I was rewiring a room in my house and noticed a ceiling light was wired black to ground connection. Apparently it has been working correctly for years. Previous owner said they separated switch from outlets to change switch to light fixture and that is only way it worked.: White to white with black to ground.
That is quite wrong for small 6-40A branch circuit wiring:
Anywhere in the world, grounds can only be green, yellow/green stripe, or bare wire. And the reverse applies: wires of those colors can only be grounds.
Anywhere in North America, neutrals can only be white or gray. The reverse isn't true in multiconductor cables - whites can be re-tasked to be hots if neutral isn't needed.
Hot is any other color.
While using a black for a ground isn't a red-alert safety problem, the question is, what's he using for a hot!!!?? Using a bare ground wire for hot or neutral is downright unsafe.
Map the wiring, find both ends of a cable, and reallocate its wires to the correct usage by changing it on both ends.
Beware: sometimes color changes like that are done as a workaround when a wire breaks.
As Ed Beal points out, none of this applies to large 50+A circuits or to feeders and service entrances.