HV Cable can have 3 different levels of insulation. Uninsulated HV Cable can electrocute mobs and players, possibly even killing them, so it is recommended to insulate the cable unless it is being used purely for defensive purposes. HV Cable can shock mobs and player within a one block radius of it.
When using uninsulated cable for defensive purposes, the cable needs to have constant EU travelling through it, preferably at high (512 EU/t) or extreme voltage (2048 EU/t). An easy way to set up an "electric fence" is to send power from an MFS Unit(powered with a HV Solar Array flower or your prefered power source) to an HV Transformer to step up the voltage. Next, set up uninsulated HV cables around the perimeter of your fence, after stepping down the current with an HV transformer send the excess EU to either an additional MFS unit then (through at least 1 piece of cable) back into the original MFS Unit to improve efficiency, or to a mass fabricator to consume the excess EU.
It's also worth noting that, despite of the massive capacity of these cables, they are actually cheap to craft, since they are made of refined Iron, a very cheaply made material.