As I have experiance of a data centre I can answer your question from the kit that we operate here - You have to be careful with any UPS derived power supply and as you pointed out you can get when you lose mains power up to twice the phase-neutral voltage across two conductors. In theory this could equal not far off 700v - the workings ? simple rms equations. the rms value of a single phase 240 circuit gives around 345v now put in two UPS units and worst case 180 degrees out then you have a voltage of 690v.
Where you can find people getting caught out with this is in simple dual fed items of kit - If you have say one side of the dual fed kit running on UPS and the other on mains of UPS then you have the possibility of having this sort of voltage kicking about. This is when people forget just what they are working with and need to be amde aware that there is a chance that they may encounter higher voltages than expected. Some test equipment may not be geared up for going across this sort of voltage if it has surge protection inbuilt and the person working with unfused probes then you could get a pretty nasty fright.
Any equipment with dual feeds and UPS sourced supplies you really need to mark up and ensure that the operator knows this, also if you are working with kit that has integral UPS units then you need to mind this if you have emergency power offs in the room - this will not kill the power and you may have a risk if someone gets connected to the power side of things and then somoene else hits the power kill button - the circuit may remain live and dangerous !
Rugged
