Service Ground questions?
If two wires are near eachother, a high voltage/current in one, will induce a voltage/current in the other.
The magnitude of the induction is a complex formula, but the result is that "objectionable currents" can be induced in the other line.
An "objectionable current" is just that. A current that you would normaly object to. For example a DVD player that normaly draws 1 amp, but is forced to take 100 amps due to a fault. The extra 99 amps are objectionable.
So if you run the circut to your DVD in the same condut as your main ground, and a T-storm hits, then you risk the objectionable currents flowing through your DVD.