IEEE 1584 - 2002 states that: "Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low impedance
transformer in its immediate power supply."

This exception has had quite a bit of discussion on the forum in the past. Do you use this exception or any variation such as a smaller transformer size for the cut off?

We use 75 kVA and below for the exception to add a little more margin.

We use <125kVA and <=240V.

125 kVA

We currently use =<240V and 125kVA. For a while we had used 4,000A bolted fault as the cutoff regardless of transformer size. We had read documents which claimed that Ferraz had been able to sustain an arcing fault at 240V and 4200A bolted fault. The problem was, we were a lone wolf in this regard. Invaribly customers would ask, "Why does one 75 kVA transformer have a Cat 3 HRC when another has a Cat 0 HRC and NFPA 70E says they both should have a cat 0 HRC?"
I'm hoping that someday that fault current will replace transformer size, but for now, wer are running with the pack.

Bob Ragsdale, P.E.

Very appropriate comment BCR 1962.

You won't be the "Lone Wolf" forever. I am head of the task group within IEEE 1584 that is reviewing this very issue.

If I have anything to do with it (and I do ) the low cut off point will ultimatly be based on current rather than kVA. This will allow the cut off to be used on long 208V circuits that may not necessarily be fed by a small transformer.

The actual cut off points that we are reviewing will likely be based on many sources including the data you cited, tests that I have been involved with as well as some additional research.

I have an IEEE 1584 meeting this weekend so we'll see how it goes. As I have posted before, the IEEE process moves very slow so I am sure this will take a while. However, once it looks like we have an agreement on a new low limit, I will post it here.

We use the cutoff but where hot work is likely to occur I do calculations to verify.

As you are on the committee(s) John, I hope you can implore the other members to simplify this stuff. I feel there is a hazard in pretending that 28 variables to be entered is 'more safe' than 14 variable to be entered.