I've several times found several feet of cable in our barn, neatly stripped of all insulation, power still on. I've never found any electrocuted rats.
There are only two possibilities that I can see:
a) They drag a piece of insulated board to stand on while they very delicately nibble round each cable, then take the board away afterwards
b) They turn the power off first, then turn it on again when they have had their fill.
Rats are very clever.