It can be a bit confusing for some people to understand how a backbone system works, but the following gives a basic description to help understand it when connecting new devices, or troubleshooting.... or to just to understand why they have a power draw when all components are off.
it doesnt matter if you have a small boat with one or 2 devices/components attached by a backbone system, or a large ship with many components in the system, the system is the system and it all works the same...
Im not sure about the generation of seatalk system you have specifically, but as close as it is to the nmea2000 (which now seems to be standard) I am reasonably certain that it operates nearly the same.
when installed on a backbone system, there are two power sources to every control head/unit. one source powers the screen and the units controls, and the other source comes from the network cable itself.
let me clarify a couple things here.
on a backbone system, its only the control head/units that need the extra power source..... everything the retrievers/senders require for proper operation, is offered thru the backbone.
most "basic" displays that have no controls that can be used to adjust the system settings, may operate as soon as connected to the backbone, as some displays use very little power and can draw what it needs thru the backbone cable..
and, any n2k compatible unit that is installed and is NOT connected into the backbone system, almost always can be used with only one power source to it.
the backbone network is designed to be powered on all the time, regardless whether any of the attached devices are on or off.
and this is why in the installation instructions it recommends an on/off switch be installed in the wire that powers up the network backbone, so that it can be shut off when the network will be unused for longer periods.
the network system still has the same power draw when all components are off, which can be a respectable amount, depending on the size of the backbone system. (some longer systems require a power source from each end to feed the system)
all " retrievers/senders" (gps antennas, transducers, engine senders, tank monitors, wind/weather instruments ect..) are still powered up and working even when the component itself is not.. (the screen is off but the internals are only in sleep mode and still alive from the system power)
the reason is, some types of boats have more than one station, and the electronics can be shut off at all stations not being used at the time, but all of those at the working station can be on and the network still powered up..... then, if one of the components at another station is desired to be monitored, it can be turned on and is immediately ready without waiting for it to initialize..
sometimes a boat may be anchored with all network devices off except for the wind/weather station, (or some other component) so the network still needs to be powered up.... but if the component is not connected into the system, it can be on while the network cable power is shut off.
as for turning it off when not needed, one should not use the circuit breaker as the on/off switch, but a dedicated switch should be wired into the system at the point where the network cable taps its power from.
a boat I purchased earlier this year (2017) had an extensive n2k network system that the PO did NOT understand how it was supposed to work, and so most of the electronics and tank monitors didnt work, even though he hired his "electronics guru" neighbor to install them, new, 3-4 years ago... nearly everything in the system is redundent ( 2 of everything) and is quality stuff, but that means nothing if it doesnt work.
I will admit that I didnt know a lot about the n2k system at the time, but as my brother bought a new boat about 4 months before I aquired this one, and he wanted his electronics connected in a system to his n2k ready motor... so I had some minor experience in building a small 2 component system on his boat.
but the system on the boat I bought was much bigger and already installed and, NOT working, so was a large headache..... until I realized there were 2 n2k backbones running thru it.... then it became a smaller headache while trying to sort one system and its components from the other.
I learned what the previous owner never did... you can connect ALL the components on to one backbone cable, AND the backbone cable needs its OWN power source.... with a shut off switch.