Nextek Power Systems... has developed a system for delivering power via DC to lights and motion sensors through a building’s metal frame, instead of through wires.
Paul Savage, chief executive of Nextek... said the current was not enough to electrocute anyone.
“If you licked your fingers you might get a little bubbly feeling, like if you put a nine-volt battery on your tongue, but it is not noticeable if you’re in a non-wet environment,” he said. — NYTimes
A 6 to 17% gain in transmission efficiency is pretty significant! Plus you eliminate the third wire.