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 Post subject: Understanding the Tables
PostPosted: Mon Jul 26, 2010 11:20 am 
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Joined: Thu Sep 24, 2009 5:46 pm
Posts: 25
For 600V class switchgear racking a breaker is a Hazard/Risk Cat 4 and operating the breaker with doors closed is a Hazard/Risk Cat 0. If there is a blast the I.E. is the same in both cases. Can someone explain this difference?
So if I complete the arcflash study and find a Cat3 on the line side for the same unit is the first operation a Cat 3 and the second a Cat 0?


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PostPosted: Mon Jul 26, 2010 7:11 pm 
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Joined: Mon Jan 18, 2010 11:35 am
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Location: Wisconsin
There is a reason the tables result in Hazard RISK Category.
The tables have seem to have little to due with the amount of energy in a flash versus the likelihood (risk) that a flash will even occur.


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PostPosted: Wed Sep 01, 2010 2:57 am 
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Joined: Mon May 03, 2010 1:41 am
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What I have noticed about the tables is when some people look at them they don't always pay attention to the notes. The notes are a critical part of the table. You may not even be able to use the table if your available fault current is too high or your clearing time is too slow. Racking a breaker is dangerous. So the tables put it at a dangerous catagory 4. Just in case. However if it is a 100 cal/CM┬▓ hazard then catagory 4 PPE may simply get you killed or worse if there is an arc fault. And if you do a study and it is determined that it is a catagory 3 hazard then the tables no longer apply. It is a catagory 3 hazard with a given number of Cal/CM┬▓ of incident energy at 18" or 24" depending on the situation. The tables are not designed to reduce safety requirements in high energy tasks. They are designed to be used on the equipment that meets the criteria listed in the notes which is typically lower available fault current and/or fairly fast clearing times.


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PostPosted: Wed Sep 01, 2010 6:31 am 
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Joined: Mon Jun 02, 2008 11:58 am
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Location: Charlotte, NC
I agree 100% with DCE.


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