single phase short circuit calulations

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dvdcmrs

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I have used the web site program for calculating the available short circuit current at end of feeders. This is a good tool and I have found it very useful. I have a question about the program in regards to single phase systems. I am aware of the higher fault currents that can occur between phase and neutral for these systems but the program seems to incorrectly calculate phase-neutral short circuit currents for very short feeders.

[ January 16, 2006, 07:41 PM: Message edited by: dvdcmrs ]
 
Re: single phase short circuit calulations

What program are you referring to, and what makes you think it is doing the calculation incorrectly?
 
Re: single phase short circuit calulations

The program is 'Fault Current Calculator' found under the 'Free Stuff' section of the web site. The phase - neutral fault current calculated seems to high because for short feeders the phase - neutral current is higher than the phase - neutral current at the source transformer / panelboard.
 
Re: single phase short circuit calulations

That sounds right. This is taken from the Cooper SPD which details one way to do short circuit calculations.

Note 5. The L-N fault current is higher than the L-L fault current at the secondary terminals of a single-phase center-tapped transformer. The short-circuit current available (I) for this case in Step 4 should be adjusted at the transformer terminals as follows: At L-N center tapped transformer terminals, IL-N = 1.5 x IL-L at Transformer Terminals. At some distance from the terminals, depending upon wire size, the L-N fault current is lower than the L-L fault current. The 1.5 multiplier is an approximation
and will theoretically vary from 1.33 to 1.67. These figures are based on change in turns ratio between primary and secondary, infinite source available, zero feet from terminals of transformer, and 1.2 x %X and 1.5 x %R for
L-N vs. L-L resistance and reactance values.
Click to expand...
You can find the whole thing at
cooper- bussman

Steve
 
Re: single phase short circuit calulations

I agree with the original poster. I don't see how the fault current could be higher at the end of a short section of feeders than it could be at the transformer. Even if the feeders are VERY short and VERY large, they still provide impedance and should therefore reduce the fault current.
 
Re: single phase short circuit calulations

To clarify the original post I am not referring to the line-line fault current compared to the line - neutral fault current. My concern is with the line - neutral fault current at the end of short feeders is calculated to be higher than the line - neutral fault current at the source transformer / panelboard.
 
Re: single phase short circuit calulations

I should of read your first two posts more carefully.

How short are we talking about here? Can you give us some example numbers that demonstrate this?

It does sound odd, but I wouldn't be too quick to assume something is wrong. I'm thinking it might be possible for some werid type of resonance or standing wave to be causing this. But I'm only guessing.

Another guess: short circuit calculations are not exact. There is more than one method to doing them, and I think most methods are really just approximations. Being approximations, they have limitations. Maybe the method used just doesn't work for lines that short. Or maybe it just isn't giving the accuracy you expect.

Steve
 
Re: single phase short circuit calulations

Just out curiosity, is there a transformer in the circuit? If you go from a panel to a step-down transformer via a short feeder, then the fault current downstream of the transformer will probably be higher than the fault current at the first panel. If there is no such transformer in the circuit, then either the program or the model (i.e., input from the user) has to be incorrect. Without attempting the problem myself, I could not tell which.

All I can suggest it rechecking your input data, or having someone else check it for you. If it still gives the same strange results, then I think it would be appropriate to inform the web site administrator of the apparent program error.
 
Re: single phase short circuit calulations

Example:
Phase-Neutral short circuit at source transformer/panelboard = 10,000 amps.

Phase-Neutral short circuit at downstream panelboard = 12,000 amps. Feeder length = 20 feet.

I realize that this is an impractical calculation, but my concern is does the program provide inaccurate (higher) results for longer feeders. I was more curious than anything else.
 
Re: single phase short circuit calulations

Good point Ryan.

I thought maybe you were talking about a difference of a few amps over a fraction of a foot.

2000 Amps increase over a distance of 20' is much more suspicious. But Ryan has one possible cause.

Steve
 
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