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 Post subject: Arc flash boundaryPosted: Sun Jan 24, 2016 4:24 pm

Joined: Fri Nov 21, 2014 10:14 am
Posts: 23
I'm performing an Arc Flash study on a 7200 v system but the arc flash boundary value is around 21 feet. What could be wrong in the calculation that is giving me that big value?
Is it normal?

Thx,

Jorge

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 Post subject: Re: Arc flash boundaryPosted: Sun Jan 24, 2016 6:49 pm
 Plasma Level

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1700
Location: Scottsdale, Arizona
jorgepaz wrote:
I'm performing an Arc Flash study on a 7200 v system but the arc flash boundary value is around 21 feet. What could be wrong in the calculation that is giving me that big value?
Is it normal?

Thx,

Jorge

It depends on a few details. What is the incident energy? Also, I'm assuming this is for equipment since an AFB is being calculated which implies IEEE 1584 equations are being used.

If IEEE 1584 is being used and this is MV equipment, at medium voltage, a value known as the distance exponent is quite low which is likely the reason assuming everything else is correct.

What I mean by this is the incident energy theoretically decays as the inverse distance squared. i.e. double the distance and the incident energy drops by (1/2) squared or 1/4 of the value.

However, based on IEEE 1584, for low voltage equipment, it is not really a squared value. It is 1.641 for distribution equipment and 1.473 for low voltage switchgear - smaller rate of decay with distance. For medium voltage switchgear the exponent is 0.973 which means the incident energy decays very slow with distance thus requiring a greater distance to hit 1.2 cal/cm2 (the energy at the AFB)

Hope that helps.

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 Post subject: Re: Arc flash boundaryPosted: Sun Jan 24, 2016 7:38 pm

Joined: Fri Nov 21, 2014 10:14 am
Posts: 23

The incident energy is around 8.13 cal/cm2 @ 36".

Thx.

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 Post subject: Re: Arc flash boundaryPosted: Mon Jan 25, 2016 8:08 am
 Plasma Level

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1700
Location: Scottsdale, Arizona
jorgepaz wrote:
The incident energy is around 8.13 cal/cm2 @ 36".

That sounds correct. I actually have a medium voltage switchgear problem in my arc flash calculations/study class that has results very close to what you have. A bit of a coincidence. The calculations use a worksheet that I developed based on IEEE 1584 equations so people can see how this all works behind the scenes. It helps to answer questions just like this rather than blindly accept a software generated solution without understanding what happened.

The calculated incident energy for the in class example problem is 8.1 cal/cm2 at 36 inches and the calculated arc flash boundary is 21.4 feet. This is very close to what you had so your results sound reasonable. This is based on the existing edition of IEEE 1584. We have a new edition still in the works.

I understand that the large arc flash boundary seems odd but as I mentioned earlier, the rate of decay with distance is quite low at medium voltage. From IEEE 1584, the exponent is 0.973 so it takes a bit of distance to reach 1.2 cal/cm2 at medium voltage. We solve a lot of different arc flash calculation problems like this in the Arc Flash Class - April 25-26

btw, the usual disclaimer - I'm Secretary of IEEE 1584 but this answer regarding the IEEE 1584 Standard is my opinion and may or may not not necessarily reflect the views of the IEEE 1584 Working Group.

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 Post subject: Re: Arc flash boundaryPosted: Mon Jan 25, 2016 8:30 am
 Plasma Level

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2173
Location: North Carolina
To answer the "what is going on", medium voltage equipment is quite deep so rather than the thermal energy spreading out in a hemispherical shape as it does for most low voltage equipment or in a spherical shape as it does for open air conditions, it tends to be a more focused conical shape. Considering that thermal energy is exactly like any other form of radiation then you can imagine how just as with a light fixture the thermal energy bounces around inside the enclosure walls but the vast majority of it radiates out in a clear "cone" emanating from the enclosure opening. The tricky part here is that the actual effect is a mix of not only thermal radiation but heat, smoke, and plasma and the enclosure is not intended to be a reflector in the same was a a light fixture so the thermal radiation spreads out more than what would be expected if we were to model it as a light fixture. Given that most medium voltage gear is roughly about as deep as it is wider and sometimes deeper (36"x36"x48", 96, or 108" are common) you can probably imagine why things would be rather focused at close range.

At the current time though there were very few tests done for IEEE 1584 that examined the incident energy at a distance. Some test work that was done at Mersen indicates that as the distance increases the shape of the enclosure disappears. However as far as I know there is no further test data to indicate how to handle "near" vs. "far" effects so today we treat everything as nearby (within the first 3-6 feet of the enclosure) which then produces somewhat "silly" results for medium voltage gear. Obviously at some point not only would we treat near vs. far different but we may treat "directly in front" of electrical gear different from "far away".

Note that once you get beyond a "substantial barrier" then you can ignore the working distance for practical purposes. This would be the case for instance standing on the other side of a substantial wall. There is no definition of what a "substantial" wall is but I can offer this. In the world of thermal processing equipment (kilns, burners, etc.), we frequently erect thermal barriers which are literally nothing more than a cheap sheet or sometimes two sheets of aluminum siding with unistrut spacers. This arrangement allows me to easily stand a few feet away from a 2000+ degree radiant heat source without injury, melting hard hat, etc. Part of the magic here is that aluminum has an emissivity of around 3% which means that 97% of the thermal radiation is reflected and only 3% is absorbed and then retransmitted (with some losses) on the "cold face". After two layers of this you can imagine why there is little to no thermal radiation left. Although aluminum is by far the best "cheap" low emissivity source, you can achieve similar effects with something more substantial such as a block wall, a couple sheets of gypsum drywall, etc. However, arc flash also includes a pressure wave and some shrapnel which would easily knock down a flimsy barrier like this but it should give you the idea that as long as the wall is substantial enough from a mechanical point of view, nearly any wall will do in terms of blocking thermal radiation.

The idea of substantial barriers is unfortunately as I just described not really quantified in any meaningful way meaning we don't have a bunch of test work on the subject so I can't point you to some kind of NFPA document. It falls down to using your best engineering judgement.

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 Post subject: Re: Arc flash boundaryPosted: Mon Jan 25, 2016 11:33 am

Joined: Fri Nov 21, 2014 10:14 am
Posts: 23
This is very interesting information.

thx a lot for your help.

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