We have a 100kW 3Φ 208V generator and ATS operating in North America. The system as installed does not allow for a test where the load can be placed on generated power when utility is available. We have solicited quotes to modify the system to allow such testing.
The suggestion from two licensed electricians who have bid the project has been to interrupt the sense circuit which the generator uses to detect utility power. The ATS contactor is two-position three-pole (single coil). I asked if the lack of "dead time" where the load is disconnected from either supply would cause problems, and got what I would best describe as a shrug.
Since the generator is not phase synchronized with utility power, I imagine this would cause... interesting things to occur. I'm not so concerned about lighting or electronics - but synchronous motors (HVAC / Elevator) do seem like they would have quite a bit of stored energy. On the other hand, I can envisage this to be equivalent to starting from a stopped state or slip as one would see from a VFD.
Should a rapid (~50ms) loaded cut between two unsynchronized sources be expected to cause problems other than the brief outage for an average office building?