Comment by nealabq

When the grippers close they probably close the loops of a coil that wraps around the wire. So it's harvesting the ever changing magnetic field that arises from AC current, independent of voltage. You can still get some power from coils that aren't wrapped around the wire but are still parallel. I think that's how wireless phone chargers work.

You can also take power using a capacitor instead of an inductor, from the changing voltage (not current) in an AC line. Like when you hold a florescent tube vertical under a hi-power line, and it lights up.