Insulator Testing

After getting hold of some EHT cable which has a 25kVDC rating, I started to experiment with insulators, using the neon sign transformer as the power source. As the NST has an output voltage of only 10kV RMS I figured the voltage would be well within the rating of my EHT cable. To test this hypothesis I connected up some cable to the NST and brought a bare electrode up to the insulation. To my dissatisfaction I saw and heard lots of little purple sparks! What's the point of insulation if it don't work, I thought! Then it struck me.

One area of Tesla Coiling is pulse discharge capacitor building. Now I'd read lots of stuff on it but until now I had not made the connection between insulators and dielectrics. AC is a very different beast to DC. My wire had insulation rated at 25kV DC but I was using 50Hz AC! The conductor in the wire was one plate and the electrode was another, along with the insulation (or dielectric!), I'd made a small capacitor! AC current can flow through capacitors!!