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 Post subject: Maximum bolted fault vs arcing current
PostPosted: Thu Sep 11, 2008 12:47 pm 
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Joined: Wed Mar 05, 2008 7:42 am
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Hi,

in Annexe D, D.6 Calculation of Incidient Energy for a Flash Hazard Analysis the standard states that the arc flash analysis should be conducted using the maximum bolted fault and the minimum arcing value.

I have no problem computing the bolted fault. But how do we compute the arcing fault. It states that for 480-V systems, the industry accepted 38% of Ibf.

Does anyone uses this value? Is this a reference where I could find a method to compute the arcing current?

Thanks

John


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PostPosted: Fri Sep 12, 2008 6:51 am 
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Location: Charlotte, NC
I use the 38% rule for field calculations only when using my 70E calculator on testing jobs where no study has been done. Basically you are using this to see if the 38% current will be below the INST trip of the feeder.

These results will be very conservative, I wouldnt use this method for an actual study at a facility, it is a quick and somewhat sloppy method with questionable results.


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PostPosted: Sat Sep 13, 2008 6:55 pm 
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John,
Maybe I missed what you were asking, but in the Annex is the IEEE formula to calculate arcing current. You will see it as Ia and you need the Ibf, gap distance, voltage, etc. Taking a percentage like 38% is very estimate as Ia can often be much higher, especially in medium voltage apps.


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PostPosted: Sun Sep 14, 2008 6:53 am 
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haze10 wrote:
John,
Maybe I missed what you were asking, but in the Annex is the IEEE formula to calculate arcing current. You will see it as Ia and you need the Ibf, gap distance, voltage, etc. Taking a percentage like 38% is very estimate as Ia can often be much higher, especially in medium voltage apps.


He is talking about the minimum requirements for the 70E method calcs on a 480V system. You do a 100% and a 38% of BF current calc, find your clearing time from the curves and ue the 70E equation. Very primitive method but it is acceptable to use, and failrl easy to do in the field in a pinch. Accuracy? Not so good, lots of assumptions made that fail in the "Safe" side so you usually waer more PPE than you need, OK for field jobs, not good for a facility study to put info on labels.


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PostPosted: Sun Sep 14, 2008 8:37 am 
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The other option that I never see mentioned is the IEEE method that is based on Ibf and molded case breaker size. Its a simple calc that gives you IE. Its in 1584 table at the back. Has anyone tried it.


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