Short circuit data & motor starting, protective relay

[SELEC]

I am doing a s-c study. The only data I got from Utility are initial fault (the fault available now) and ultimate fault value( the fault after the future expansion).

My questions are,

1.Can I use initial fault as Min fault contribution to conduct motor starting and protective relay study? Or I should ask for the Min and Max data available for now before expansion?

2. I am assuming that we should use ultimate fault value to conduct the max fault study and rate the equipment. Is that right?

I am new to North America. I look forward to being aware of the common practices here in order to do a good job.

Thank you guys in advance.

 

[davidbeach]

Be glad that you actually got the initial fault number, use it for any arc-flash calculation, and use the ultimate fault value for all of your equipment sizing. That is usually the only number a utility will make available. You could also keep the initial number in mind when doing a coordination study and make sure your design works for those faults as well as the ultimate. In general, which ever number gives you the worst results (highest cost, most difficult coordination, worst starting, highest voltage drop, etc.), that is the number to use.