Thanks Andrea - great drawing.
This came up because I'm switching from a fuel cell back to a standard tank and in so doing, I need to hook up the fuel gauge to a sending unit. First attempt didn't succeed and it was thought the old gauge had previously shorted out. On further examination I noted that it had not been grounded with the black wire (or any wire). I wondered if such a wire was redundant and that the gauge would be grounded to the instrument fascia when it's bolted in place.
ciao