utility unwilling to provide minimum available short circuit values

[malachi constant]

Hi all,

Working on an arc fault study for a facility in Xcel Energy territory. I have asked the utility engineer for maximum and minimum values of available short circuit current. They have replied that they are not legally required to provide the minimum information and so as a rule do not. That the maximum values they publish in their standards book are the only data they will offer.

The maximum values are based solely on transformer size. Meaning if I have a 300kW transformer located at the end of a run in a rural setting, I'm going to have to use the same value that I'd get with the same transformer sitting 500' away from the largest substation in their ten-state territory. That's how I understand it.

I assume to find the worst case arc fault scenario I am going to have to plug in the max value and record the available incident energy at given points in the system. Then plug in 90% of the max value and record values. Then 80%, 70%, 60%, etc until I see a discernible pattern and can estimate worst-case values.

Is that a reasonable approach? What would you do in this situation?

 

[jim dungar]

Is that a reasonable approach? What would you do in this situation?

Your idea is consistent with good engineering practice.
Don't forget that Xcel says they have the right to supply a larger or smaller transformer, therefore you need to consider those fault currents when evaluating the SCCR of your equipment.