Arc Flash Studies

[PSU Electrical]

We just recently finished a new addition to an industrial plant. All of the power distribution equipment for the addition was taken all the way back to the primary substation for the plant. So we have a 4kV feed from the primary substation switchgear lineup to a new 4160/480VAC Unit Substation and a few MCC and Panelboards beyond that. The client requested whether we can get an Arc Flash Study (and Short Circuit and Protective Device) done for the new addition. I would like to meet this request but all of the studies I have done were for the entire facility whereas in this situation, I'm only doing a portion of the overall power system. Can I do the Arc Flash, SC and PD Study for this new addition (and do it accuratly of course) by only modeling the new equipment back to the primary service switchgear or do I truly need to model the rest of the plant? I understand the plant could have motor contribution and such but if I simply took the remaining plant load and total motor contribution, can I just put that in as a lumped load? What, if any, would be the difference in Arc Flash and SC levels if I DO and I DON'T model the rest of the plant? Any feedback would be appreciated.

Chris (ETAP user)

 

[jim dungar]

We regularly model 'portions' of a facility or campus installations.
What you are describing is not a lot different than only doing (1) building at time.

 

[charlie b]

I agree that you can do this. But I would think through the lump load model a bit before doing the analysis. For example, if the rest of the plant has 20 motors ranging from 1/2 to 10 HP, I would not model it as a single motor with the total HP of the 20 motors. That would artificially create a larger motor contribution to the fault than you would get by modelling the 20 motors one by one. I would draw a line at 1HP, and disregard motors that small or smaller, and include a model of each of the other motors. Depending on the software you are using, you may be able to group them (i.e., 5 motors at 2HP, and 7 motors at 10 HP, etc.).