It's easy for me to feel the mains frequency while gently rubbing the top surface of the MacBook while it's plugged in. Really feels unsafe, but neither me nor the computer have suffered any serious injuries yet.
That's due to interference suppression capacitor in the PSU. The safety standard puts the "touch current" limit at something like 300uA (0.3mA), which is definitely in "painful but not dangerous" territory. You do need to exercise caution when plugging in other devices that are also connected to the mains, since that amount of current and voltage can certainly damage sensitive electronics.
Old but good page on such measurements: http://www.aplomb.nl/SMPS_leakage/Doc_ie.html
Like for instance the magic mouse. I've completely destroyed three magic mice by sometimes accidentally touching the mouse to the laptop. It'd not do much of anything initially, but at some point touching them together would kill the connection for a couple seconds, and over time it evolved into the mouse just refusing to connect altogether.
I'm glad my boss pays for this hardware because I'd be incensed to have a mouse THAT expensive break that quickly.
I haven't been a regular Mac user, but I've had maybe 3 work MacBooks since 2010 and I recall having this issue with all of them.
Why haven't they fixed it?
It can be avoided by using a grounded power supply, but because there are large countries that have ungrounded outlets in common use the most designs are ungrounded.
It happens with other 2-pin chargers on both MacBooks and other laptops, but it depends upon various factors how strong the leakage is
so what I mean is maybe house electricity grid is not grounded.
And almost no other device I've ever used. My aluminum Framework does not do it. My wifes magnesium LG Gram does not do it.
I have felt it on other things but only extremely rarely. It's bizarre that whatever it is they're doing different, it's probably wrong, and they've kept doing it in every device for decades.
To describe the effect in more detail for anyone who doesn't already know: It's like the case is alternately grabbing and releasing your skin at 60hz.
It's a bit like chatter, ie the periodic friction you use to ring a wine glass by wetting the rim and then running your finger along it. It rings because the combination of the friction, the lack of friction from hydroplaning, and the rubbery give of your skin, makes your skin alternately grab and release 30,000 times a second. Only in this case you are only barely touching the case not pressing enough to make any friction or make a squeal noise. It's like static electric charge attraction. Just touching the case you feel nothing, but move your finger along the surface and you feel it vibrate your finger without any friction to explain it.
It's unsettling and displeasing, which are strange words to expect from an apple device at least when you are only talking about the design and not the tech stack or corporate behavior. It makes me think of cheap electronics from a country with no consumer safety regulations that will probably burn down every 3rd house they wind up in.
It's probably harmless, but then again a lot of things that are harmless in short infrequent doses turn out to have been harmful after you did it for 10 hours a day for 20 years.
if you take the plug part from the brick you'll note that there's only two pins but the button-like thing is a ground
as noted in a sibling, the power adapter extension cable does plumb the ground through (https://www.apple.com/shop/product/mw2n3ll/a/power-adapter-e...)
US Version: https://www.apple.com/shop/product/mw2n3ll/a/power-adapter-e...
Seriously though, that does not sound safe at all.
