Just a quick update. I was on another job this week to find a short circuit. Turned out the short was in a 1960's era raceway in the slab, likely EMT but possibly RMC. In any event it likely rotted out and damaged the wires in the process. This time, the circuit was protected by a Murray breaker. In the troubleshooting process, I had my amp clamp on the circuit and I saw the same current rise although slightly slower this time as it was not a dead short. The current rose to about 50 amps before the breaker began to buzz and finally trip. It took much less time for that to occur than the FPE, less than 10 seconds. Definitely an interesting contrast in the course of a week's work.
Apply a dead short to a SQ-D QO timed at the instant of a voltage peak, and the magnetic trip function will occur in less than a millisecond, by my 50 year old memory. Once the latch trips it takes milliseconds for the contacts to fully open. This time is determined by F = M*A. The moving mechanism mass, the spring force, and the distance to move.
i can not presently find my notebook with information from experiments using six 6 V car batteries, a mercury switch, welding wire, a 15 or 20 A QO, and a special current shunt. I might have reached 5000 or more amperes essentially instantaneously