The UPS protection and switching design has to follow the IEC rules for disconnection and protection.
In particular, Disconnector switches shall be installed when a disconnection is required.
Circuit breakers in series would instead, create protection issues and be dangerous for the operator.
Accessories like handle blocking coil, auxiliary contacts are powerful yet simple solution to make a safer UPS.
The neutral should be rated according to the expected level of 3rd harmonics.
The neutral reference to ground can be lost temporarily, the earthing system being changed into a temporary IT system, without any harm for the equipment (UPS, servers, etc).
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White paper: Protection, switching and neutral issues - IEC
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