On 1/27/21 1:42 PM, Carlos E. R. wrote:
My experience with UPS may be limited, but back in the day I ran a 
10-KVA UPS to power a group of Sun Microsystem servers.  It was a 
nice UPS with a ferroresonant transformer that conditioned power as 
well as running the 10-KVA load for 30-minutes.  In my experience,
we experienced more unplanned power outages caused by the UPS itself
than if we directly connected to the mains.  After we retired the
Suns and switched to SuSE I once managed to keep the main server up
and running continuously for a bit more than 4-years, without the
UPS.  Without a reboot!  It was a busy server too, with a hardware
RAID controller.
Do you remember how did the UPS fail? I'm curious.

The UPS never actually failed, except for a time or two when the
battery bank needed to be replaced.   The unscheduled outages
were more due to me fiddling around with it and messing it up.

It was a Best Ferrups 10-kva model.  Looks like they're still made:

https://www.eaton.com/us/en-us/catalog/backup-power-ups-surge-it-power-distribution/eaton-ferrups.html

The battery bank was in a separate container as big as the main
unit.  They were free-standing, not rack mounted.  It would power
the load through the saturated transformer and when power dropped
it would use the battery dc (switched I guess) to serve as the
AC input to the primary.  The load would never see a glitch, until
the battery bank discharged.  The saturated core of the transformer
served to regulate and smooth power to the load. 

Regards,
Lew