For a ESS I use AC-IN with Grid connection and Main AC OUT 1 for critical loads. In the following I mention a few facts to make sure, I understand everything correct. Pls. comment and correct me, if there is anything false in my understanding.
1) I assume, I must never feed in any energy in to the MP2 out terminals. OUT1 or OUT2. I do not intend to connect additional PV inverters at AC OUT using the 1:1 rule.
2) To make the critical loads non stop running, there is the idea of a bypass contactor from MP2 AC-In to MP2 AC-Out1. See the MP2 block diagram, with 3 external contactors. K-In, K-Out and Bypass what disables the internal MP2 redundant K-sync contactors at the MP2 input.
3) Once the bypass switch is closed, the AC OUT1 terminals should be disconnected as soon as possible. The release time of a few msec of any used external contactor will be sufficient to avoid any gap for the critical loads. (early make, late break)
4) Using a 1-0-2 switch to select the source for critical loads between grid and MP2-Out1 always produces a short gap. It cannot replace the nonstop switch with external contactors.
5) As critical loads are supplied by the grid, we may open the input contactor K-in at any time to replace the MP2 unit by another one.
6) After MP2 replacement is installed, I reconnect battery what starts up the inverter. For this moment, AC Out1 is out of sync with the grid, why we must not close the K-Out output contactor.
7) To allow inverter phase sync, I close the AC in contactor. The inverter needs some time for phase shift. If in phase, the internal redundant K-sync contactors close.
8) Same time the internal K2 closes to supply the loads on AC Out 1, if any exist. In my case, there are no loads on AC Out 1, but it is possible to use this output for indication that the inverter reached a stable state with phase sync.
9) After phase between MP2 inverter and grid is in sync, it is allowed to close the output contactor K-Out and open the Bypass contactor with a early make and late brake. The overlapping time should be as short as possible. A few msec are accepted. If bypass is closed for a longer time and grid fails in this situation, the MP2 would be not able to disconnect from grid.
10) Again, the critical load does not see any gaps in supply and the new MP2 is now in operation.
11) The used grid is not a public one why I am allowed to keep the neutral an PE lines always connected.
