@digitalnomadjc
Yes any IT equipment will fail if power has been momentarily removed for >10 ms for a 50 Hz line frequency and about 8 ms for a 60Hz.
The reservoir caps in any power supply, at best, are rated for half the line frequency hold up. Usually they are quite underrated.
Almost any ATS is always a break-before-make type, since accidentally turning on both the static switches inside will short the two independant sources.
I suspect that the 26 KVA ATS you mentioned about is only a transfer switch designed for a delayed turn on to a secondary source.
I am not sure, but AFAIK generator transfer switches are (mostly) delay based transfer switches.
@skimask
The "hold up" time you are referring to is at the output of a DC PSU.
The OP, i understand, is interested in the holdup time of the input AC supply.
:-)