Frank, you actually measured, rather than calculated, the short circuit amperage of the PH135 and found it to be 77 amps??? I'm surprised the internal resistance isn't such as to limit it to less. I wonder whether the contacts of most breakers could handle more than one or two such breaks.
The internal breakers on a Z4000 open quicker than 24 seconds. I have never had the Z4000 external 10-amp breakers open.
