Getting Electrical Shock From Touching Computer Case

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Grandmastersexsay

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May 16, 2013
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I've got an old system that I was planning on re purposing. The only problem is when I touch any unpainted surface on the case, I get a small shock. I pulled out my DMM and found -12V from anywhere on the case to ground. The computer itself still runs fine.

The case is pretty dirty, and in definite need of some compressed air. Is it possible this is just dust, or am I going to need a new PSU? This thing has been running 24/7 for about the last 5 years straight. Anyone have any experience with this?
 
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RetiredChief

RetiredChief

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Feb 22, 2007
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1) Very few people can feel 12V. A few have been know to test 9V batteries using their tongue - LOL.

2) Have you checked the ohmic resistance from case to Ground (For USA, Case to round prong on outlet) should be a short (< 1 ohm).

3) Did you try to measure the "AC" voltage on case

4) PSU does have a Neg 12V output: HOWEVER, if the case is properly grounded and the -12V was shorted to the case the -12 V rail would have been shorted to ground. All Other DC rails are Positive.

5) Have you verified that you are NOT picking this up from the outlet.
 
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Grandmastersexsay

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First I checked resistance to ground, which was a short.

Then I checked AC voltage. Never thought to do that for some reason. The meter said 57V!!!

I guess I need a new PSU.

Thanks.


 
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RetiredChief

RetiredChief

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When you said you could feel it, reason I asked if you checked AC.
Are you in US, Reason I asked is :
AC Ground Should should go to Case ground. The wall outlet ground is connected inside the PSU to PSU chassie ground. Attaching the PSU to case should then put case (chassie) also at ground.
If You are measuring 57 VAC from case to Ground Then AC ground is NOT going to case ground.

Question what are you using as your ground reference.

Have you also checked the Neutral to ground at the outlet.
With the PSU rocker switch in the off position do you still measure this AC voltage.

Almost sounds like one of two things.
1) Your AC ground is not connected to the chassie, or
2) Your Wall outlet is not properly grounded.

Bottom Line, Not sure the PSU itself is the culprit.
 
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Grandmastersexsay

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Yes, I am in the US.

Now that I am looking, my UPS does have a building wiring fault light that is on. I checked another computer plugged into the same UPS, and that came up with more than 40 VAC from chasis to ground. Ground is just a piece of angle iron I placed on the concrete floor.

I guess I'll take off the receptacle an see what's going on.

I don't quite understand why I would be measuring such a large voltage. I thought ground was only used for static discharge?
 
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RetiredChief

RetiredChief

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Ground is a point that considered to be "earth" ground. A piece of angle iron would not be ground - Your water pipes are generally considered a good ground (unless plastic).
For a couple of bucks they have the little 3 prong testers at Lowes and Home depot that check proper outlet wiring.
 
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Grandmastersexsay

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For testing purposes, when nothing else is available, a piece of angle iron laying flat on a concrete floor is a pretty decent ground, as evident by the large voltage reading I was able to get.
 
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hunter315

hunter315

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May 12, 2009
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Not necessarily, that angle iron could be floating at any voltage relative to true ground, you are just measuring the voltage differential between it and the case, they could both be floating at 40 VDC with the case 12 V higher. Proper grounding for a building is achieved by pounding a long metal rod about 12 feet into the ground, its the only way to get it proper dissipation, the resistance between the angle iron and the cement floor it is sitting on is too high to ensure that it is actually losing all of its charge. Its not actually all that easy to make something have zero potential, that is why there is all that fancy ESD protection gear, you tend to float a few tens of volts above true ground even if you aren't intentionally insulating yourself, you just don't realize it because you aren't that sensitive to voltage.

If you have the computer plugged in it is likely into a two outlet receptical, measure relative to the ground prong on the other outlet so you have true ground to work with. Also the wiring fault light may indicate that your neutral line is not connected back to ground at the box, or that your ground lines aren't connected to ground at the box which would cause them to float at an arbitrary voltage. Since you have the warning light, pick up one of the testers and find out if it is a wiring issue, as long as you have the PSU screwed into the case it is hard to not have a grounded case.
 
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