Interestingly, in the 10 or so years I've had a Z4000, I've never had the externally resettable breakers for the handles ever pop; it's always been the internal electronic. Had forgotten they are there.
You are correct Frank in your comments on the ZW having the possibility of an unprotected short between handles. This is a good reason for putting a breaker or fuse in every output (A,B,C, or D) of a ZW (or virtually any other standard transformer) even if you're absolutely certain the internal breaker (on U circuit only) is perfect.
