Short Circuit Info from Utility & SKM

E

electrocat13

Member
Location
Houston, TX
Occupation
Electical Engineer
Hi! I do alot of arc flash studies in SKM, and would like help understanding something I come across. Why don't the SKM calculated load-side short circuit values match with the values given to me by the utility company?

So I will reach out to a utility company to get a site's short circuit information, and sometimes I receive the short circuit current values for the line side AND load side of the utility transformer. I also request X/R values and the utility transformer's KVA, Z%, V_LL, and upstream fuse. Then I plug the LINE side SC values and XF values into my SKM model and run short circuit. Typically the SKM calculated short circuit values on the secondary side of the transformer is much much higher than the values given to me by utility. Why is that?

I would think less about this if both values were similar, but typically they're very very different. For example 900A Vs. 3200A. Is it just a result of "real-life" losses vs. theoretical calculations? Is there another value I should request from the utility company to more accurately calculate the secondary values? Should I start adding a tolerance to Z% and adjusting the SC options to include the tolerance to get a closer secondary SC value? Is there an another solution to showing the utility values at the transformer secondary that I'm just missing completely? lol

I know I can lower Z% to adjust the values, but I don't really want to show a discrepancy between the model values and what the utility company gave me. I'm sure y'all know how enough discrepancies, even minor and with good reason, can make a customer raise an eyebrow. Plus I'm just curious.

I've seen other studies just remove the utility xf, and simply put the secondary SC values as the "utility". However, I think it's nice to have the utility xf and fuse information showing in the one-line if possible. Sometimes in the field its impossible to see what the XF size is, or and what fuse is being used.

Thanks in advance!
 
SKM can perform SC calculations using several different methodologies, but that would not explain such huge differences.

Most likely you are getting different data from the utility. The primary side data is likely close to accurate while their secondary side data is worse case data based on a transformer that the utility might install someday.

For short circuit studies use the worst case value for selecting equipment ratings.
Use the lowest value for arc flash incident energy calculations. You will need to run two different scenarios.
In either case you can always show the utility fuse.
 
Last edited:
jim dungar said:
SKM can perform SC calculations using several different methodologies, but that would not explain such huge differences.

Most likely you are getting different data from the utility. The primary side data is likely close to accurate while their secondary side data is worse case data based on a transformer that the utility might install someday.

For short circuit studies use the worst case value for selecting equipment ratings.
Use the lowest value for arc flash incident energy calculations. You will need to run two different scenarios.
In either case you can always show the utility fuse.
Click to expand...
We run calls right at the secondary bus, no wire. Also a larger transformer. Average 1 size
 
jim dungar said:
SKM can perform SC calculations using several different methodologies, but that would not explain such huge differences.

Most likely you are getting different data from the utility. The primary side data is likely close to accurate while their secondary side data is worse case data based on a transformer that the utility might install someday.
Click to expand...
Interesting! I did not know that, thanks.
jim dungar said:
For short circuit studies use the worst case value for selecting equipment ratings.
Use the lowest value for arc flash incident energy calculations. You will need to run two different scenarios.
In either case you can always show the utility fuse.
Click to expand...
Ah! Completely forgot about making a different scenario lol. So I'll make my base scenario use the XF and Line side SC values. and I'll make a second scenario where the site is not connected to the utility XF, and is instead fed from a different "utility source" that uses the Secondary SC values given by the utility company. Then use worst-case for Arc Flash values.

I'll print the One Line using my base scenario, and the load-side SC discrepancy will still show, but at least I have a better understanding to explain if asked!
 
jim dungar said:
SKM can perform SC calculations using several different methodologies, but that would not explain such huge differences.

Most likely you are getting different data from the utility. The primary side data is likely close to accurate while their secondary side data is worse case data based on a transformer that the utility might install someday.

For short circuit studies use the worst case value for selecting equipment ratings.
Use the lowest value for arc flash incident energy calculations. You will need to run two different scenarios.
In either case you can always show the utility fuse.
Click to expand...
But the OP said SKM is showing much much higher secondary current than the values given to him by the utility,
 
don_resqcapt19 said:
But the OP said SKM is showing much much higher secondary current than the values given to him by the utility,
Click to expand...
It happens when he inputs the utility primary side values then compares his calculations to the utility secondary side. This is partly due to the utility secondary side values being based on a different transformer.

This is one reason I ran many different scenarios (more than 15 was not uncommon) in order to find a likely worst case arc flash incident energy.
 
Last edited:
I have never run skm, but how does it handle the line side (primary side) fusing? That could also be playing a part?

Does SKM pull or allow the user to put in fuse tables / charts for different fuse manufacturers and types?
 
Elect117 said:
I have never run skm, but how does it handle the line side (primary side) fusing? That could also be playing a part?
Click to expand...
Fuse are not considered when conducting short circuit calculations.
I have yet to see a utility fuse sized to protect anything on the customer side. Their fuses are usually employed to remove a damaged transformer from their grid.
Elect117 said:
Does SKM pull or allow the user to put in fuse tables / charts for different fuse manufacturers and types?
Click to expand...
Kind of; it is easier to send your fuse data to the software manufacturer and have them create a TCC rather than DIY.
 
You must log in or register to reply here.
Top