Should you determine that you need to divide your layout into power districts with the Digitrax Zephyr Xtra as the command station then I have four suggestions of value:
1) Be sure to set the trip current of the PM42 to 1.5 amps,
2) Be sure to set the trip speed of the PM42 to the fastest setting,
3) Be sure to set the command station to the slowest trip setting of 1/2 second,
4) and if you can afford it use the DCC Specialties PM42 breakout board which makes wiring the PM42 easier.
Note here that there are other circuit breakers available but should you choose one of those other circuit breakers you will still need to set the current to below that of the command station capabilities, set the circuit breaker to be as fast as possible, and set the command station to respond slower than the external circuit breaker.
Also make sure that the wiring between the command station and the circuit breaker and the circuit breaker and the rail bus is at least the same size as the rail bus.