Generally, my expectation for older obsolete equipment is to open in ten cycles or less for faults in the instantaneous trip rating of the OCPD. 166 milliseconds or less. Ten cycles is my number for equipment I consider to be slow opening under fault and unsafe. At ten cycles of fault current, there will certainly be catastrophic (blast) damage at and near the fault location, and it's a minor miracle the breaker can open under fault without exploding itself. Ten cycles of fault current is unsafe. Something after the high SCCR bus must have current limiting capability and open faster, unless it's old and junk.
My guesstimate would be after ten cycles the breaker would be open but not able to extinguish the arc, if it needs a half second to clear. A faulty OCPD.
I appreciate the discussion but I don't see where any equipment is expected to pass 30 cycles of service entrance high SCCR fault current. I would see the OCPD's at fault if they did not open fast, not the conductors for being under rated.
Is that 1/2 second clearing time a standard that some equipment must have that withstand rating? I would think the utility primary fuses would open before then.
edit: oh, 1/2 second or 1/2 cycle. I should be napping. 1/2 cycle blast damage will still likely be catastrophic at high SCCR, but not to the egc.