^That looks seriously dangerous.
There should be a main shutoff of the panel to the street, I hope.
Take it slow and safe.
AFAIK, Common returns to the source.
Ground is to a copper ground rod stuck in soil. Literally. A last resort safety measure.
I once read about a SF BART electric train station messing up an entire surrounding neighborhood because of a common vs ground problem.
(there is a lot of juice flowing those electric trains)