RG-8 Coax Cable Failing Under High Voltage

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Maxwell_Epstein
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RG-8 Coax Cable Failing Under High Voltage

Post by Maxwell_Epstein »

Hello all!

Following the advice given to me on my last post, I purchased a length of RG-8/U cable for use as HV cable from my 30kV 10mA Hipotronics power supply. As I have tested this cable, however, it has been failing at around 20 kV. As of now, I am grounding the outer sheath around the copper conductor, and at ~20kV with no load, electricity arcs through the insulation and ruins the cable. I have placed a fuse between my variac and the input to the Hipotronics transformer to prevent this short circuit from damaging the transformer.

I am posting this to ask, is there something I am doing wrong that the RG8 cable is failing? Should I simply not ground the outer foil sheath? If not, is there a different type of cable that I should use instead of RG8?

Thank you!
Max E.
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Richard Hull
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Re: RG-8 Coax Cable Failing Under High Voltage

Post by Richard Hull »

Spellman uses very special RG-8

https://www.spellmanhv.com/en/Technical ... HVPS/AN-07

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Re: RG-8 Coax Cable Failing Under High Voltage

Post by Dan Knapp »

Do you know where it is failing? If it is at the end of the shield, your problem may be solved by field shaping to reduce the local field strength. Spellman has a note about this. One method is to fold the shield back over an o-ring and tape over it to eliminate the sharp edge on the shield.
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Re: RG-8 Coax Cable Failing Under High Voltage

Post by Maxwell_Epstein »

I have not checked the latest failed cable, but the earlier failure occurred a decent distance away from both ends. Thank you very much for your advice, though! I will be sure to shape the ends of the foil as you suggest on my cables.

Thank you for the link as well!

Reading through it, I am wondering where I should go from here as far as cables? I was recommended to use RG-8/U cable in an earlier post*. Would I still be able to use this cable if I did not ground the outer sheath? I would, of course, have the understanding that in that case the cable would no longer be nearly as safe to touch or place near other wires. I have some spare rubber hose that is about half an inch thick that I could run the cable through to provide additional insulation. I know that this approach is not as proper as finding a better cable, but when considering cost and other factors, is it an option that I could safely choose to take if I remain cautious of the ramifications?

Thank you!
Max E.

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Dennis P Brown
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Re: RG-8 Coax Cable Failing Under High Voltage

Post by Dennis P Brown »

Your solution is a good idea.

I too had an issue where as my 30 kV cable failed (through at 30 kV. Not your issue but should not have failed.) So I treat the cable as a naked high voltage danger and suspend it in air so it cannot short to anything. I did use an insulated plastic pipe to feed it out of my power supply case - not unlike your rubber hose insulation solution.
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Re: RG-8 Coax Cable Failing Under High Voltage

Post by Rich Gorski »

I just purchased 2 meters of 22AWG, 50kV rated cable on Amazon (no ground shield) for $15. There's no ground shield but if you believe the 50kV rating printed on the cable you can always run it through a grounded metal pipe. If you limit it to 30kV my guess it will be OK. My installation doesn't have a ground shield but I have the cable routed so there's no easy access to it and the terminations are shielded with thick plastic tubing. So far I've had it up to 35kV without problems. Be careful how you end the cable at the terminations as that's where the bare wire is exposed.

Rich G.
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Richard Hull
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Re: RG-8 Coax Cable Failing Under High Voltage

Post by Richard Hull »

I have always run 60kv silicone "noodle wire" unshielded in suspended pvc pipe having an elbow over the fusor and a naked drop in air to the HV terminal on the fusor. Worked and never arc'd at 45kv since 2003!

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Liam David
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Re: RG-8 Coax Cable Failing Under High Voltage

Post by Liam David »

I've used RG-8/U coax (from Amazon, I believe) up to 70kV without issue. It's possible you got a poor-quality cable or perhaps I got lucky since these cables aren't rated for such voltages. You say that the ground shield is foil... does it also have a braided copper component, or is it just foil?

