I attached schematics of my system.
DISCLAIMER:
I am not an electrician and located in Germany. Please check with you electrician local rules.
My grid connection is protected by NH00 63A (not shown) followed by SLS 35A (shown).
The grid goes two ways parallel:
Left: directly to left side of 1-0-2 manual transfer
Right: via MP2 to right side of 1-0-2 switch
The MP2 is protected by MCB on ACin and ACout
The loads are connected after the transfer switch and protected by RCD and MCB.
Cases
Switch pos 1, A & B open: maintenance
loads are grid connected, MP2 can be maintained
Switch pos 1, A closed: grid parallel ESS
MP2 and loads are parallel, loads are served by grid and supported by MP2. if grid failed, loads are not supplied
Switch pos 0: loads are disconnected
Switch pos 2, A+B closed: ESS with critical loads
Loads are on ACout supplied even if grid fails. ESS is working.
Switch pos 2, A open, B closed: off grid
Loads are served by MP2 but disconnected from the grid
In my system the N is never connected to ACin and ACout same time.
A 4p 1-0-2 switch works like:
Switch pos 1: 3p+N connected on 1
Switching from 1 to 0: 3p disconnects first, N later
Switch POs 0: nothing connected
Switching from 0 to 2: N connects first on2, 3p later in 2
Switch pos 2: 3p+N are connected to 2
The switch is a 1-0-2 not 0-1-2.