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 Post subject: Problematic Arc Flash Study
PostPosted: Thu Sep 06, 2012 11:22 am 
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Joined: Thu Sep 06, 2012 10:57 am
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Recently, I received results back for an arc flash study that seems not to make sense. Here are two things that I came across that do not make sense to me. I am probably more critical of it because the last one had problems. I am by no means an expert on arc flash calculations.

1. There are RPP’s that are feed by 2 main breakers. There is one that is fed into a side compartment along with a tie breaker that is separate and another that is fed from top of the panel which is in the same cabinet. They have posted 4 stickers, 1 for each break and 1 for the panel. They have varying arc fault calculations, even though you have 2 items in one cabinet.

2. Also, these RPP’s are fed 120/208Volts fed by 225Amp breakers with a tie breaker that can change sources. This is the odd part, I have some that require Cat 0 PPE and some that require Cat 4. This seems odd to me. I know things can change when they conductor length increases and will cause less current for causing the breaking to trip instantaneously. When I spoke to the engineer about it, he told me that was why. Although, it seems really strange you would go from Cat 0 to Cat 5 with an extra 30 feet of wire.

Any help would be greatly appreciated.


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PostPosted: Fri Sep 07, 2012 2:59 am 
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Joined: Wed Jul 04, 2007 7:53 am
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Could you provide a sketch or one line of this? It sounds odd but can't be sure of the details. Also, what is Cat 5? The standard categories go from 0 to 4.


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PostPosted: Fri Sep 07, 2012 10:39 am 
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Location: Charlotte, NC
K. Wilson wrote:
Also, what is Cat 5? The standard categories go from 0 to 4.


Although never official I believe there was a CAT 5 proposed in maybe the 2000 edition of 70E that didn't make final cut. One major auto manufacture I know well put a lot of HRC 5 labels in several different plants. I was not popular guy when I pointed out no such thing existed.


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PostPosted: Sun Sep 09, 2012 6:25 pm 
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OK, now I would be suspicious. A category from a 2000 proposal. 4 labels on a piece of equipment. I have never seen more than 2 labels on equipment. The 2 label cases that I have seen were for:
  • Switchgear - one label on the main and one for the feeders
  • Equipment with a normal and alternate case (which can be confusing)
Sounds like you need to receive a detailed explanation.


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PostPosted: Mon Sep 10, 2012 11:06 am 
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Given that the results spanned from Cat 0 to the highest range, and that we are talking about a 120/208 V level where I have seen similar results in the raw data, I am curious about whether or not the engineer applied any maximum trip duration to the arc flash incident energy calculation. If the decision was that it was not prudent to do so, then it is entirely possible that you will see this wide span. This is because a smaller level of fault current of one source compared to your other source with a higher amount can result in a very large increase in the amount of time before the breaker in play will trip, and this can result in a much larger AF incident energy in this scenario as compared to a scenario using a source with a lower impedance.

This could explain the widely varying results. Just food for thought. Obviously a detailed explanation from the actual engineer would be better.


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