Ran into this today, and I thought I would post it here for comment before I decide to recalculate this entire facility. We have:
1MVA utility transformer with 54,700ARMS available on the 240 volt 3 phase, 3-wire corner grounded secondary coming in to
MSD, Pringle switch with KTU 2000 fuses, bus to feeder section then
KAL26600 600 ampere 2 pole feeder breaker with
340' feeder, 2x300kcmil copper conductors per phase to
MLO Saflex panelboard. In panelboard is a 200A fusible switch with FRN-R-200 fuses then
250' feeder, 1x3/0 copper conductors per phase to
QO MLO loadcenter. In loadcenter is QO2100 circuit breaker with 10' of 1x#3 copper per phase to
30kva 240x208Y/120 transformer, 5.6% impedance
208V secondary tap is 1x#4 copper per phase running 50' to a 24 circuit MLO load center (yes, there's no secondary protection and this is not at all NEC compliant).
Arc flash label applied by engineering firm on 24 circuit MLO panelboard:
Flash Risk at 18"
Min Arc Rating 66cal/cm^2
Flash Protection Boundary 208"
Glove Class 00
PPE: DO NOT WORK ON LIVE
The label at the service entrance described above indicates 12cal/cm^2
Is this incident energy possible? Did they simply assume infinite bus on the transformer primary?
Even using the category method and Table 130.7(C)(15)(a), assuming<25ka at that point, we're at Category 1 PPE.
I would appreciate any feedback, similar findings, comments or corrections on my thinking before I remodel this whole facility in ETAP!

How are you getting 208Y/120V with a single phase primary?

If I'm following you correctly I would expect the available fault current at the 208V board would be so low that at 208V an arc would not be able sustain itself long enough to achieve 66 cals, if at all.

I just ran a quick 3 phase version of this and I'm getting 1.5 cals, not 66 cals using all defaults.

Also, I don't see a 600A KAL breaker available. Only up to 250A, but I didn't look very hard either.

It's a 3 phase primary, not single phase. The system is 3 phase 3-wire corner grounded delta (we call it "grounded B phase around here). That second to last load center is a QO130L225, main lug only. It LOOKS, at first glance, like a single phase panelboard with phase A (black) terminated as L1, phase C (red) terminated as L2 and phase B (white) terminated on the "neutral" bar. Thus, to feed the three phase primary of the transformer, phase A and phase C come from a 2-pole 100 ampere circuit breaker with phase B coming from the "neutral" bar. This was Square D's system for low amperage corner grounded delta systems and required "H" circuit breakers, such as a QO2100H, which have a straight 240 volt rating rather than the standard 120/240 volt rating. I should have explained that in more detail in my original post! Thank you for the reply!

Ok.

I used to work at a university that had ungrounded 240V delta, high leg delta and corner grounded delta services. The corner grounded delta still used 3 pole breakers. Never considered the possibility you describe. Thanks for the clarification.

A 600A trip breaker would be a type MA or an LC, depending on age.

Is it possible the fault current is so low that the OCPD never trips and the OP duration was set to 1000sec?

Nice catch on that first Square D feeder circuit breaker - you are both correct that it can't be a KA. I took that catalog number from the previous engineer's evalution spreadsheet We'll have to pull that gear open to get all the correct cat #'s when we can schedule a shutdown; then we can check settings as well, if there are any. I [u]believe[u] we are talking mid 1970's for manufacture date of the MSD and feeder section.

If it was from the 70s then it would be a type MA. The LC didn't come out in that size until about the late 90s.