Bad wiring or defective PC power supply: Why am I getting shocked?
July 12, 2007 11:04 AM   Subscribe

Bad wiring or defective PC power supply: Why am I getting shocked?

Okay, so they are rewiring the apartments in my building and after they replaced my breaker box and I powered up the server I keep in my closet it failed to boot. When I took the cover off it and then leaned on the metal shelving unit it sits on I got a shock. One quick multimeter check later I noticed that the AC voltage between the ground on the machine and the metal cabinet was 60 volts, which seems like a lot.

My question is whether the power supply in the PC is shorted or if they miswired the ground on the outlet. Normally I would just call the electrician back, but the fact is I drilled a couple of unauthorized holes in the wall to get power into the closet in the first place, so I don't want to attract attention unless I have to. How can I check if they socket is grounded correctly using only a multimeter?
posted by frieze to Home & Garden (14 answers total)
It's probably a bad ground.

Here's a tester from home depot for $8:

It's probably better than messing around with trying to open up the wall plate and possibly shorting out yourself as well as the shelf...
posted by jenkinsEar at 11:13 AM on July 12, 2007

You can use your multimeter, but be very careful. The wide slot is neutral, the narrow slot is hot and the hole is ground. There should be little if any difference between neutral and ground and 120 vrms between hot and neutral. Also, there should be no difference between your shelving and ground.
posted by caddis at 11:30 AM on July 12, 2007

Your best bet is to call apartment management and tell them that there's bad grounding in your apartment and that you got shocked. It's their job to fix this, and in my experience they're pretty quick about this kind of thing because of the legal liability. Once I called about getting shocked by an electric stove, and the next day they actually replaced the whole thing.
posted by rolypolyman at 11:35 AM on July 12, 2007

Am easy way to check if it is the PC or the outlet is to plug the PC into another outlet, or plug a different PC into the closet outlet.

I had a computer shocking me once and I checked the outlet with a tester and it read fine. I ran an extension cord to another room and the shocks went away so despite the reading it turned out that the outlet was in fact not grounded properly.
posted by mikepop at 12:13 PM on July 12, 2007

What caddis said, step-by-step:
(assuming USA 120V 60Hz)

1. Measure outlet ground to shelf (or some other grounded surface) Try on AC voltage scale. Should be 0VAC. If not, call electrician- this is dangerous.
2. Do same measurement on ohms scale- should get approx. 0 ohms. If not , call electrician.
3. Measure outlet ground to neutral on AC scale- should be 0 VAC. If not , call electrician.
4. Measure outlet ground to hot on AC scale- should be around 120 VAC. If not, call electrician.
5. Might as well measure outlet neutral to hot on AC scale- should be around 120 VAC. If not, call electrician.

If any of these steps fail, the wiring is bad and possibly dangerous. If they all pass, call us back.
posted by MtDewd at 12:45 PM on July 12, 2007

Hmmm... steps 1 & 2 are assuming the shelf is grounded. It may not be. Try to find a known ground, like a (unpainted) radiator or metal water pipe.
posted by MtDewd at 12:48 PM on July 12, 2007

but the fact is I drilled a couple of unauthorized holes in the wall to get power into the closet in the first place,

So this server is plugged into a box you added? Maybe the electrician corrected a long standing wiring problem, and your box is now incorrectly hooked up.
posted by Big_B at 1:18 PM on July 12, 2007

The fact that the shelf has 60 vac on it may have NO bearing on the server not booting. Take the server out of the shelf and try it in a known good outlet, at a friends place if necessary.

What other gear is on the shelf? Some gear, especially older stuff, has small bypass caps from each side of the AC line to the chassis. In an ungrounded situation, these caps cause the chassis to read half the line voltage (60 vac) . It's not usually at a lethal current, but can give you quite a little surprise. Very common with guitar amps...

measure the AC voltage between the shelf and a known good ground - water pipe, radiator. If you still get 60 vac, then the shelf is ungrounded and some piece of the gear on the shelf is creating the 60v potential.

you'll have to disconnect all gear from the closet outlet, then measure between the outlet ground and the pipe/rad. If you get 60 v at this time, then the outlet isn't grounded properly and it's electrician time.

if the outlet measured 0 vac, then it's one of the pieces of equipment.
posted by Artful Codger at 1:35 PM on July 12, 2007

There are a couple of previous questions that covered leakage current: Eject the core, A DVD player, a TV, composite cables, and a spark. And there is this very interesting article: I'm shocked, why am I getting a tingle?
posted by Chuckles at 2:38 PM on July 12, 2007

Does the shelf have any two prong (aka double insulated) equipment on it (like a receiver, or a TV..)?
posted by Chuckles at 2:40 PM on July 12, 2007

FWIW, a modern high-impedance multimeter can happily read nonzero voltage between two isolated (floating) objects, in situations where an older d'Arsonval-type meter would provide enough of a current path to eliminate the voltage difference. Such voltages wouldn't be dangerous (or even give you a shock; there's practically no energy available). I can't think of a simple safe way to distinguish though. Since you receive a shock, there's clearly a bad connection somewhere, but possibly not actually connected to the shelf.

Anyway. By "drilling a few holes" do you mean you just ran some cords through the wall, or did you do some actual wiring which you could have done wrong (e.g. hooked a ground wire up to the wrong place)?
posted by hattifattener at 8:28 PM on July 12, 2007

I can't think of a simple safe way to distinguish though.

You can measure the current to ground, if you know it is really ground (don't measure current to live, or even neutral, that will be shorting the shelf to the AC line - bad, bad, bad). I'd use the 250mA scale (after checking the current rating on the fuse), so the fuse can blow immediately if there is a substantial current flow.
posted by Chuckles at 9:40 PM on July 12, 2007

Yeah, I'm not entirely sure that counts as "safe", if there really is a wiring fault. I suppose it's unlikely to do more than destroy your meter. (Sure, the fuse should protect it.)
posted by hattifattener at 9:53 PM on July 12, 2007

I did run through a few different wordings actually, involving cheap multimeters and under fusing..

There really doesn't seem to be any reason for all that, but I guess "no plan survives first contact with the enemy", and all that..
posted by Chuckles at 10:52 PM on July 12, 2007

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