Some advice please

[u/Academic-Ad5274]

First of all, im running a 6uf 10kV cap bank with regular rg58 coax. The coax is just rated for 2kV/5kv peak but it's holding up great so far, 8months a lot of discharges, no issues. It's polyethylene between the inner and outer conductor and according to the Internet the dielectric break down strength is at least 20kV/mm. The insulation is 3mm thick. So in theory 60kV but officially it's rated much lower. As I said it's handling short 10kV pulses with no problem. I'm wondering if 20kV would be fine two. Thinking about ordering bigger caps (20kV). Do you think it will be fine, at least according to the break down strength of the PE it should be, still worried because the cable is rated much lower. So would you guys try it?

Secondly, any better way to measure high frequency DC output more precisely? Currently it's a HV DC flyback, diodes and charging resistors, 2 small caps for smoothing the wave a bit and then the main cap bank. The voltage divider is connected to the main cap, 33V on the display of the multimeter should be 10kV in reality but I always have the feeling, it's close but not a 100% accurate.

[u/Academic-Ad5274]

Really nobody? I know nobody can tell for sure but according to my math and the thickness of the PE it could be fine for short pulses. Going to test 17kV tomorrow ( hooking two 10kV caps in series). I need to know because I don't wanna spend the money on bigger 20kV caps if the coax will fail. I know the right way would be to get thick HV rated coax but it's hard to get and expensive. If someone here ever did something similar or over voltaged coax. Please let me know.

[u/Paradigmnoia → u/Academic-Ad5274]

I use 28 gauge CCA telephone wire (individual insulated wires from the bundle) with up to 45 kV and 320W and they don’t even arc to each other when touching under load. It really depends what you are doing that determines how much insulation is enough. The tighter you bottle it up the more it fights to get out.

[u/Academic-Ad5274 → u/Paradigmnoia]

I know but with coax the distance between the two conductors is fixed, I can't space them apart and regular non coax cable would increase the inductance which would choke current rise rate.

[u/Paradigmnoia → u/Academic-Ad5274]

It sounds like you know what you are doing so not much more I can add. My Vevor 50 kV powder coater just uses radio-rated coax, like for shortwave radio, if that helps.

[u/DmitryE]

Things like this are very often deliberately rated well below the physical limit when breakdown occurs. It is simply good engineering to leave a safety margin - you really don't want that cable to arc over while you're running high power RF through it since that would be dangerous. I'd be reasonably happy running 20kv through it but then again your mileage may vary, since it is guess work how much margin the manufacturers left in. Another potential issue is partial discharges or corona, if it doesn't arc today it might arc later when you're least expecting it. Oh and if I remember correctly, higher frequency currents tend to be more stressful on the insulation

[u/Academic-Ad5274 → u/DmitryE]

I know but I double checked the insulation and I don't know how I remembered 3mm, the insulation between the inner and outer conductor of the coax is just 1.1mmPE. should still hold 40kV in theory but in reality, one weak point, one bad bend etc and it could go wrong. I'm either leaving the circuit like it is and just adding 2 more coax in parallel for cutting inductance down further or I need to get rg213 U mil spec and the 20kV cap. That's the only 2 options. I don't have a power issue, it's more I like running long lead cable which chokes the current rise. Just getting 12-13GA/s with 60ft lead cable. Still why more than an exploding wire needs but not the crazy 20-100GA/s the industry reaches for ultra fast ns discharges. Of course I could shorten the coax but I don't want. I like running long cable. Inductance is enemy number one. The current performance isn't bad but there's always room for improvement.