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 Post subject: Short-Circuit Cutoff Levels
PostPosted: Wed Jun 11, 2008 11:08 am 

Joined: Tue Mar 25, 2008 1:00 pm
Posts: 32
I've been given a spec by a customer that states that short-circuit and arc flash analysis can be ignored below 14kA @480V and 10kA @ 208V. Would anyone know where these numbers came from?

My main question though is this: At the secondary of a transformer stepping from 480V to 208V, I am calculating a short-circuit current of 3kA, which should be excluded from the study by the spec, but I calculate an IE of 9.6 cal/cm^2, or PPE level #3.

Can anyone provide some insight here?
Thanks.


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PostPosted: Wed Jun 11, 2008 12:02 pm 
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I have a guess. "Standard" AIC ratings for 208V breakers is (well used to be anyways) 10kA and 14kA for 480V. Higher fault currents require higher interuption ratings for the breakers. So maybe they are saying you dont need to do a S/C study below those values.

In a way that makes sense. However, that has nothing to do with arc flash, as you already found out. There can be some pretty series Ei's below the paramaters they set.

Unless I am missing something here, you need to go back to the customer and explain this. If they still want you to ignore the Arc Flash study below those levels, make them sign a waiver of liability.


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PostPosted: Thu Jun 12, 2008 10:37 am 
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Jim posted about an IEEE exception for 208V system fed by 125KVA and less transformer being excluded. I don't know about the 480V.


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PostPosted: Thu Jun 12, 2008 5:55 pm 
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I think Zog hit it on the head here. On the surface their logic of 14 kA and 10 kA might make sense but there is still some debate on how to handle low magnitude arcing currents. Most people have probably seen that very low currents can yield very long device clearing times which translates into more incident energy. The question is: is any of this realistic? There are many schools of thought such as the 125 kVA xfmr / < 240V exclusion clause in 1584, 2 second cut off, will the low level arc even sustain itself etc. Unfortuanely there is not a clear cut answer so people are "being creative" to justify minimizing the effort. If they get too creative, it might be difficult to explain in a legal setting.

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PostPosted: Fri Jun 13, 2008 5:17 am 

Joined: Tue Mar 25, 2008 1:00 pm
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I'm glad its not just me. I'm just going to ignore their cutoffs and do the analysis for the whole system. Thanks everyone!


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PostPosted: Fri Jun 13, 2008 6:28 am 
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brainfiller wrote:
I think Zog hit it on the head here. On the surface their logic of 14 kA and 10 kA might make sense but there is still some debate on how to handle low magnitude arcing currents. Most people have probably seen that very low currents can yield very long device clearing times which translates into more incident energy. The question is: is any of this realistic? There are many schools of thought such as the 125 kVA xfmr / < 240V exclusion clause in 1584, 2 second cut off, will the low level arc even sustain itself etc. Unfortuanely there is not a clear cut answer so people are "being creative" to justify minimizing the effort. If they get too creative, it might be difficult to explain in a legal setting.


A friend of mine at Westex has done some arc flash testing at KEMA on 208V 125kVA and says he has seen arcs sustain themselves below the IEEE 1585threshold level, I dont remmember what Ei's he observed but they were high enough.

I guess there needs to be a cutoff point somewhere, and I am sure IEEE did thier homework on that level, but I can see that changing in the future. However, you need to find a point where the "Risk" level is low enough to ignore the "hazard", we will never, ever, be able to protect all workers from arc flash injuries. There is a chance, my computer battery could explode as I type this message, it has happened, but should we require PPE when working on a notebook PC?


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PostPosted: Tue Jun 24, 2008 10:58 am 
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Zog, you used the key word. "Risk" If we are suppose to protect eveyone from everything, we'll be walking around in bubble wrap, kevlar, nomex and not be able to move. Risk and the practical side of all this needs still seems to need some work. :D


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