George,
I would think (mainly speculation) that the sizing of the OCPD would have to be done on a case by case basis, depending on what the load and application is. In your fire pump example, locked rotor current makes sense because that is the maximum current that could be drawn by the motor in a pre-failure condition. After failure, whether phase to phase or phase to ground, the available fault current would come into play which could be anywhere from 5000 amps to 25,000 amps or even (4) times that depending on many many variables. That should be enough to open the circuit (provided the OCPD is rated for the available fault current).
It might not be a bad idea to check available fault current to make sure you use the proper class breaker or fuse, but I guess that's the case for all of us no matter what the application.
