I worked for a company doing mostly on-premises (wireless) carrier software years back but they still wanted our own server room to be 3 nines for reasons lost to me now. They had installed new electrical circuits high on the walls so that we could survive minor flooding incidents.

So our lead Ops guy is unplugging half the redundant power supplies and plugging them into the new circuits, but a few critical servers are single PSU still, on UPS units. This is how he discovers that one of our PSUs has rotted and only has about five seconds of reserve power in it. Big outage, no bueno.

Exactly that, we too suffered a power loss at our DC that was due to faults in the power supervising and switch-over circuits; we test both generators weekly.