Controlled Voltage Environments
A Controlled Voltage Environment is basically one where you aren't.

The testing voltage for the cable is ideally 5,000V.  You don't want to be testing it with that sort of voltage by hand.  For safety reasons you shouldn't hand-test with anything over 30V AC (or 60V DC).

Above 30V AC it is possible to get an electric shock, which is to be avoided if at all possible, for obvious reasons.

Therefore testing with voltages above that, up to the upper test limit of 5,000V (I assume this is the voltage at which the insulation would start to break down), must be done in an isolated "Hands Off" environment.

Such an environment has strict controls on the flow of the high voltages - both physical (breakers, safety switches, current limiters, dual controls, etc) and procedural (Yes, RTFM :} ), and only trained operators will be able to access and use such facilities.