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| Utility X/R https://brainfiller.com/arcflashforum/viewtopic.php?f=23&t=1964 |
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| Author: | LHall [ Wed Jan 04, 2012 2:46 pm ] |
| Post subject: | Utility X/R |
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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| Author: | Jim Phillips (brainfiller) [ Wed Jan 04, 2012 4:26 pm ] |
| Post subject: | |
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. |
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