I think the question goes deeper than this. Under serious circumstances, a German electrician would in fact calculate, whether branch circuit conductors of a certain size would actually allow an overcurrent protection device with a certain current rating and trip curve to trip within the required time based on the loop impedance/short circuit current.
This is most interesting where breakers/fuses are used as earth fault protection (which is the fact in all Canadian and US circuits that are not on a GFI). An extensive run of small wire, fed by a small transformer might actually have such a high impedance that an earth fault wouldn't be noticed as a short circuit by the breaker any more but merely as an overload.
BTW, I can't remember if we've covered this before, but do all US/Canadian breakers have both a thermal and a magnetic fast-trip? If there's only a thermal trip, trip time is likely a moot point anyway.
