I was just testing this since I blew up a Raspberry Pi when hot connecting its ground to another ground (or minus) of another power source.
Now I tried connecting the rectified output ground of which the source is a variac with a switching power supply ground (or minus). Both devices are in a socket that has a ground pin but the variac is not connected with it. Having some ~45 volts on the rectified (with 400uF cap) variac output and the switching power supply set at around ~12 volts, I connected the grounds and something blew up in the switching power supply (it still seems to work although I could smell something in there burned).
I read some questions on this matter here and I still wonder what I did wrong. I connected these 'grounds' together before and it worked fine (driving some large capacitive load with the variac using an RPi output pin for control on a large transistor.
- Is it necessary for both power supplies to start at 0 V when connecting their ground and turning any voltage knobs?
- What may have been blown inside my power supply that it is still functioning? My guess is some output filter cap that briefly took some voltage from when they were still floating compared to each other?
- What I also saw is a huge (~120V) AC (50Hz) difference between the 2 negative terminals before connecting them. I guess this is normal and will not produce any current after connecting.