am
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Location: Mid-West
Application Specific

I believe this is applicaiton specific. We recently evaluated a short cirucit / coordination / arc flash study that was completed for a client by a third party, and one thing caught my eye. They utilized the two second rule. For the application I thought this may be a bad Idea would like to get your thoughts.

Design intent:
EE locally laied out a system of a bus duct mounted 45' above the floor adjacent to a catwalk. The bus plug was caculated at a level 3 utilizing the two second rule. Without the two second rule the breaker clearing time was upwards of 8-10 seconds (seems like an eternity to me).

Some argued that after two seconds the "arc" would not be able to re-strike. I am still struggling with this one, however.

My concern where do you go on a catwalk? Bigger concern the blast pressure was estimated somewhere at 16K per sq. ft. meaning that if there was an incident it may be the fall that causes a fatal injury from 45' up. As it turns out by changing some settings on the main breaker trip unit (taking out the I^2 and setting the short time lower) it picked up the fault exponentially faster at the 85% or worst case level, and still provided good coordination with downstream devices. It was amazing what a couple of setting changes can make.


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PostPosted: Fri Mar 11, 2011 12:28 pm 

Joined: Mon Feb 07, 2011 9:45 am
Posts: 7
Is there a voltage limitation on the 2 second rule? Presently our company standard it to us 5 seconds for above 1000 volts.

Thanks.


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PostPosted: Fri Mar 11, 2011 8:46 pm 
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Joined: Mon Mar 29, 2010 5:02 am
Posts: 136
2 seconds in the electrical world is HUGE......


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