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The scenario:

I attached a desktop to a power strip that was connected to a UPS and then all the other components on the UPS shut off. I thought I might have accidentally clicked the kill switch on the power strip, but it turned out I didn't. I unplugged the desktop and tried to turn on all the components that shut off. Everything turned on except a desktop that was already connected to the power strip. So I took this 'broken desktop' over to another wall outlet but it still did not turn on. The next day this desktop started to work again.

Does anyone know what might have happened here? Or recommendations for a better computer layout? I have a few ideas but I thought I would check in with the Super Users.

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A UPS has a maximum power output, and when that is exceeded, it shuts down by disabling the oscillator or an internal circuit-breaker or fuse may blow. Evidently, the startup current for the last desktop added exceeded that limit. It is doubtful you exceeded the limit for the power strip because those usually handle ~1,880 W, far beyond what two computers and monitors draw.

Your setup looks reasonable: you only have required devices on UPS such as a PC and associated monitor. The only way to save a few watts would be to plug the displays directly into a wall outlet. Though a power glitch would temporarily lose the display, the PC would still be protected. This is not optimal, though, and might set up a ground-loop, as all items are not plugged into the same circuit.

A better solution is to get a second UPS for desktop 1/monitor 1.

As for why the PC did not restart immediately, but did later, my guess is that on retry, the OS did some recovery "magic", e.g. abandoning swap and hibernation files damaged by the abrupt outage... just be thankful it did recover.

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  • Seriously, big +1 for mentioning the (somewhat esoteric) ground-loop issue.
    – misha256
    Commented Nov 30, 2015 at 22:39
  • Thanks for the response. I agree the computer is definitely doing some magic that I really would like to understand. Although, I am thankful it did recover.
    – Robbie
    Commented Dec 1, 2015 at 1:13
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A possible cause of this is a voltage drop. While it doesn't explain the misbehaving desktop computer, it would explain the rest: As you may or may not already know, a computer (and most other kinds of hardware) use a lot (relatively speaking) of power when it's initially powered on. I'll spare you the electrical details, but it basically boils down to a very brief spike in current running into the hardware, and in this case, out of your UPS. Enough hardware turning on at the same time, and the current drain will be enough for the UPS to being able to keep the voltage up, resulting in a brief moment when the hardware shut down from being underpowered.

As for the desktop computer: Any chance it has a faulty PSU?

Also: Are you sure the UPS is rated for the theoretical max load from the hardware?

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  • Thanks for your response. Yes, I think the PSU could be faulty. It is a $500 HP Pavilion. I am guessing they are pretty low quality. The UPS is probably not rated high enough for all this hardware.
    – Robbie
    Commented Dec 1, 2015 at 1:10
  • On startup, most power supplies demand a surge of current. There is no reason to suspect the Pavilion per se.
    – DrMoishe Pippik
    Commented Dec 1, 2015 at 1:41

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