Both arcing faults burned more than 20 minutes I was told.
They are not high current bolted faults. They were shorting across an air gap supported by carbonization deposited on the busbar supporting insulator. The conductive shorting path was the plasma and the deposits on the insulator. You have to ask yourself how much current can you push across a 1" air gap using plasma and carbon deposited on ceramic or Bakelite as the conductor. Whatever the current was, it was not over the threshold of the breaker trip curve and it was not all 60 Hz, probably mostly not 60 Hz.
Or put another way, let's guesstimate the arc was drawing 200 kW in a localized fireball and the breakers were rated to carry more than 200 kW for normal loads. As a premise, maybe the fuse was more sensitive to the high frequency components. Let's guesstimate or carry as a premise that the arc had 100 kW of 20,000 Hz or greater energy. The breaker ignored it but the fuse saw it and opened, but it took more than 20 minutes to respond.
An arc has myriad high frequency components all the way to and beyond radio frequency range. If you're quoting something calibrated for 60 cycles, components of the current are above 10,000 Hz. The calibration of the time current curves are going to be off. The question is what is the sensitivity to high frequency current above 10,000 Hz generated by the arc and how many kW of that RF will the protective device carry before it opens.
Peak current was limited by the current density of the plasma, the availability of charge carriers crossing the gap at the same time
Yes, the main and feeder breakers stayed closed, so the normal power remained on until the utiliiy's fuse opened.
