My opinion is that you MUST have minimum 2 wires. If you do not, the signal
from the "source" will not be detectable by the "destination" circuit, since
it has no point of comparing that signal.
So, in this aspect, imagine you have a battery power circuit, completly
isolated from ground, lets imagine it is over a thick rubber base, ok? Well,
you can conect a wire to the AC wall outlet and connect it anywhere in your
circuit with no damage to the circuit (yes, perhaps, to yourself:) but the
circuit will not be harm, since the 120VAC are not referenced to any
ground/neutral connection....

That's my opinion, perhaps someone will produce a much more clear
explanation.

Mauricio

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