I'm reviewing the AFC analysis of the previous engineer and have noticed that he only used the utility service transformer AFC for all downstream calculations. We have a (massively oversized) standby generator onsite that also supplies power to all the same locations in the event of an outage. The only place he used the AFC from the generator (which is double the AFC from the utility) is for the ATS panel, however am I correct in thinking that this value should have been used for all downstream calculations as well to get a MAXIMUM available fault current?
Generator fault current
- Thread starter EC Dan
- Start date
- Status
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
If the ATS is open transition, and the short circuit current (I don't know what you mean by AFC, to me it means arc fault current) from the gen is different, an arc flash study will have different results when the resulting currents are higher / lower because the breakers act slower / faster. We calculate both ways.
- Location
- Illinois
- Occupation
- retired electrician
It would be unusual for the generator to be able to supply a fault current even remotely close to that of the utility. In many cases a generator can't even supply enough fault current to get into the instantaneous trip range of a circuit breaker.
mayanees
Senior Member
- Location
- Westminster, MD
- Occupation
- Electrical Engineer and Master Electrician
... which is why the generator scenario needs to be used for the arc flash calculations because the incident energy is usually worse when on generator.
If the Power Study was done correctly the worst-case incident energy between those two scenarios would be what's listed on the label.
- Location
- Illinois
- Occupation
- retired electrician
I agree, but post 3 says "available fault current"
Ultimately I am reviewing both SCCR for panels and incident energy for arc flash PPE, but my original question was aimed at the maximum available fault current from an SCCR perspective. I know for a fact he used the utility fault current for all downstream calculations and my concern is he should have used the genset fault current.
I know fault current from a genset would normally not approach utility, but to give y'all some numbers, we have approximately 10 kA fault current available at the utility transformer and 24 kA from the genset. The genset is 1.5 MW and like I said, vastly oversized (was meant for a much larger plant that never materialized). I'm still trying to figure out how the previous engineer got the genset fault current, but it seems reasonable given the power and if you assume he used a low sub-transient reactance (around 9.1%).
Last edited:
mayanees
Senior Member
- Location
- Westminster, MD
- Occupation
- Electrical Engineer and Master Electrician
HAH! And I read it as Arcing Fault Current, but after re-examination it looks like he was referring to bolted fault current.
In that case Mr. OP it's a safe bet that the Utility is providing much more fault current than the generator so the analysis can just look at the Utility contribution.
You'd think so but we have abnormally low utility fault current (service transformer only 300 KVA) and an abnormally large genset (1.5 MW). 10 kA fault current for the former, 24 kA for the latter.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Then original study was done wrong. Effictively, 110.9 says you need to use the highest possible value regardless of the source.
That was my inclination as well, thanks for confirmation. Glad I'm doing this review! I also discovered all the panels were only rated 5 kA SCCR (default) so this makes that issue much worse.
xptpcrewx
Power System Engineer
- Location
- Las Vegas, Nevada, USA
- Occupation
- Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
Call me pessimistic/negative but I believe there are too many unqualified companies/persons performing power system studies. Anybody can oversimplify modeling a system, click on “run study”, then print out the report. Doing a quality study that produces meaningful results is so much more involved. The main issue I see with power system studies is laziness, no attention to detail, and bad assumptions on top of bad assumptions. Garbage in garbage out… what they should have done is modeled the system for each operating scenario or switching configuration, or systematically reduced the system to minimum and maximum scenarios.
Are you referencing power distribution panels ? Curious, how old are they ? I've not seen a 5ka panel in ages !
Who made your units? I have a 300 kVA transformer here and it says 16.8 kA @ 480V! A 1.5 MVA with a %Z of 12% would also be giving 15 kA.
No I was referring to control panels, however the distribution panels may also have issues since 22 kA breakers would not be sufficient.
The service transformer is ABB with 3.91 %Z. The ~10 kA number is the reported value from the utility. As for the generator, I'm still trying to replicate the previous engineer's number, but if you assume worst case sub-transient reactance of 0.09, then ~25 kA is not unreasonable. He seems to have used 0.091 as his value, but I can't get good info on the alternator so not sure what the actual value is yet.
Ok so I was able to track down the alternator specs to get the information I need to calculate fault current from the gen set. Previous guy seems to have significantly over-calculated fault current, but maybe someone can check my math. Alternator sub-transient reactance is 0.16 p.u. with a 2281 kVA rating. Gen set is 1875 kVA. That gives a fault current of 1875 / (480 * 1.73 * (0.16 * 1875 / 2281)) = 17.2 kA. I'm guessing he used an ultra-conservative sub-transient reactance in his calculation, since back-calculating his number gives 0.091 p.u. His number was going to be very problematic if correct, since almost every 480 VAC branch circuit breaker is only 18 kA rated. Still a problem for the default 5 kA SCCR 480 VAC control panels, but at least it's not a mass replacement of breakers.
And maybe it's down to 5kA by the time it gets to those control panels. Cable lengths do amazing things for reducing fault current.
xptpcrewx
Power System Engineer
- Location
- Las Vegas, Nevada, USA
- Occupation
- Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
It cancels in your math, but why are you using the genset kVA? Shouldn’t this be based on the alternator base rating?
- Status