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I'm theoretically aware of grounding issues that can come up when using an oscilloscope [e.g. as explained in this video], but I can't make sense of this one:

I have a device under test (imagine an Arduino board) plugged into my laptop, which is plugged into the wall via a brand new Apple power supply which has only two pins (no ground). If I connect my probe ground lead to the shield of the device's USB connector, and then probe the device's GND my oscilloscope shows a 152.0V sine wave at 60Hz, offset by -10V!

When I unplug my laptop, this signal goes away – unless I happen to be touching both my laptop and the metal end of its power cord! In fact, my oscilloscope shows this "110VAC" signal when I probe the metal end of the Magsafe adapter.

Why would the outer "ground" of my MagSafe adapter have mains-level VAC on it relative to my oscilloscope's ground? They are plugged into the same outlet — is this expected or could there be something wrong with this building's wiring?

UPDATE: I realized I had a three-prong extension cord handy to swap out for the "duck bill" on this Apple power adapter. When I use that, the AC voltage measured on the outer part of the MagSafe connector goes away. Still disconcerting that it would be there when using the two prong plug, or is it just some sort of harmless low current leakage when connected that way?

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  • \$\begingroup\$ Can you be more clear about your measurement? It sounds like you clipped scope ground to circuit ground, then probed circuit ground and found it to be 100 V. That doesn't make sense. Is your scope probe ground broken? \$\endgroup\$ – tomnexus May 14 '15 at 2:01
  • \$\begingroup\$ Apple power adapters are notorious for passing AC to ground. If you look around you'll find lots of people asking why their macbook mildly shocks them. Every time I've seen this come up they have been grounded and using the two pin connector. Annoyingly even in countries where there is no two-pin equivalent the apple duck (no cable) adapters do not connect ground. \$\endgroup\$ – caskey May 14 '15 at 5:57
  • \$\begingroup\$ It is not only Apple, every ungrounded laptop supply internally is connected through a cap to the mains. \$\endgroup\$ – ilkhd May 14 '15 at 8:39
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    \$\begingroup\$ @natevw Thanks. I guess the USB shield isn't grounded. You're measuring the difference between the scope chassis and the laptop chassis, which is floating at VSupply/2 (RMS) because of the capacitors in its EMI filter. Clip the scope ground to the Arduino ground, and the cabin pressure will return to normal. \$\endgroup\$ – tomnexus May 14 '15 at 17:47
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    \$\begingroup\$ The laptop can't float at more than the whole supply voltage, it could be more that half. Just to check you're clear about this though: 110 V AC RMS has a peak voltage of +-152 V and so a peak to peak of about 300 V. \$\endgroup\$ – tomnexus May 15 '15 at 17:56
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I guess the USB shield isn't grounded. Strange but not impossible.

Without a ground reference, the scope will measure the difference between its own ground or chassis, and the laptop ground. If the laptop has only a two pin plug, then its ground will be floating at about VSupply/2 (RMS) because of the capacitors in its EMI filter.

Clip the scope ground to the Arduino ground to force the laptop ground to the same as mains ground, and the scope will start to read more normally. A small current will flow, it's limited by law to perhaps 200 uA, but might be a lot less.

In general on an experimental bench, you should connect the equipment ground to mains earth. The 100 V at 100 uA won't hurt you, and your laptop will be protected against it, but the Arduino isn't, and you can easily blow things by touching them.

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  • \$\begingroup\$ It seems that the shields of ± "everything" on my laptop are grounded — iff the power adapter is grounded. (Which makes sense.) Thank you for explaining why those shields float at such a high voltage when it's not grounded! I'll probably stick with the 3-pin plug (and/or unplug laptop to isolate things as necessary) but good to know seeing 100+ VAC in my circuit is ± expected simply due to filter caps. \$\endgroup\$ – natevw May 15 '15 at 18:17
  • \$\begingroup\$ Yeah, apple power bricks are weird in that regard. They have a grounding connection and yet only connect it when using it with their mains lead, not when being used "wall wart style" or with a normal figure eight lead. Even stranger at least my one has the "double insulated" symbol but it's inside a box labelled "apple japan". \$\endgroup\$ – Peter Green Dec 13 '15 at 4:39

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