I am sure the answer must be implementation dependent.
AFAIK the reason for a CB in the field current is
- to enable the pilot to prevent the alternator from generating any power, but without having a BIG (up to say 100A rating) switch in its output line
- to protect the field circuit in case of a shorted field winding
Neither CB should pop on an overvoltage condition, so long as the alternator is basically working.
If there is overvoltage protection (how common is that?) that should be implemented inside the voltage reg, which should open-circuit the field winding when some preset figure (e.g. 35V on a 28V system) is exceeded.