The glib answer is to say contact the cable makers. But more practically, for all but the thinnest plastic insulation, the break-over voltage is determined by the quality of the ends and terminations, and not by the insulation of the cable. PVC or polyethylene will hold off many tens of kV across a mm thickness of dielectric.

Air will only hold off about 1kV/mm and any surface which is open to air will start to support tracking at a few hundred volts per mm, depending on surface finish, humidity etc.

Mike.

I've a feeling that there was a 'deemed to comply' thing for cables (or perhaps wiring systems) above certain rating - but I can't find it at the moment. I'll have a dig if no-one else comes up with a better answer first....

   - Andy.

An interesting question and there is probably no real answer other than how the voltage rating for a wire or cable is determined.

The typical sequence is to test samples at increasing voltage until there is an almost instantaneous breakdown. The tests are repeated at lower voltages where the samples will take an increasing length of time to fail. The voltage/time to failure curve is then plotted to find the long term voltage withstand for the product.

This value then has various safety factors applied to give the published voltage rating. For typical PVC equipment wire the instantaneous breakdown voltage is above 20kV, the long term withstand voltage around 6kV and the rating is 600V. In this case a 4kV impulse would not cause a problem to the wire or cable itself but possibly for the terminations.