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 Post subject: Arc Flash Analysis Results
PostPosted: Mon Dec 12, 2011 9:23 am 
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I've recently obtained the short circuit analysis and coordination studies for 20+ of our buildings. All buildings are provided electrical utility by 13.2kv pad mount x-fmrs to 1600 - 2000A switchgear. Beyond that, multiple panelboards and a few hundred motors < 3hp. My question/problem is that the study reports, except in the case of the main switch, that 80 - 90% of these locations are Cat 0, in a rare instance or two, Cat 1. Also, the panels throughout these locations are now labeled Cat 0. I questioned the company performing the study and they tell me that the studies were performed utilizing "limited mitigation". If I perform hand calculations on some of these circuits they seem to be more common to the NFPA 70E tables, especially with open covers, energized and testing. After further questioning the company they returned a new result with "mitigation removed" and now all of my buildings are Cat 3, 4 or dangerous..... My question is:

1. Shouldn't these panels have been labeled to the task with the highest potential?
2. Why are they manipulating the model with a "mitigation factor"?

Any help would be greatly appreciated!! Keeping my people safe is my number one priority!


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PostPosted: Mon Dec 12, 2011 9:55 am 
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Location: Wisconsin
My experience is that achieving 80-90% of locations as Cat 0 or Cat 1 is usually possible.

Once you have a reliable study done through away the task tables.

Our reports are presented with "As Recommended" results, based on our simple mitigation suggestions (i.e. adjusting an existing breaker or replacing a fuse type). When requested we will also provide "As Found" results.


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PostPosted: Mon Dec 12, 2011 10:04 am 
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Location: Spartanburg, South Carolina
Our reports show the IE without mitigation, describe mitigation options with the IE reduction possible. Labels should not be applied with reduced IE until the mitigation is implemented.


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PostPosted: Mon Dec 12, 2011 10:20 am 
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jghrist wrote:
Labels should not be applied with reduced IE until the mitigation is implemented.

Absolutely.


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PostPosted: Mon Dec 12, 2011 6:13 pm 
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Thank-you responders

I was pretty certain of the response I would receive, just needed others expert opinions. I've read this site for some time, a great source for education and guidance...... Thanks again!!


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PostPosted: Fri Mar 09, 2012 4:50 pm 
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Location: Salt Lake City, Utah
I am putting together a training powerpoint for our staff, to be used in our Arc Flash Program. Is there a simple way to describe the method of calculating the Incident Energy? Could someone direct me to some help in this area? Thank you.

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Priscilla Anderson :D


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PostPosted: Sat Mar 10, 2012 6:55 am 
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Location: Spartanburg, South Carolina
phanderson wrote:
I am putting together a training powerpoint for our staff, to be used in our Arc Flash Program. Is there a simple way to describe the method of calculating the Incident Energy? Could someone direct me to some help in this area? Thank you.

The most common calculation method is the IEEE-1584 empirical equations. You could simply state that incident energy is usually calculated from curves derived from experimental results, using available fault current, voltage, fault clearing time, arc gap, and working distance.


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PostPosted: Sun Mar 11, 2012 11:27 am 
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Thank you so much, I will use that. I appreciate your help, and this forum, it is very helpful.

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Priscilla Anderson :D


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PostPosted: Fri Apr 13, 2012 9:02 am 
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Hello gentlemen,

Can you shed some light on a issue I have concerning the coordination study ? The study it's self, if I'm correct is determined by a software program ( various programs on the market). There's my question.... the study isn't complete until the system is actually tested, (opening times of the breaker mechanically) or the information from the software is enough? to provide the required information.

Thanks for your thoughts,
steve


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PostPosted: Fri Apr 13, 2012 1:56 pm 
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Location: Connecticut
The coordination study is based on the TCC "assumed" trip time settings of the protective device. The settings such as long time, short time, inst, grd fault will determine when the device will operate. Fuses have a melting time curve. The software determines the settings based on your system requirements. In the old days we used breaker TCC curves. Unless the breaker is tested you really don't know for sure.

In order to operational test a breaker it needs to be done with a direct injection tester which applies current to the breaker stabs and recorders the time vs. current opening time. It can also be done with a secondary injection tester that connects to the breaker trip unit substituting the CT's.

I'm a big supporter of breaker testing. I've seen many breakers that failed to operate during testing due to bad trip units or mechanical failures.


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PostPosted: Fri Apr 13, 2012 4:48 pm 
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Location: North Carolina
You can still get TCC curves for almost any fuse or breaker that is or has been on the market. I might be old fashioned but I routinely pull them out and look, whether software generated or a printed version (paper or electronic). It's just so much faster to look at a TCC curve and figure out what the effect on an arc flash rating is than going through several iterations with some of the software packages out there. With the software, you can easily look at several curves in a row.

If you look on S&C's web site ([url="http://www.sandc.com"]www.sandc.com[/url]) although it is intended for medium voltages, S&C has an online coordination curve calculator which works pretty nicely for doing a very basic analysis. I use it all the time but I also have a lot of S&C gear (we have a lot of 4-23 kV equipment).


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PostPosted: Mon Apr 16, 2012 10:46 am 
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Thanks for your thoughts guys.


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