I have several DL380 G5 servers that instantly shut down when UPS switched to battery. These are all pure sine wave UPS units. two of the UPS units are APC and one is Eaton. they are 1500VA units.
One interesting thing is that out of 3 servers, 1 of the 6 power supply's will hold during the switch-over. that is, when I move this power supply to different servers and UPS units, the server stays running during the switch-over.