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 Post subject: X/R RatioPosted: Mon Dec 13, 2010 11:22 am
 Sparks Level

Joined: Tue May 18, 2010 7:03 am
Posts: 53
Working on a large model that was originally started years ago by someone else. I noticed the X/R ratio for the utility used is 20. Actual info received from the utility is 5.5 for one circuit, 7 for the other. What difference should I expect in the study? My concern is having to change every label as a result of this change. I have seen articles recommending a value of about 12 for the X/R ratio when the utility doesn't provide it so 20 seems high.

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 Post subject: Posted: Tue Dec 14, 2010 4:28 am
 Plasma Level

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1637
Location: Scottsdale, Arizona
mountaineer wrote:
Working on a large model that was originally started years ago by someone else. I noticed the X/R ratio for the utility used is 20. Actual info received from the utility is 5.5 for one circuit, 7 for the other. What difference should I expect in the study? My concern is having to change every label as a result of this change. I have seen articles recommending a value of about 12 for the X/R ratio when the utility doesn't provide it so 20 seems high.

The X/R of 12 was in the 1986 IEEE buff book based on an example from the Chief Engineer at Square D in the 1970's. (where I worked at one time). The idea is to select an X/R that produces a large impedance angle. i.e. Tan-1(12) = 85.23 degrees.

When the source impedance is added to the first transformer or conductor impedance in the system, they might have a lower angle than the source. This means that when the assumed large source angle is combined with potentially a lower transformer / conductor impedance angle, the resulting total impedance is LOWER and the short circuit current is HIGHER. (which is conservative for a short circuit study)

In your case, reducing down to 5.5 or 7 might technically lower the short circuit current but I bet it would be such a very small amount it is not worth the effort.

As far as arc flash, other than the extremely small change in short circuit current, X/R does not enter into the arc flash equations (yet). There is some thought that the X/R may have some minor (?) effect but nothing has been done as of yet.

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Jim Phillips, P.E.
Brainfiller.com

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