Have you thought about using multi-mode optical fiber cables for the tether? Either with a battery on the robot or in addition to the 2 wires you are using for power.
If you use an optical fiber for the data, you might be able to send only 12v (or 24v) instead of the 120v of power through the cable which should make it much safer.
The 120v (or 240v over here in Europe) seem to be a big safety issue if you take this project for research at universities or even schools. Electrocuting half a school class doesn't sound like good publicity.
Additional benefits would be the data rate and the cable length. At shorter distances (up to 100 meter) you could go as fast as 100 Gb Ethernet - not sure why you would need that, but 1 Gb Ethernet would be useful if you have more then one HD camera attached. With 1 Gb you could go up to 1000 meter in cable length.
An additional benefit would be you have a stronger tether if you use two cables (2 wires power + optical) in case something breaks.