I reran an existing arc flash study with the newest update from SKM. I was very surprised to see a 208V panel that was previously Cat 0 is now Category 4. This panel is fed from a 150kVA 208V transformer. The bolted fault is 9.44kA. The previous version of SKM reported as cat 0 if bolted fault was < 10kA. WOW - big difference.
The original study did not print a label for this panel nor did they model any of the subpanels that it feeds. How far do we have to go with the model and labels? Is this determined anywhere? Is there a common sense stopping point?

Check your Arc Flash Study Options. There is still an option to report as Cat 0.

I've seen a couple of times in this forum where it is stated that if the fault current is <10kA the hazard category would automatically become 0. I can't seem to find this in 70E. Have I overlooked it? If so, could you direct me to where it is found. Thanks for your help.

Take a look at the Notes at the bottom of the Task Matrix PPE table. Several of the categories reference this note. Don't have it in front of me, but it basically says that if the fault current is under 10K amps, then you can reduce the PPE for that task by one level. Doesn't necessarily mean all tasks are Level 0, but those that were Level 1 would be now Level 0 - those Level 2's would drop to Level 1, etc.

There is no code reference that says all tasks subject to less than 10K amps is Level 0.

Not anymore it dosent, that was removed in the latest revision.

That is correct, and I have not seen it here on this forum either.

The question has still not been addressed. How far into a system do we need to take these studies? At 208V fed by a 150kVA transformer do we stop at the first panel immediately down stream, or continue on to sub panels or disconnects or motor starters?

Maybe we do stop at a certain current level, but what is it?

Good question. I would love to be able to stop the study at a certain fault current level, but as far as I can tell this is not addressed in NFPA or 1584. This seems to be what the different codes are implying by excluding 208 volt systems fed by a small transformer, but I can't find where we can stop at a certain current level.

I model my systems down to the disconnects unless it is a 208 volt system fed by a single transformer less than 125 kva.

IEEE 1584 says: «Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger lowimpedance
transformer in its IMMEDIATE power supply.»

We now need to debate on what is the definition of «immediate power supply»

[quote="McFlash"]The question has still not been addressed. How far into a system do we need to take these studies? QUOTE]

[quote="JPEG"]We now need to debate on what is the definition of «immediate power supply» QUOTE]

I agree this goes back to my previous question how far into a system. What is considered Immediately. One device after the transformer or everything downstream.

I agree this goes back to my previous question how far into a system. What is considered Immediate. One device after the transformer or everything downstream.[/QUOTE]

I believe most of this goes back to the evolution of the standards. When IEEE 1584 was developed and published back in 2002, the tests were conducted with a very minimal budget. It answered many questions that were previously still hanging out there such as arcing current calculations, effect of grounding, gap etc. In the end, it was understood that lower fault currents at 120/208V would likely not sustain themselves very long and a blanket statement about 120/208V circuits below transformers < 125 kVA was added. That was it. No details.

I don't think the word "immediate" had any special meaning and probably was not the correct word choice. I believe the intent was, any circuit that falls into the criteria of 120/208 < 125 kVA does not require calculations and that was it.

There has never been a formal short circuit cut off defined. Even with the <125 kVA statement, it implies a low short circuit but does not define what that is. There have been a few tests on this but nothing officially published in a standard. One set of [url="http://www.brainfiller.com/documents/PGETestingbrainfillerposting.pdf"]tests by PG and E[/url] which I posted somewhere else in the forum and added a link above, shows various 120/208 V fault levels and their duration to self extinction. Another lab that I work with conducted tests that refute some of this depending on the bus orientation and phase barrier placement. Their tests indicate the arc flash will only extinguish for very low faults of just a few thousand amps - I don't have anything to post from them.

The bottom line, is there is no "official" answer at this point in time about where to stop calculations based on a minimum fault current.

Someone might look at a typical fault from a transformer < 125 kVA which would likely be 112.5 kVA as the next size and draw a comparison. Howver, they would have to accept the liability for making this judgement on their own and I don't of anyone that would do this.

Many people actually cut the study off at 75 kVA or lower due to the uncertainty.

But NFPA 70E does not say immediate power supply. It says (130.3 Exception 1)

Correct - NFPA does not use "immediate". I think NFPA realized it was not the best word to use like I mentioned in the post - it does not add to the intent and it was not included. The concept from IEEE and now NFPA 70E is:

..you do not need to include circuits below 125 kVA transformers at 120/208V. There is nothing else that can be read into it.

Splitting hairs about the word "immediate" or anything else does not change the intent. It would be nice if there was a low short circuit cut of and I think everyone probably agrees there is a cut off out there somewhere but there is no definitive research or standard to quote or reference (yet). Anything other than the 125 kVA 120/208V is going to have to be someone's "engineering judgement" (and liability)

I think the word "immediate" in IEEE 1584 means in the same voltage level. If you have a 500 kVA 4160-480Y/277 transformer feeding a 100 kVA 480-208Y/120 transformer, which then feeds a panelboard, the transformer in the "immediate" power supply of the panelboard is the 100 kVA transformer. The 4160-480Y/277 transformer is larger than 125 kVA but is not in the immediate power supply.

Correct!

I agree! I just wish there was more clarification as to a SC current cutoff. Maybe after the next round of testing we will have something more definite.

Alright, I'll bite.

I've got a 500kVA 13.8kV - 208/120V transformer. So far I've only posted the switchgear (31 cal/cm2 @18") bussed into the transformer and nothing below. It feeds two 400A MCC's and one 250A panel, each off a separate breaker.

Running the numbers, I come up with <3 cal for all of these lower buses. Should I go ahead and post them? I'm not confident that they would even be this much of a hazard. Stepping down below the main on each bus, we are at less than 1 cal.

What is the SC current at the downstream MCC's? Just curious. I would definitely say to label those pieces of equipment. I would venture to say that if you went beyond your MCC's and Panel your hazard would go up in calories.