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I have a laptop, connected to the wall socket. A separate USB charger is also connected to the (same) wall socket.

I have noticed, when my arm touched the USB ground, it hurt a little bit.

When I measure the difference between my laptop aluminium plate, and my USB charger ground, the difference is 84.5V AC. AC uA measurement is 76 uA.

Why does this happen?
Is this dangerous?
Do I need to do anything to alleviate this "issue"?

enter image description here

Gizmo
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  • Y-capacitor leakage. Set it to measure current instead and move the lead. It should be in the 100 uA range. – winny May 25 '21 at 12:48
  • USB charger may be isolated not grounded, and you may be seeing AC via its RF suppression capacitors. IF so, this is normal and on the AC current range you'll see a few (tens of) microamps. This is normal. If not... –  May 25 '21 at 12:49
  • it seems to measure 0.3 uA DC, 0 on AC. – Gizmo May 25 '21 at 12:50
  • Does this answer your question? [How does low impedance measurement reveals ghost voltage?](https://electronics.stackexchange.com/questions/278684/how-does-low-impedance-measurement-reveals-ghost-voltage) – winny May 25 '21 at 12:51
  • is the laptop actually conductive? ... have you measured the resistance between two points on the laptop? – jsotola May 25 '21 at 12:51
  • Your multimeter may not be able to measure that low. Anyway, it’s nothing to worry about. Causes him in audio systems sometimes though. – winny May 25 '21 at 12:52
  • @jsotola there are worn-off spots on the laptop that are conductive. Ohm measurement between two of these spots shows 0 ohms. The laptop is connected via a 3-wire (into socket) charger (2 wires into laptop). The USB charger has only two wires (prongs?) into the wall. – Gizmo May 25 '21 at 12:54
  • Correction: It's 76 uA in AC (I forgot to connect to "mA"). – Gizmo May 25 '21 at 13:00
  • According to this question, it would be interesting to see if reversing the way the 2-prong USB charger is plugged into the mains socket changes anything -- the buzz feeling was more pronounced one way than the other: https://electronics.stackexchange.com/questions/55285/how-to-safely-ground-a-switching-power-supply-with-floating-outputs – ErikR May 25 '21 at 13:03
  • Reversing the USB charger in the wall socket seems to result in 20VAC difference, but still hurts a bit when "touching the wire accidentally". – Gizmo May 25 '21 at 13:05
  • 76 uA is Y-caps for sure. If you have access to true ground, you can ground your minus output to it and the “problem” will go away. – winny May 25 '21 at 13:18
  • Allright, the suggestions are welcome! I will try to find a true ground – Gizmo May 25 '21 at 14:59

2 Answers2

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Maybe you don't have real ground to supply your laptop (unconnected in wall socket). It's potentially dangerous for devices. I had similar situation with my PC when ground wasn't connected in socket - it was 180V :)

Neptun
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This is normal RF leakage winding capacitance current through SMPS floating charger. If you wish to avoid this , don’t touch it with most sensitive part of body or connect a VGA port to an earth grounded monitor or similar retrofit of earth ground. It is not unsafe.

Tony Stewart EE75
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