Published X/R Ratios

Grouch1980

Grouch1980

Senior Member
Location
New York, NY
The following link is an article on x/r ratios.

If you look at page 4, there is a graph at the top (Figure 4), and a table at the bottom (Table 1).

For Figure 4, which is a graph of the x/r ratios and (peak asym current / RMS sym current), does anyone have this in tabular form, that way you don't have to estimate the value on the curve, but can see actual numbers?

For Table 1, is there a more complete table with the test x/r ratios? For example, showing the test x/r ratios for other equipment (switchboards, distribution boards, transformers, etc.). ARE there even test x/r ratios for switchboards / panels? There is no information on Google. And I do not have SKM unfortunately.
 
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Regarding the Figure 4 graph, it's based on a equation, so it can be calculated for any value of X/R. If you are looking for a table, Conrad St. Pierre's book "A Practical Guide to Short Circuit Calculations" has a good table (3-1), but I don't have it in electronic form. If you're interested in the topic, it's a good reference book. I think you'll have to contact EasyPower to buy a copy (long story). But keep in mind the Square D graph is comparing PEAK current to RMS symmetrical.

Regarding the test X/R ratios, the Square D table is pretty good, since these are taken from different standards.

Is there a specific issue you are dealing with or just for general knowledge? If you are checking breaker ratings, there are a lot of variations in how the calculations are done, depending on the breaker standard that applies.
 
Thanks.
What’s the equation for figure 4? I prefer the equation over the table, so you can calculate for any value.

Do you have a link to the Square D table for the ratios? Also, are there test x/r values for switchboards or panelboards?

No specific issue.. this is for general knowledge, and trying to do things by hand.

Information on all this is so scant. I can’t find anything.
 
Grouch1980 said:
No specific issue.. this is for general knowledge, and trying to do things by hand.
Click to expand...
Distribution equipment is not considered in SCC calcs.
Note Bus Duct is a feeder system...so will need to account for its length in a given calculation.

You will need to get utility X/R & SCC amps at service point as your start.
You seem familiar with X & R values.
Visualize your one-line and work on developing a way to capture these values across the one-line.
Every segment will have an X & R associated with it...combined as a phasor will yield your Z value.
Don't let "phasor" panic you...many videos to refresh or bring you up to speed to work the math.

Assume you want to include motors, as you should. Search for the IEEE formula that will allow to develop each of your motor values.
Transformers have standard published X/R values.
You will need to practice how to move your calculation either up or down the one-line THROUGH the transformer depending on what POINT on the one-line you are calculating.

When doing your calc with NO motors, it is just a straight line from source to fault...combing ALL the Z's along the way.

The fun begins when you start adding in motors across your system.
Quick example...main panel A with two panel branches AA & AB. Say panel AB has a motor...M1.
You are calc'ing fault at panel AA.
Motor M1 is a source and its contribution needs to carried up the one-line to main panel A, combined with utility contribution and then work your way down the one-line to panel AA.

You can see how this can get messy in hurry. Consider adding an Excel sheet to do the calculations for you.

You will need to be able to calc Z values in series and parallel...
Hint for later ... 1/Z.

Unless you want to take a power systems class on per unit...switch to Ohmic Calc were you are working with ACTUAL system values.
Once you grasp the math and can assign correct values to ALL your components your calcs will be much easier.
 
MyCleveland said:
Every segment will have an X & R associated with it...combined as a phasor will yield your Z value.
Don't let "phasor" panic you...many videos to refresh or bring you up to speed to work the math.
Click to expand...
Yep, got this. Followed most of your post.

So a switchboard or distribution board or panel... their breakers and fuses have test x/r ratios, but the boards don't?

Also, what is the equation for figure 4 as I mentioned above?
MyCleveland said:
You can see how this can get messy in hurry. Consider adding an Excel sheet to do the calculations for you.
Click to expand...
Oh yes, I know it can get messy :)
 
Grouch1980 said:
The following link is an article on x/r ratios.

If you look at page 4, there is a graph at the top (Figure 4), and a table at the bottom (Table 1).

For Figure 4, which is a graph of the x/r ratios and (peak asym current / RMS sym current), does anyone have this in tabular form, that way you don't have to estimate the value on the curve, but can see actual numbers?

For Table 1, is there a more complete table with the test x/r ratios? For example, showing the test x/r ratios for other equipment (switchboards, distribution boards, transformers, etc.). ARE there even test x/r ratios for switchboards / panels? There is no information on Google. And I do not have SKM unfortunately.
Click to expand...
 
Grouch1980 said:
I'd like to be able to do this by hand, or at least practice it... and that graph in Figure 4 is not the best tool to use. There's no way to get an accurate value on the x-axis.
Click to expand...
All you need to concern yourself with is the table 1 test X/R values for the various AIC rated OCPDs.
When you do your calc for a fault at any point on your one line you end with a x & r values for the at this location…your x/r is your calculated values X & R.

Let’s say you have a SCC of 8k, and you are hoping to use a 10k rated panel.
Now let’s say your x/r is higher than the test value of 1.7…say 2.2.
You now need to derate the 10k down because you are placing it into a circuit where it is exposed to a higher asymmetrical current.
Yes there is a formula for that also.

SKM is looking better yes?
 
Grouch1980 said:
Shucks :(. No formula then.

I'm familiar with that formula: MF = (I asym @ X/R calculated) / (I asym @ X/R tested)

One day! Trying to convince a higher up :).
Click to expand...
What good would the formula do?
Your calc x/r will give you everything you need….you compare that to your selected OCPD test x/r.

Are you an EE or EC?

Was only kidding about SKM.
EE or EC…either should be able to build a spreadsheet to generate the needed results.
 
Grouch1980 said:
Thanks.
What’s the equation for figure 4? I prefer the equation over the table, so you can calculate for any value.

Do you have a link to the Square D table for the ratios? Also, are there test x/r values for switchboards or panelboards?

No specific issue.. this is for general knowledge, and trying to do things by hand.

Information on all this is so scant. I can’t find anything.
Click to expand...
Take a look at this link for a simplified equation for calculating the asymmetrical current from the symmetrical.

www.siemens.com

Siemens TechTopics No. 44

Anatomy of a short circuit
www.siemens.com www.siemens.com
 
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