An open alternator output, for example 50a breaker tripped or disconnected wire could cause the field breaker to blow only if the regulator suppling full voltage to the field draws enough current to trip the 5a field breaker. This should not be possible but if the 5a breaker is old and tired and, say, tripping at 3a your could see it trip. I have never seen this happen though.
Don’t overlook wire fault as a cause, insulation worn through and he wire touching something grounded etc.
Ok, great news.
Now disconnect the Field wire from the back of the alternator and leave hanging.
Does the 5A break still blow? If it does it's a bad regulator. If it doesn't blow now, then we may have a shorted field in the alternator.