I am wondering to find a clear advice how the mains (AC-IN, AC-OUT 1 and 2) of the multis in a 3-phase system should be protected/coupled:
Via 3-pole or 3+N-pole circuit breaker/fuses and why?
In my case it is not essential to have 24/7 power, i.e. isolate one-by-one multi in case something is to maintain, is not necessary (hopefully this has to be done seldom anyway 
Thanks!
To my knowledge a 3-phase system only runs when all inverters are online.
So you might as well use the 3+n breaker.
I don’t have an iron-clad explanation for you, but for my 3-phase utility connection I use a 3-pole circuit breaker for phases A, B, and C and pass the neutral through. There is no harm using a 4-pole breaker to disconnect the neutral.
And @Ludo is correct, all inverters in an inverter group have to be online for the group to function. If you take one Multi down, the entire group will halt until all group members are available.
Thank you @Ludo and @OGPS for your support!
Oh yes, it really make sense the other multis will also switch off in a 3-phase system to prevent problems with 3-phase loads (motors…).
In my ESS setup normally domestic grid will be connected to AC-IN, everything is powered via AC-OUT 1.
In case of maintainence of the multis/ESS, I will switch via 4-pole bypass (1-0-2 switch) to power everything via grid.
Therefore the disconnection to AC-OUT1 will be done 4-pole via bypass anyway, so it doesn’t matter whether a 3-pole or 3+N pole breaker/fuses disconnector is installed.
On AC-IN side it will be better to use also 3+N to have a clear disconnection in case of maintenance the multis/ESS. In order to make sure the AC-IN N of multis is 100% potential free.
However, I find it strange this topic seems to be hided pretty well or is it just too basic
Have a good one!