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?
Best Answer
The problem is most probably caused by "Y" filter capacitors between the P & N mains lines and the ground line in one of the supplies.
If the ground line is grounded no voltage occurs compared to mains ground. If the ground line is ungrounded it floats at half supply due to the capacitor voltage divider formed by the two Y caps.
As the impedance is high the voltage you see when you measure it depends on the meter design and the value of the Y caps. eg a supply with 0.001 uF Y caps is more liable to be 'loaded' by a meter than one using 0.01 uF Y caps.
The impedance of this floating voltage is such that an unpleasant sharp feeling may be felt if the ungrounded ground is touched but the currents are well below those usually deemed dangerous. However, equipment can be damaged by exposure to this AC couple voltage.
A solution is usually to ensure that all "grounds" are grounded to a common ground. If this grounding occurs when an interdevice signal cable is plugged in it may cause damage as connection is made. Long ago I had a brand new printer destroyed in this way the first time it was plugged into a PC.