HV Cable is the highest tier of Cable. It can tolerate up to Extreme Voltage (2048 EU/t), although they have an extremely high distance-related energy loss. Quad-Insulated HV Cable loses 0.8 EU every block traveled, which is higher than even Double-Insulated Gold Cable, as it only loses 0.4 EU per block traveled.
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.
NOTE: Do not complete the circuit unless wearing suitable armor (Nano , Quantum, Dark Matter, Red Matter, or Gem) to avoid being killed by shock.
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.
Uninsulated HV Cable
Insulated HV Cable
Double-Insulated HV Cable
Quadruple-Insulated HV Cable
File:Tekkit - How To Make And Use All Cables (Basics)
File:Tekkit Tutorials - IC2 2 - EU Storage, Transfer, and Conversion