Maine Sail- what exactly do you mean by "in a fault"? Is that a term for a specific kind of circuit?
Any sort of "fault" that would trip the breaker such as a short, internal problem in an AC appliance, over load of the circuit or other problem...
Unlike land based applications we have many, many, many cases of reverse polarity issues on boats. Thus the white and black are both interrupted in the event of a fault and a reverse polarity indicating lamp is installed.
Muffy & Skippy fire up the new 42 foot Sea Ray toss the dock lines and proceed to yank the dock pedestal out by the roots. The marina sends Darrell and Darrell down to fix it but Darrell confuses the silver screw for the brass one. These types of issues are why we break both the hot and the neutral in marine applications.
If the pedestal or your boat are improperly wired so hot and neutral are backwards, and you have only a single breaker tripping HOT, as shown in the original pic, the neutral, NOW HOT, has no circuit protection other than the main breaker which is way larger than the wire is rated for. Also the neutral bus will always be hot with no way to turn it off short of unplugging. A turned off double pole breaker isolates the neutral bus but not when it is wired for breaking just the hot wire.
What if someone is working on your boat? They assume they have turned off the main breaker but unless they physically unplugged the boat the reversed neutral is still live due to the reversal of polarity. This is also why reverse polarity indicators are so important. A boat with an AC system should have a reverse polarity light and a double pole breaker that interrupts both hot and neutral.