Oh man I went through this a few years ago...though I doubt it's the same problem.

I was a custom integrator and one of my clients had a beeping UPS. Unstable incoming voltage. Ok, not the first one of these to kick the bucket so I do the RMA.

Later that same week I'm at a different client for the same thing. Call my boss and tell him we should start looking at other manufacturers, he agrees.

RMA arrived and I install it. Later that day the client calls...the beeping is back. I call my boss and he's at yet another house dealing with the same thing.

"Wait...you're at [client]'s? That's a few blocks away. And so are [other 2 clients]. Are we sure it's the UPS...?"

TURNS OUT IT WASN'T! There was something fucky with the neighborhood grid (the transformer? node? I don't know, I'm not an electrician). We talked to ComEd, they and a few weeks later they did some repairs. No more problems.

Could be battery. That is usually the first thing to swap no matter what.

It could be whatever internal component the UPS uses to measure the line voltage is going bad. If that is the case your probably looking for a new UPS :(. Unless you want to do some very indept troubleshoot and potentially board level repairs.

Other option is to try the UPS on another circuit, or if available, another building entirely.

and it gets nowhere near those thresholds, the voltage fluctuates in the 224-234 range.

I'm assuming you measured hot to neutral, and didn't find any transients.

Did you put your oscilloscope on hot to ground as well? A faulty ground could cause what you are seeing.

Do you have any CFL bulbs anywhere in your house? I once had a dead CFL bulb in a fixture in my basement. Anytime I turned on my basement light, every LED lamp in the house would strobe.

I would wonder about shared loads on the mains causing undesirable power factors or other electrical noise. I'd check that the mains frequency remains within specification as well: 50/60Hz ± 3Hz according to APC.

It could also be noise on the line, try introducing a ferromagnetic filter (low pass filter) to see if the situation improves.

Check also that the occurrences are not linked with the activation of an electric motor. I one bought a meat grinder that evertyime was on the analogical radio got only noise. And they even were not connected to the same circuit.

I had one that switched on constantly. I changed some sort of sensitivity setting and now it only switches on with actual power loss.

My laser printer being on the same circuit causes my UPS to trip unexpectedly even though they aren't connected. I've had to move it to another breaker.

Don’t know for sure but the first thing I’d do is get a new battery. 3 years ago means it’s overdue, and if the battery is damaged or not putting out proper power levels it could be the cause. I always know it’s time for a swap when my UPSes start tripping for no good reason, although I’ve never had one go nuts like yours is. They usually just stop working if I let them go way too long between swaps.

I’m dealing with something similar but not as bad.

Our best guess is brownouts for our instance, but it doesn’t sound like what you’re dealing with. Good luck. Let me know if you’re able to find out. Hope the hardware isn’t failing.