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 Post subject: Arc Flash Protection Boundary
PostPosted: Thu Feb 18, 2016 8:55 am 
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Location: Fot McMurray, Alberta, Canada
Working on the Arc Flash Analysis i found that for a Low Voltage MCC Buss the AFB (10.56 inches) is less than the Limited Approach (42 inches) and Restricted Approach (12 inches). What could be wrong in my model?


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 Post subject: Re: Arc Flash Protection Boundary
PostPosted: Thu Feb 18, 2016 8:58 am 
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jorgepaz wrote:
Working on the Arc Flash Analysis i found that for a Low Voltage MCC Buss the AFB (10.56 inches) is less than the Limited Approach (42 inches) and Restricted Approach (12 inches). What could be wrong in my model?


What's the incident energy? I'm betting it's well below 1.2 cal/cm2.


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 Post subject: Re: Arc Flash Protection Boundary
PostPosted: Thu Feb 18, 2016 9:23 am 
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Location: Fot McMurray, Alberta, Canada
Yes it is 0.5 cal/cm2


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 Post subject: Re: Arc Flash Protection Boundary
PostPosted: Thu Feb 18, 2016 10:51 am 
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jorgepaz wrote:
Yes it is 0.5 cal/cm2


There's no reason that isn't possible. Can't say for sure without having your model.

The other boundaries you mention deal with shock hazard, and have nothing to do with arc flash hazard.


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 Post subject: Re: Arc Flash Protection Boundary
PostPosted: Thu Feb 18, 2016 11:54 am 
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Location: Fot McMurray, Alberta, Canada
thx a lot.


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 Post subject: Re: Arc Flash Protection Boundary
PostPosted: Thu Feb 18, 2016 12:58 pm 
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jorgepaz wrote:
Working on the Arc Flash Analysis i found that for a Low Voltage MCC Buss the AFB (10.56 inches) is less than the Limited Approach (42 inches) and Restricted Approach (12 inches). What could be wrong in my model?


what is the arc duration? are you calculating the arc flash boundary based on actual incident energy to 2nd degree burn or based on 1.2 cal/cm^2?

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 Post subject: Re: Arc Flash Protection Boundary
PostPosted: Thu Feb 18, 2016 1:43 pm 
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jorgepaz wrote:
Working on the Arc Flash Analysis i found that for a Low Voltage MCC Buss the AFB (10.56 inches) is less than the Limited Approach (42 inches) and Restricted Approach (12 inches). What could be wrong in my model?


The original source for the shock protection boundaries is IEEE 516. NFPA 70E, NESC, and all subsections of OSHA regulations refer back to IEEE 516. I suggest reading this if you want to know the excruciating details. Below a certain voltage the limited approach boundary is fixed at 3'6" for everything except overhead lines (where it switches to 10'). Eventually it increases as the underlying flashover distance increases. The restricted approach boundary is fixed and somewhat arbitrary below 1 kV. Above that point it is calculated using the flashover distance plus a constant that accounts for inadvertent movement.


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 Post subject: Re: Arc Flash Protection Boundary
PostPosted: Mon Feb 22, 2016 9:32 am 
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jorgepaz wrote:
Working on the Arc Flash Analysis i found that for a Low Voltage MCC Buss the AFB (10.56 inches) is less than the Limited Approach (42 inches) and Restricted Approach (12 inches). What could be wrong in my model?


This is not uncommon at all. The Limited Approach is a SHOCK boundary based on IEEE 516 with the idea that Limited is for UNQUALIFIED persons to remain safe since they do not know which parts are energized and could potentially TOUCH energized parts and be shocked. There is never correlation of Limited Approach and AFB.

Different hazards of the same device.

Hugh Hoagland
e-Hazard.com


Last edited by wbd on Tue Feb 23, 2016 5:32 am, edited 1 time in total.
removed personal contact info


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 Post subject: Re: Arc Flash Protection Boundary
PostPosted: Mon Feb 22, 2016 10:23 am 
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Location: Fot McMurray, Alberta, Canada
Thanks everyone for your response.


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