Electricians of AskMe: Help, my desk keeps shocking me!
November 30, 2014 2:40 PM   Subscribe

A few months ago I moved into a new apartment, and decided to get one of these industrial shelving units - something like this to use as a desk/bookshelf/musical gear shelf. Everything works great, except when I plug my macbook into its charger and start typing. If my hands are on the aluminum body of the macbook and my wrists brush up against the metal frame of the shelving unit, I get a sharp shock, nothing too bad, but painful and annoying.

Is there something I should be doing to ground or otherwise keep my metal shelf from shocking me? Is this a particularly dangerous situation given the number of old pieces of electronic musical equipment sitting on this shelf?

Any help in diagnosing/solving this issue would be greatly appreciated. Thanks!

Would be happy to MeMail pictures of the setup on request.
posted by taromsn to Home & Garden (6 answers total) 5 users marked this as a favorite
 
Are you using the 3 prong grounded cord for the macbook charger? What else on the desk is plugged in?

This sounds like a ground loop, and like something else on or contacting the desk is plugged in "backwards". Either your Mac charger, or the power strip or outlet it's plugged into has an improper ground.

I've had huge problems with this, macs, and music gear. You can also get a ground tester at a hardware store for like $4 if you are indeed using the grounded power adapter cable for the computer.
posted by emptythought at 2:58 PM on November 30, 2014


A/C devices for home use have two or three wires connected: hot, neutral, and (optionally) ground. If you have a recent wall socket, the hot wire should be the narrow blade, neutral is the wide blade, and ground is the round pin beneath/between them. Neutral and ground should be close to the same potential, and you shouldn't get a shock between them. Hot to neutral is where the device is supposed to get its power, and hot to ground means something bad is happening. (If you have GFCI protectors, hot to ground should trip them.)

I would expect an undamaged MacBook A/C adapter to be pretty robust, so for troubleshooting your setup, I'd focus on the other old electronic gear. I'd start by unplugging everything but the Mac, then see if you still get shocked. (Keep track of which way the other gear is plugged in!)

Next, plug each piece of gear in individually, with the plug in its original orientation, and see if you can figure out which piece is causing problems. (This is probably somewhat dangerous, although no more so than just using everything as you are now.)

If you can narrow it down to a few culprits, and they have two-blade plugs, try turning the plug over (thus switching hot and neutral). If one way zaps you, the other way shouldn't.

If this fixes your problem, then you really ought to replace the power cord on the bad device with at least a polarized cord (neutral blade wider than hot blade) and preferably a 3-prong grounded cable.

Another thing to check, as emptythought notes, is the wiring of the wall sockets. A cheap, simple tester will tell you if the ground is disconnected or the hot and neutral wires are swapped in the wall. If they are, you should fix it (if you know how, including turning off the breakers & etc.) or have it fixed. Miswired sockets are dangerous.
posted by spacewrench at 3:11 PM on November 30, 2014 [2 favorites]


(We bought a house where the first floor had a bunch of three-prong outlets that didn't actually have a ground wire. Needless to say, we had the outlets rewired, but you never know what someone might have thought was a good idea.)
posted by leahwrenn at 4:01 PM on November 30, 2014


A couple of months ago, JackFlash pointed out something interesting in his answer to a similar question:
The typical AC adapter has a small capacitor between AC neutral and DC ground which is necessary to reduce the transmission of undesired high frequency common mode electromagnetic interference on the AC lines. This capacitor passes a very small AC current, a fraction of a milliamp, that can cause a slight tingling sensation.
The aluminum body of your macbook is connected to DC ground, so when you touch it, you are connected through a capacitor to AC ground.

Now if there happens to be a lot of time-varying induced EMF hanging around in your metal shelving unit, as you might expect with all that equipment on it, and you happen to brush your wrists against that unit, you will have provided a path to ground for that EMF, and it will go right through that capacitor (to the limits of its capacitance) since it is time-varying.

I'd guess that's what you're feeling.
posted by jamjam at 4:17 PM on November 30, 2014 [2 favorites]


Thanks for the responses, I did as spacewrench suggested and unplugged everything but the macbook with 3 prong charger, no shock.

I'm going to try adding and subtracting items from the power strip and see what else it might be, will report back shortly
posted by taromsn at 4:48 PM on November 30, 2014


This is the kind of tester spacewrench is talking about. IMHO it is worthwhile for everyone to own one to test their household outlets for wiring problems and to periodically test their GFCI outlets.
posted by Poldo at 8:14 PM on November 30, 2014 [1 favorite]


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