You most certainly can degrade insulation using high voltage insulation testers on conductors. Depends though. Old style crank ones , if there was a weak point in the insulation and you had it set to 1,000 volts you could ''pop'' thru the old insulation at the weak point. The digital readout electronic ones mostly used now don't push the voltage hard enough to do much similar damage.On the conductor, they are clearly label 600V. Megger is a insulation tester that sends high voltage with little amps to test the resistance. Since the voltage that is sent through the conductor is more than 600V, does that degrades the insulation?