From my experience, I would not use a fuse on the alternator field circuit.  I have a B&C voltage regulator that will pop the breaker once in a while.  I had it updated by B&C to the latest design and ran a dedicated short wire for voltage sense per their recommendation. What happens is that a small bit of electrical noise can fool the over voltage circuit and the crow bar circuit shorts out the field thus popping the breaker.  It used to happen quite frequently, but with the update it still happens,but very rarely.  Just reset the breaker and it won't do it for months.

 

Tom Byrnes

Lancair IV N54TB