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Do you modify the results of arc flash study if equipment is underrated for available fault current?
Yes, my software does that automatically
Yes, I do this manually
No, my software treats arc flash and equipment evaluation separately, so I do too
No, I've never thought about this before
I don't know what my software does, I just plug in the info and run with whatever it spits out!
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ekstra   ara
 Post subject: Effect Of Underrated Breakers On Arc Flash
PostPosted: Thu Feb 16, 2012 7:25 am 

Joined: Fri Jan 27, 2012 9:27 am
Posts: 11
An arc flash hazard analysis determines (among many other things) the amount of time during which arcing fault current is flowing before a protective device trips.
A short circuit study/equipment evaluation determines (among many other things) if your equipment is rated to interrupt the maximum available fault current the protective device may see.
If you are doing formal studies, and a device fails your equipment evaluation, how do you handle its effect on the results of your arc flash study?


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PostPosted: Thu Feb 16, 2012 7:29 am 

Joined: Fri Jan 27, 2012 9:27 am
Posts: 11
(I meant to put the subject as
Effect Of Underrated Breakers On Arc Flash-POLL

Can a moderator change that so it doesn't look like I'm overposting?)


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PostPosted: Mon Feb 20, 2012 11:01 am 

Joined: Wed Aug 25, 2010 7:15 am
Posts: 24
Location: St. Paul, MN
diakonos1984 wrote:
If you are doing formal studies, and a device fails your equipment evaluation, how do you handle its effect on the results of your arc flash study?


We do note that the breaker is underrated and we recommend replacing the breaker. However, the vast majority of underrated breakers that we see are underrated for the bolted fault but are not underrrated for the arcing fault.


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PostPosted: Mon Feb 20, 2012 11:23 am 

Joined: Thu Apr 30, 2009 12:18 pm
Posts: 6
An Arc Flash Hazard analysis ultimately produces labels that signify the potential incident energy released during an arc fault event. This incident energy figure is then converted into a Arc Flash Hazrd Rating for establishing minimal PPE required. How can anyone place a label on any peice of electrical equipment that is powered by an overdutied (in AIC terms) upstream breaker? Every model in the arc flash software makes the assumption that the breaker will perform at the level the manufacturer designed and modeled in their time-current curves. If the breaker is overdutied, the time current curves go out the window, ie useless. If you ignore overdutied breakers and place a label on a piece of electrical equipment that states the Arc Flash Hazard Category, in my opinion, you are treading on thin ice. Fix the AIC issues, then complete the AFH analysis.


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PostPosted: Mon Feb 20, 2012 11:55 am 

Joined: Wed Aug 25, 2010 7:15 am
Posts: 24
Location: St. Paul, MN
Cowbell wrote:
Every model in the arc flash software makes the assumption that the breaker will perform at the level the manufacturer designed and modeled in their time-current curves. If the breaker is overdutied, the time current curves go out the window, ie useless.


If you have a breaker that is rated for 14kA and your maximum available bolted fault current it 15kA, the breaker is underrated and you are abosolutely correct that the TCC is useless for that fault. But if you run an arc flash study, and the protective device arcing fault current is 7kA, the TCC is not useless for that fault condition. In fact, the arc flash label you created would be useless if you had an arcing fault that is anywhere near the rating of the breaker.

I agree that the owner should take care of the under rated breaker, and we recommend that they do that, but I wouldn't refuse to finish the study or give them the arc flash results for a breaker that is properly rated for the calculated arc fault current.


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PostPosted: Mon Feb 20, 2012 2:08 pm 

Joined: Wed Dec 02, 2009 4:46 pm
Posts: 29
Location: CT
bvadams wrote:
We do note that the breaker is underrated and we recommend replacing the breaker. However, the vast majority of underrated breakers that we see are underrated for the bolted fault but are not underrrated for the arcing fault.

I would suggest it may not be a good practice to consider the rating of the device with respect to arcing fault current. If the CB is underated for bolted fault it is underated and should not be relied upon. Some firms will stop the analysis for that circuit and circuits downstream. The argument could even be made that to be in proximity to that device is more hazardous than for other circuits as it may fail if attempting to interrupt a fault. I would suggest the arcing fault, and or the arc flash analysis should not be considered in isolation or completely separate from other electrically caused hazards. Depending on how the arcing fault develops currents may not flow exactly as the 1584 model predicts. The fact is that the device is underated and its succesful operation is in doubt. It should be replaced or the hazard should be mitigated in some other manner.


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