I recently bought a UPS for my iMac and it runs just fine.

However, sometimes I also run a MacBook Pro nearby and when I have my hand in contact with the MacBook and reach across to touch the aluminium keyboard attached to my iMac I can feel electricity running through my finger!

It doesn't happen when the MacBook is running on battery power, only when it's connected to the mains. It also doesn't happen when the UPS isn't connected.

The MacBook Pro is plugged into a standard wall outlet, the iMac is plugged into a UPS and that's in a similar wall outlet (not the same one).

I'm not very electrically minded, so am puzzled as I'd like to plug the MacBook into the mains again! Does anyone have any ideas as to why it's happening and how I can fix the problem?

  • Does this happen when you plug the MBP into the UPS?
    – IconDaemon
    Jul 30, 2013 at 12:05
  • 1
    It sounds like a grounding issue. I would purchase an electrical ground checker amazon.com/Electrical-Receptacle-Outlet-Ground-Tester/dp/… and first be sure that all three sockets (2 wall and 1 UPS) are correctly grounded.
    – hoss
    Jul 30, 2013 at 13:15
  • I don't have the correct cable at the moment to connect the MBP into the UPS, and will try that as soon as it arrives in a day or two. I'm hoping that would sort the issue. Jul 30, 2013 at 13:18

2 Answers 2


Usually, ground loop current is experienced on portables where often no ground wire is connected - just the neutral and live (or hot) wires.


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 magsafe connectors and only DC power going in to them. It's harder to be sure an iMac or other device that has 110/220 AC power directly into the case, so I would rather be too cautious and have you get an electrician to look at the outlet if your Mac works properly on other outlets or a technician to look at the Mac if it leaks current no matter which receptacle you plug it in to.

You might be able to find a grounding issue and solve this yourself, but if you have any doubts as to safety, I'd rather you chose to get expert help rather than trust our guesses based on how things should work.


And note that all the MacBook questions are low risk situations where any iMac one would be a little more danger (potentially).


I would not worry about it.

Those are blind currents and not dangerous.

The UPS is grounded (has 3 pins wall socket)

Your Mac is not grounded (has 2 pin wall socket)

There is always some current leaking to Ground wire, it could be number of places in your household (it could be the toaster ?).

To prevent damages in wet areas like bathrooms there is a GFI (ground fault interrupter) circuit there. It is there to save your life if you drop the fan in to the bath tub.

If you are concerned then use a GFI to plug your UPS in to.

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