Flash Boundary

That is one of the "holes" in IEEE 1584. As you already found out, lower short circuit currents can produce long clearing times and the equations treat this the same as a high short circuit current and low clearing time. Other than the 2 second cut off which can still produce very large results, there is not much else in writing.

The newer reseach is lookng at arc extinction times since the arc might extinguish long before a device trips.

I was just in the lab this week "blowin stuff up" fpr some independent testing and we had a hard time gettting a 30 kA bolted / 15 kA arcing current to sustain more than 1 or 2 cycles. It was phase-to-ground testing. Three phase sustained quite well - we even managed to throw a bus bar placed on top of the phases about 20 feet with that low of current.

There is still a possibility for some equipment that a phase-ground arc flash can go three phase quickly.

With all that said, there really is not a good answer yet. I understand the new IEEE / NFPA testing might finally be looking at energy per time which is what it will take to distinguish between incident energy from low current / long clearing time and high current / low clearing time.