This was originally posted on APC forums on 4/19/2012
Yesterday my son called me at work and said "the computers are beeping all over the house." My wife called me on the way home an hour later and said "the phones and the Internet connection are down."
When I got home I found that 3 of my 4 APC UPS units (Backups 550 x2, Backups 350 and Backups ES 350) were beeping and clicking like crazy. Only the older Backups ES 350 was not beeping. Looking at the log for one of the computers I saw that the computer thought that AC power was going away and coming back about every second. This was happening on 3 of the four APC UPS units scattered in different rooms in the house. This problem had been going on for over an hour. I asked my wife and son if any of the lights in the house and been flickering or doing anything strange at all and they told me "no." I checked power line voltage and frequency and found them to be fine while the UPS units were still acting up. Batteries in all units were replaced 6 months ago.
My best guess as to what went on was that there was some sort of power line noise or other AC power line anomaly that managed to get the UPS units into a bad state. I suspect a software defect in the UPS units allowed them to stay in this bad state after the initial power line problem was no longer an issue. Even if the power line quality had still been a problem, the cycling back and forth between AC and battery didn't make any sense.
These units had all worked fine for five years before this without any issue. Since this problem I've sent sensitivity from "high" to "medium." I doubt the issue will happen again anytime soon, but I am afraid I've lost confidence in the APC UPS units now. I'm concerned because these units back up telephone, alarm system, Internet, etc. Has anyone ever seen anything like this happen before?