The village where I live was sat under a thunder cloud for most of Friday, and we suffered a few power fluctuations (specifically, what seemed to be split-second outages). When I got back home from work, I found that my PCs had shut down during one of these outages. When I went to boot one of them back up, I couldn't get anything to display on screen, nor did the boot seem to complete correctly. I tried a number of things - unplugging different bits of hardware, swapping graphics adaptors, etc. - to no avail. I thought I was looking at a fried motherboard or CPU. Power seemed to be distributed correctly to the peripherals (the drives all appeared to be working) so I figured it couldn't be the PSU. Eventually I unplugged it from the mains and left it overnight (approx 12hrs unplugged). I tried it again this morning, and it booted up correctly. Woo-hoo!

I have all my equipment protected by surge-protected power strips, so I don't think a spike caused these problems. Obviously it has something to do with the power fluctuations, and maybe the PSU in the problem machine got itself confused somehow.

The questions are, for future reference and to help people with similar problems:

  1. What are the likely causes of the boot failure I experienced?
  2. Is a UPS a simple and cost-effective solution, or might other things help prevent this happening in future?
  3. What UPS can you recommend (my budget is limited)?
  • duplicate of superuser.com/questions/7766/…
    – Josh Hunt
    Jul 18, 2009 at 12:49
  • @joshhunt My question isn't an exact duplicate of the one you linked to, as my computer was still able to POST, I think. I got a couple of beep codes out of it, e.g., when I took out all the RAM.
    – alastairs
    Jul 18, 2009 at 23:37

3 Answers 3


If you have a brown-out, you may lose the power to one of the power rails, but not the others. This could cause it to appear off, even though parts of the power supply are still on. This could especially be the case if you have a power hungry Video card. It would pull all of the power off the 12V rail, but the CPU, doesn't generally need the 12V rail to actually function. If this is the case, you might see the Num-lock light on your keyboard.

I've had the reverse problem, where the 5V rail lost all power, because of a faulty USB cable. So the computer wouldn't respond, but the screen still looked the same, because the Video card still had plenty of power.

If you suspect this as the problem, unplug it from the wall, and press the power button. This should be enough to empty out the capacitors.

I think you wouldn't have had this problem, if you had it on a UPS.


I have an APC ES 550. It was about $60, and seems to work fine for me.


APC, excellent UPS'. Make sure you use the protected outlets for the computer. Most have protected and unprotected outlets.


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