Great answers thanks. But what of the grounding posts on much equipment, including grounding blocks and common grounding of equipment I have seen mentioned? Are these only for systems with poor electrical grounding?
The electrical ground that comes from the service panel is for safety. Millivolts of difference in a ground potential won’t affect the safety features.
The problem with a lot of audio equipment is it ends up with multiple "ground" references, with some or no relationship to the outlet, and the transformer center taps fighting to be "correct."
The worst such problems often come from PC’s which have a very different "ground" for the electrical interconnects than the power plug. This is why proper grounding may become an issue. It has nothing to do with the quality of the wiring in the home, and yet, the quality of the ground in your home is important for life safety.
IMHO, the best configuration for high quality audio is to float the signals with no relationship to the earth/safety ground. My Luxman integrated does not use the AC ground at all. This is probably breaking US/UL regulations but as a result what I do not have is a ground loop. :D
You may notice some DAC makers make a point of touting the incoming copper connectors (USB and coax) are galvanically isolated. Sadly, not all DACs do this.