Don, I'll answer your last question first. Breakers and fuses should always be on the transformer outputs, so they protect everything downstream.
A fuse or breaker in the U circuit has one purpose, to protect the transformer. A postwar ZW shouldn't be subject to more than 10 amps. FYI, the ZW internal breaker is in the U circuit, but is very slow acting. It provides no protection if there is a short between any of the ABCorD outputs, which is possible.
A current of less than 10 amps can cause considerable heating on the layout. Breakers or fuses should also be put in each of the output circuits to protect the layout. They should have the lowest amp rating that will operate your trains, which is usually less than 10 amps. On my layout, I run 6 TIU channels. Some channels control areas where probably not more than 1 or occasionally 2 trains will be moving at once, such as yards. These get 5 amp breakers. Mainlines where I might have 3 locos running get 7.5 amps. But I do not turn smoke on.
No fuse or thermal breaker opens immediately upon the current being exceeded. There is a lag while the thermal elements heat up. By using smaller amperage breakers, they open sooner on a short. In another thread, Gunnrunner John mentioned certain magnetic breakers available from Digikey. Magnetic breakers should open as soon as the rating is exceeded. Electronic breakers, such as in the Lionel PH-180 Powerhouse, also open upon exceeding the rating. I am considering replacing my thermal breakers with magnetics, but have some higher priority matters to deal with.
FYI, I use Blue Sea marine push button breakers, available on-line from Defender Marine. They are in the $5-$6 range and have worked well.
Hope this helps.
RJR
