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I've purchased a Line Interactive UPS 1200VA. It's supposed to give 8~20min backup (as written in the manual). This UPS has transfer time < 10ms. It works fine when the mains is active.

But the problem is that when the power went off, the PC also went off for a fraction of a second and started rebooting.

The strangest thing though is that it has to be a proper power failure by the service supplier. If I just turn off the power at the wall plug, the UPS kicks in immediately and the PC carries on working fine.

What's the solution of that? What should I do? My hard drive was physically damaged because of improper shut down & restart. I've 5~8 times power outage in my country so I badly need an UPS.

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Do you have any other extension lead between the UPS and the PC or anything like that? – Bora May 15 '11 at 15:03
This is usually caused by a low-quality or overloaded PC power supply. It's required to hold up the DC voltages for 30 milliseconds until the UPS is able to catch up. Very poor power supplies, or overloaded power supplies, are unable to do so. – David Schwartz Jul 15 '14 at 22:56

Does your UPS also properly protect against surges as well as power drops? If not it could be that the power outage is preceded by a brief surge which rather then smoothing the UPS reacts badly too (or allows through and the PC's PSU reacts badly to it). I would expect that must UPS devices are also designed to smooth out surges as well as protecting against outages, but it might be the case that either not all do, or that the power problem your incoming supply presents is outside the range of what your UPS is intended to deal with. It could also indicate that your USP is faulty and not correctly dealing with circumstances that it should be able to brush off.

It would be a good idea to add the make+model of UPS to your question. Also, the country+area you are in may make a difference to how likely certain electricity supply anomalies are, so you might want to specify that information too.

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Depending on the model of the UPS, you might be able to get some information from the monitoring software that comes with it. Connect the USB cable that came with the UPS and install the software. Usually there will be a log inside the UPS that it can read that will show you when and why it failed. Contact the manufacturer and see if they have any suggestions.

My suspicion is that the UPS isn't switching fast enough for the computer.

A line-interactive UPS handles brownouts without relying on the battery by using an adjustable transformer - the transformer adjusts the incoming voltage to the proper 110 or 220V that you normally get. However, while it is adjusting you lose power for a few milliseconds. This rating is probably NOT the same as the transfer time (where it switches to battery). Usually it's less than 10ms, but it could be that yours is slower or simply defective.

In most cases, it switches fast enough that the connected devices don't notice. Power supplies usually have big enough capacitors in them to run for a few milliseconds after they lose power. But if you have a weak or cheaply made power supply in your computer, it may not be able to handle it.

I can't think of any way to test this except to wait for the next brownout. Plug something like an LED light in and see if it turns off for a noticeable amount of time, longer than the time it goes off when switching to battery. If so, we found the problem.

If it is a defect, the manufacturer should replace it for you free of charge. If not, I'd recommend either replacing the power supply in the computer (if it is old, cheap, or underpowered for the system it's in), or getting a better UPS.

If you live somewhere with a lot of power issues, you may even need an online UPS, where it is constantly converting the AC power from the wall to DC and charging the battery, and converting it back to AC to power the computer. An online UPS doesn't have a switching time at all, and won't cause the computer to reboot.

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This might be related to the output generated by the UPS.

I had a problem where some of the PC's in my network would shut down when the power went off. They were all using the same UPS's, but each had its own unique power supply. This was a few years back - I found a forum post that explained it perfectly but I can't be bothered to hunt it down again.

My client had a generator so the UPS's were simply to stop the PC's going off until the generator kicked in.

I found this thread, posted March 2014:

Active PFC and stepped sine waves do NOT play well together. If you get a new computer, or a new power supply, it's possible it might not work with your old UPS. If you buy a new UPS, you should make sure it produces a true sine wave output, not only because you're current computer might need that, but because your next computer is even more likely to require it.

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What happens when you unplug the UPS from the wall socket?. If the output power is interrupted then the device isn't working correctly - I'd contact the supplier/manufacturer and see about getting a replacement.

If the power isn't interrupted then something else is happening. Could it be that that power is surging rather than dipping and this is what's causing the problem? You might need to make sure you've got some surge protection in there as well.

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