I noticed that I was getting mild shocks while touching my plugged in laptop and touching a metal shielded USB device plugged into the USB port of my Raspberry Pi 3 at the same time. I used a multi meter to measure the AC voltage between the USB grounds on my laptop and Rpi3, and found there to be 130V.
My country uses 240V mains.
The laptop charger is a DELL USB C charger with ground AC pin.
The Raspberry Pi 3 charger is the official 5V supply and this does not have a ground AC pin.
Both are rated correctly for 240V.
I checked the voltage between the USB C ground of the DELL USB C charger to mains ground, and it's zero. It's also zero with the Pi 3 5V supply microUSB ground to AC ground. But only between the USB C charger and Pi 3 supply grounds there is 130V.
I also tested with another Pi 3 supply of the same brand, and that one gave me 25V between the two.
Anyone have any idea what could be going on here?