Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

My friend showed me his dead computer. When I powered up, the green LED on the motherboard didn't even come on. He had bought a new $40 500W power supply and he wanted me to install it. I did, and now the green mobo LED did come on, but nothing else powered up, not even briefly. (I guess I mean the CPU fan not starting, and no beeps. What other signs would there be?) I unplugged all drives and removed all PCI cards. The power supply is plugged into the mobo in 2 places, the 20-pin connector and the 2x2 connector. Still dead.

Then I noticed that I got a small electric shock when my left hand touched a metal sink (ground) and my right hand touched the computer case. I measured the voltage difference to be about 75V! I unplugged the Power Supply from the mobo with no change. I removed the Power Supply from the computer completely, so the only connection was the power cable from the wall to the Power Supply. The case of the Power Supply still measured 75V! When the Power Supply was switched off, still 10V.

I decided to try the old Power Supply, plugged into only the power cable, and the same voltage measured on the case! Why is this happening? Is this normal?

I did drop the new Power Supply once. I don't know what may have happened to the old Power Supply.

Would this high-voltage case harm the motherboard? Maybe it wouldn't, because the mobo is electrically isolated from the case.

At this point, I'm not sure if the motherboard is dead, but first I want to clear up this high-voltage question.

Did the motherboard kill the new Power Supply, and that's why it's shorting?

Both power supplies pass the paperclip test (ie their fan starts when I connect its green wire to its black wire).

The motherboard is P4C800, and the system had (before I unplugged everything today) 4 hard drives, 2 DVD drives, 3 case fans, a video card and a network card.

share|improve this question
Home built computer? Were the correct amount of stand offs used for the board formfactor (ATX, Micro, etc)? Too many may be touching something to the case and mobo that was not intended. – Carl B Aug 21 '13 at 1:47
75V AC or DC? If two PSUs have "hot" cases, then it's probably not the PSUs! If you really know how to use a voltmeter and take precautions with line voltage, then take three AC voltage and three DC voltage measurements at the AC wall outlet: hot to neutral, hot to ground, and neutral to ground. Repeat the measurements at the PSU-end of the power cord with the other end plugged into the wall socket. Be careful! – sawdust Aug 21 '13 at 9:11
Have you verified that the outlet is wired correctly? Use one of these. The fact that you have multiple systems measuring a hot ground indicates that either the outlet is miswired (possibly neutral and hot switched) or you have 3 different systems which all have the same defect (unlikely). – Darth Android Aug 22 '13 at 17:34
@user200692 - All information should be contained in this single question. We should not have to view multiple questions to view your information. If you don't clarify the question this question WILL be closed as its currently not clear. – Ramhound Aug 22 '13 at 22:52
Thank you @sawdust for doing this test! Now I understand that this is apparently typical for Power Supplies (or is it only lower-quality models?). I guess most people don't realize it because their systems are grounded. One thing I'm wondering is how much power is wasted if there's a high voltage difference and low resistance, but this is not a forum. I can now get back to trying to revive this dead computer. I'd like to mark the last reply as "The Answer"; how? – user200692 Aug 27 '13 at 1:37

Your experience may not be unusual nor indicate anything wrong with either PC power supply. Your house has probably always been like this (no ground for third prong), but only now you noticed it.

I connected a PSU to the wall outlet through a 2-prong cheater plug to lift the ground pin.
With a Fluke multimeter and the other unused wall socket, I measured:

43VAC between hot and the PSU case,  
77VAC between neutral and case, and   
77VAC between ground and case.  

That 77 volts between ground and PSU case is essentially the same as your measurement of 75 volts !
I'd guess this is voltage leakage through the AC coupling capacitors that is lifting chassis ground.

Personally I would not operate this computer equipment without a proper safety ground!
Or at least replace the outlet with a Ground Fault Circuit Interrupter (GFCI) breaker outlet.
Similar advice is given here.

share|improve this answer
Why do these PSU even power on without a true ground? – Dan D. Aug 27 '13 at 7:27
PSUs don't need a "true ground" (if by that you mean the safety ground) to operate. Nor would it be easy to distinguish between a "true ground" and a ground that was connected to neutral at the outlet. If you somehow made that work and enforced it, then the significant number of old homes in the US that don't have grounded outlets would be unable to use PCs. – Jamie Hanrahan Mar 6 '15 at 22:39

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.