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 Post subject: Utility X/RPosted: Wed Jan 04, 2012 2:46 pm

Joined: Wed Aug 01, 2007 4:05 pm
Posts: 11
Hi there. I saw the post about the service transformer X/R but I have a slight variation.

I have been trying to obtain the X/R for the source from the utility. They have provided the short circuit current but no impedance or X/R. The software assumes a value for me. Is the exact X/R that important for the arc flash study?

Thanks for the help!

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 Post subject: Posted: Wed Jan 04, 2012 4:26 pm
 Plasma Level

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1637
Location: Scottsdale, Arizona
LHall wrote:
Hi there. I saw the post about the service transformer X/R but I have a slight variation.

I have been trying to obtain the X/R for the source from the utility. They have provided the short circuit current but no impedance or X/R. The software assumes a value for me. Is the exact X/R that important for the arc flash study?

Thanks for the help!

Your case is similar to the other X/R question regarding transformers. Utilities don't alway have the X/R ratio (although many do). It will likley have a minimal affect on the arc flash study since the X/R defines the impedance angle not the actual impedance.

It plays a roll in how the X and the R are combined with other impedances. If you don't have an X/R a pretty common assumption is 12 or 15.

The inverse tangent of 12 is 85.2 degrees (the X/R is also the tangent of the impedance angle) so if you use 15 or even a larger value, you are talking about the last few degrees between 85.2 and 90 degrees which would be an impedance of all X and no R.

Good luck and check back if you need more help.

_________________
Jim Phillips, P.E.
Brainfiller.com

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