I think the biggest problem is that the fine wires used in dcc

equipped locomotive will not handle 5 amps, if there is a short.  At the modular club I belong to, we need 5 amp breakers to allow lash ups of 4 or 5 power units to pull long trains, but if a locomotive derails and causes a short, often it is the wires in the locomotive that is the "fuse" rather than the circuit breaker tripping off.

I see no need to install a CB

I see no need to install a CB between the power supply to the Command station or Booster unless the wires between them is a long distance. In that application you want the CB at the power supply. It would protect the long wires from a fire if lets say you drove a nail thru them and shorted. Depending on size of layout, I would consider Electronics CB(s) such as DCC Specialties on the downside of the Command Station and any Boosters.

Trip order

You need to think about what order you want the various overload devices to trip in.  You want the short circuit protection device closest to the train to trip first, and the house circuit breaker to trip last, and everything in between only tripping if something goes wrong with the things closer to the train than it is.  (Loosely speaking - a bunch of 1 amp electronic breakers powered by a 5 amp booster could each be seeing less than 1 amp, but in total they could pull enough to trip the 5 amp booster.)

So if you’ve got a 5 amp booster, it’s not going to trip until it’s drawing a little more than 5 amps.  If the booster input voltage is only a couple volts higher than the track voltage, and you put a 5 amp breaker between the power supply and the booster, it’s a little bit of a tossup whether the breaker you added trips first or the booster’s overcurrent circuit trips first.  Chances are the booster will go first because it’s trip time will almost certainly be faster.  But you should test it.

By the same token if you’ve got a 5 amp booster powering a bunch of electronic breakers each set to 1 amp, you want to make sure the trip time on the 1 amp breakers is at least as fast as the trip time on the 5 amp booster so if you get a solid short on the track, just the 1 amp breaker powering that section of track trips, rather than the booster cutting power to all of the 1 amp breakers.

Use the quarter test at each step to make sure the right thing trips.

Does your power supply not

Does your power supply not have a fuse or circuit breaker on its output?

If not, additional protection is a good idea. The 5A rating might be a little close, as the value is nominal.  

Since Digtrax uses "rate of change" to protect the booster's output, it can disconnect the track power at one or two amps should a short occur.  If the current suddenly spikes it will disconnect.  That will happen much faster than a thermal breaker. 

Does your power supply not

I was wondering why a breaker between the power supply and the DCC command station is necessary.  I built a power supply based on a kit that was offered years back by Spring Mill and it included a breaker or fuse.  I would think it would be unusual for any power supply to not have one or the other.

Per another topic on wiring a layout for DCC, I'll be using the PSx breakers to break up the layout into 3 or 4 power districts.  Assuming that is a "norm" (breaker/fuse protected power supply and district breakers)  for moderate to larger layouts, wouldn't that be sufficient?

Power supply protection

Usually power supplies have a fuse or breaker on the input (wall plug) side, as a last ditch protection against pretty much anything going wrong inside.  Assuming your supply has that, then yeah no need for a breaker between the supply and the command station.  The command station itself should have it’s own overload protection on the track output.  And if you put PSx breakers set to lower trip currents between it and the track to get separate power districts, that even better.

But if it makes you feel good, putting a breaker between the power supply and the command station is fine, as long as it’s rating is high enough that it doesn’t trip before the command station’s track output protection trips first when you short it.

FUSE

Some years ago I experimented with using a circuit breaker and found they do not trip very fast compared to a regular fuse. I gave up the idea after performing some test and now just use fuses. 

Mark

What Power Supply(ies) and Fault Protection

If and what protection could be needed between the power supply(ies)  (PS, PSs) and booster(s) will depend upon which power supply(ies) you are using and how many boosters one PS may power. All of the PSs provided by Digitrax, most PS bricks purchased online and most OEM Switch Mode Power Supplies (SMPSs) include over current and over temperature protection. So for most normal installs with one PS powering one booster no additional fuses or circuit breakers would be need. If you powering multiple boosters with one PS like a Digitrax PS2012E (20 Amp Power Supply 13.8-23VDC) or OEM SMPS you will probably want to add a fuse for each booster. With 50 Amps at 24 VDC available I chose to install fuse blocks between the PSs and the modules in my power box. One modular layout I work on powers eight DIY boosters with one 15 VDC 20 Amp SMPS. Each of the booster inputs is fused.

But!!! Which Fuses or circuit breakers to install? This is actually not an easy question to answer and depends upon what faults you are attempting to mitigate, the functional characteristics of the loads (boosters) under normal and abnormal conditions (faults - eg. shorts) and more importantly the characteristics (specifications) of available protection devices (fuses, circuit breakers). I have seen properly specified 5 Amp fuses run at about 5 Amps for thousands of hours where other 5 Amp fuses in the same circuit opened almost immediately. So just grabbing a fuse at the local hardware store or online may or may not provide adequate protection or may provide a lot of opportunities for troubleshooting frustration. Although I am a fan of circuit breakers, specification and selection of appropriate breakers gets even more complicated, and IMHO for this application should be avoided.

You can find fuse and circuit breaker guidance (orconfusing) literature online, but a good starting point for fuses would be The LittleFuse Fuseology Selection Guide.