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 Post subject: How often do you rerun a study?
PostPosted: Mon Oct 10, 2011 11:03 am 

Joined: Tue Mar 02, 2010 1:50 pm
Posts: 26
This is probably more of a utility question, but I welcome any insight.
My arc flash studies are typically on tranmission connected generation. Now, since there are changes going on all the time on the grid with various utilities upgrading their lines and changing the available fault at my generator, it is impractical to continually update the study.

Thus the question: How often do you review and update your study for changes? Every 5 years (as stated in the NFPA 70E) or more often?


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PostPosted: Mon Oct 10, 2011 6:45 pm 
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Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1558
Location: Scottsdale, Arizona
One way to approach this is to try different utility contributions to see how it affects the PPE rating. i.e. if the utility fault current is 50 kA (as an example) try 55 kA, 60 kA etc. and see if the same arc rating can be used with higher fault currents.

This will help you find an upper limit of fault current before you have to concern yourself with a change to the PPE. That way you know you have defined a bit of a margin.

It is also pretty easy to ask for updated utility data every few years. If the data has not changed much (or is in the margins that you defined) and if you did not make any changes to your system, then the study should still be valid.

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Jim Phillips, P.E.
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PostPosted: Mon Oct 17, 2011 11:59 am 

Joined: Fri Sep 25, 2009 5:00 pm
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brainfiller wrote:
...It is also pretty easy to ask for updated utility data every few years. If the data has not changed much (or is in the margins that you defined) and if you did not make any changes to your system, then the study should still be valid.


We have used this approach. Every 3 or 4 years we will revisit the utility data. It ususally does not change much.

However, many years ago (around 1990) they began to build a substation 2 miles from here. We contacted the utility to see if it would affect us. No surprise, their short circuit current went up about 15 to 20%. They were refeeding distribution circuits (ours was one of them) from the new substation.

I guess seeing a substation under construction nearby might be a clue. :cool:


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PostPosted: Fri Dec 02, 2011 2:04 pm 

Joined: Tue Mar 02, 2010 1:50 pm
Posts: 26
Good replys, thanks. I do run studies with higher fault current levels as part of various scenarios to determine sensitivity, but I think I might make it a bit more conservative, mabye 5 or 10% over what the system protection group gives me.

Thanks,

Casey


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PostPosted: Mon Dec 05, 2011 1:51 pm 
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Joined: Tue Mar 08, 2011 11:23 am
Posts: 34
Location: Utah
Lower Available Utility Fault Currents:

[font="Calibri"]Youshould also run some scenarios using lower available utility fault currents. With lower fault currents the clearing timecould increase allowing more energy let through thus equaling greater hazard.[/font]


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