I tested the ground connection from iMac to the wall socket, and there is continuity, so the grounding seems fine.
However, when I tested the printer’s USB, I measured about 90V AC coming from the printer https://imgur.com/a/kMTRIFa
Also, when I place my hand on the iMac, I feel a strange tingling sensation.
This is my brother’s computer, and he told me he has been using it like this from years.
I think this is extremely dangerous. Am I right?