@rplst8 posted:Breaker sizing is to protect the secondary winding in the transformer, and that is all. Getting a good magnetic "instant" trip breaker is to remove current as quickly as possible after a short. This can help to reduce the chances something gets damaged by wild voltage swings when a derailed loco is shorting, then not, then shorting again during a derailment.
There is no silver bullet, but a TVS diode combined with a breaker like above or one of the PSX1 devices is your best bet to protect engines with sensitive electronics (and your TIU).
For the Z4000, I'm not positive, but I thought it had pretty good circuit breakers, someone else can comment on that part as I do not own one.
The only fast-acting breakers on the Z4K are 5he fixed voltage outputs. They trip instantly. The breakers for the track power outputs are very slow and I wouldn't rely on them to break an overloaded circuit. I was very surprised to learn this after I bought 2 brand new Z4Ks.
