Wouldn't it be logical to assume that the ACU just gets the field current from the output side, which is energized when you switch on the alternator?
The alternator field cb most likely trips on overvoltage, not overcurrent.
Otherwise having one breaker that disconnects the alternator would suffice, wouldn't it?
But i don't have the schematic, the above is inferred from some rudimentary knowledge about electrical systems in general.