UPS suddenly started oscillating between mains and inverter (it popped).

[vgamesx2]

The title sums up everything, I was standing around not doing much when my UPS started making TikToks with the relay, I was trying to troubleshoot/reset it to figure out if it was just being silly when one or more capacitors have guaranteed blown and it started smoking, I quickly disconnected it and the battery, thankfully no flames or anything dramatic but do guys you have any idea what would've caused that to happen?

[kjr18]

Probably constant power cycling caused by not stable supply. It also depends on what type of UPS it was, online or offline. Offline starts supplying from battery when mains disappear, online runs all the time. They also can normalise  to some extent output voltage, by connecting different taps to output transformer, at least mine was doing it.

[james_s]

I'd bet bad capacitors are the root cause of the issue. Bad cap causes the internal power supply to be unstable and confuses the UPS.

[vgamesx2]

Probably constant power cycling caused by not stable supply. It also depends on what type of UPS it was, online or offline. Offline starts supplying from battery when mains disappear, online runs all the time. They also can normalise  to some extent output voltage, by connecting different taps to output transformer, at least mine was doing it.

Nope, I can confirm we actually have fairly stable power here, it's fairly rare to be bad enough to trip a UPS much less full outages and surprisingly even surges have been a non-issue, can't remember one ever taking out any unprotected equipment.

I'd bet bad capacitors are the root cause of the issue. Bad cap causes the internal power supply to be unstable and confuses the UPS.

Yeah, that's what I was thinking, wasn't sure and I don't want to open it right now though.

[Vovk_Z]

Bad electrolitic caps are reason #1, because it is usually hot inside UPS.
And if UPS is old enough it may need to 'recap' relays too. Dead relays is reason of UPS fail number two.