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I've seen some online threads and at least one StackExchange question on this, but the answers don't seem to apply to my situation.

I have a 2009-vintage white 13" MacBook that's always been in great shape except for needing the relatively common bottom case replacement a while back. I did the replacement myself without apparent incident.

Thereafter, though, I noticed that the exposed metal screws on the bottom of the case can burn/tingle my exposed skin if the machine is plugged in. And I do use the three-prong grounded (earthed) power cable.

I actually first noticed this while charging my 2nd-generation iPod Touch from the USB port while the laptop was charging; the whole back of the iPod had a live feel to it -- a little stuttery resistance as you passed your finger over the metal, which went back to being perfectly smooth as soon as I unplugged it. And I also have a pair of earbuds with metal casings that are downright painful to wear when plugged in to the laptop.

All these things only happen when the laptop is plugged in. Remove it from "mains power", as the non-Americans would say, and all these effects cease immediately. Happens with two different power cables I've tried, and in all different locations. (Well, given that all those locations are homes and offices with standard U.S. wiring.)

I've tried retightening the bottom case screws, but they're as tight as I can comfortably make them without worrying about stripping the head or the threads.

So -- anyone have any new insights on this one? Thanks!

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2 Answers

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If you are using a grounded plug and feel ground loop current you should either have the outlet serviced or the computer serviced.

It's normal (and safe) for ground loop current to exist and feel lightly tingly on Macs with ungrounded power supplies since there are spinning fans - especially inside the thermoplastic white Macs. It's also almost inconceivable that the low current DC voltage that a Mac battery and power adapter provide could be a safety risk, but it could be a sign of internal damage and worth having an expert check out the wall socket and or the Mac plus it's power adapter.

Since your comment reinforces the level of detail you are seeking, I would say that any Apple repair shop that received that Mac for repair might use a multimeter to see if they can sense an voltage on ground pins and/or the case. They might place a new DC in / magsafe board in and re-test to see if that's the cause. They would of course test with their adapter and then remove the components in the main unibody section to look for loose screws, scortch marks or other signs of burns or liquid damage to the components that might cause this abnormal current situation:

  • battery
  • rear vent
  • fan
  • optical drive
  • logic board

Once you've gone over these - hopefully you can visually see the problem or remove whatever is grounding current / voltage to the frame.

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As I said, I've swapped out power adapters and wall sockets already, so it's got to be the internals of the MacBook itself. And yes, I'm already planning to take it in for service. Was just hoping someone might be able to explain specifically what was going on. Thanks! –  Jenn D. Mar 15 '13 at 13:12
    
Hmm - there are several grounding pins that should contact the bottom case, so I'd start there. It also could be a screw loose or another component that needs shielding or needs an electrical ground connection and a minor jostle has changed the alignment inside the case. As you can see when the bottom case is open - the tolerances inside the MacBook line are often less than half a mm of air gap and that Apple uses plastic stickers and kapton tape to shield some components for electrical reasons. –  bmike Mar 15 '13 at 13:30
    
Thanks again. I always like to have as much clue as possible before talking to repair folks...even if it only helps me understand what they're talking about. –  Jenn D. Mar 26 '13 at 18:27
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Electrical faults: If the cause is not obvious to you then take it to someone who will know how to find it - for a Mac, that is your nearest Apple repair place.

Please don't stick live, even slightly live, electricity in your ears!

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Hee hee. Yes, I've definitely stopped doing that. :) –  Jenn D. Mar 15 '13 at 13:12
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