If you go the unshielded route, It would be good to add a ground shield to prevent static buildup.
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Dennis P Brown
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Re: RG-8 Coax Cable Failing Under High Voltage

Post by Dennis P Brown »

Liam is correct about the charge buildup on an ungrounded shielded cable; my shielding in the cable builds up a charge whenever I run the system. I carefully ground both the power supply output and shield whenever I intend to touch the cable - and I often get a spark from the shield on the cable.
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Re: RG-8 Coax Cable Failing Under High Voltage

Post by Maxwell_Epstein »

Thank you all for your advice! Although I've been using it with my NST setup, I had somehow assumed that the cheap silicone "noodle wire" was insufficient for true high voltage. I suppose not! I have a decent amount of 30kV silicone wire that I will switch to for my negative output from the transformer (if I end up going near or over 30kV or think that the wire is likely to fail, I'll upgrade to 50kV or greater).

As far as insulation, I have a length of ceramic tube that I think might offer sufficient insulation for the output of the Hipotronics transformer (the hipotronics transformer has a ~6in hole into which a long cable plugs in. I would stick the ceramic in the hole with the silicone wire inside it so that I don't get any arcs through the silicone to the grounded edge of the hole of the output). Do you think that a ceramic tube offers that sort of insulation when combined with the silicone insulator of the wire already? Where the ceramic ends, I will then transition to a vinyl hose to surround the silicone wire all the way to the feedthrough standoff.

Finally, a lot of you mention grounding an unshielded wire before I touch it. How would I do that with my setup? I can't imagine its as simple as carefully clamping a grounded wire to the vinyl or silicone insulation. Should I include an ungrounded conductive foil around my insulator and then ground that when not in operation or when I want to touch the cable? I think it is best to use an unshielded wire for standard operation, just as far as dissipating static charge when not in operation.

I am sorry for all the questions. I just want to be absolutely sure that I am doing everything properly before experimenting with potentially deadly amounts of voltage. I will be sure to have an electrician friend look over my setup before I bring it into operation as well.

Thank you!
Max E.
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Re: RG-8 Coax Cable Failing Under High Voltage

Post by JoeBallantyne »

The correct way to use RG8/U with that Hipotronics transformer, is to strip the outer covering of the cable AND the ground wrap from the end of the wire for the full 6 inches or so that the cable inserts into the transformer. Leave the inner insulator that is ~1/4 inch in diameter intact around the core center conductor. Solder the center conductor to the appropriate connector plug for the transformer (in many cases this is a male banana plug). Connect the appropriate screw on connector to the ground wrap so that when the stripped cable is inserted in to the transformer, you can screw that grounded connector onto the connector on the transformer, and the bare insulated core and plug will be properly inserted all the way into the transformer.

You can many times see pictures on ebay of similarly prepared cables. They are used not only with Hipotronics, but also with some older Spellman supplies. If you look at enough high voltage power supply listings on ebay, you WILL see pictures of a few of those style cables.

Look at those pictures closely and you will get a better idea of how the cables are supposed to work.

If you aren't fully stripping the outer ground and insulation from the RG8 where it inserts into the transformer, you aren't using it the correct way, and that may have something to do with why you are getting shorts.

On some supplies the length of the stripped back cable end can be almost a foot or more.

Properly forming the shape of the outer grounding sheath at the point where you put the screw on connector onto the cable will also help minimize the probability of arc through at that location as Dan Knapp mentioned already earlier in this thread.

Joe.
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Re: RG-8 Coax Cable Failing Under High Voltage

Post by Maxwell_Epstein »

Thank you, Joe, for the advice.

That is how I have been using my RG8 cable, I stripped away the black outer insulation and foil sheath (which also had braided, very thin copper wire around it), leaving only the white insulation surrounding the copper stranded core of the wire. I crimped on a male banana plug and used heat-shrink to seal the insulation (the heat shrink has not failed in any tests). I used a metal connector screwed onto the black insulation about 6 inches up on the cable to attach it to the transformer as you suggested here as well. Unfortunately, the places that the RG8 cable failed were in the middle of the cable and directly under the metal connector, even after I removed the foil shield completely and tried again with a new cable. It seems likely that I just purchased a low quality cable.

I will absolutely keep your advice in mind and be careful to properly consider field shaping concerns. I will attach pictures of my cables once I get home to provide a better idea of what I'm working with.

Thanks!
Max E.
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Re: RG-8 Coax Cable Failing Under High Voltage

Post by Richard Hull »

Yes, the core around the central wire needs to be hard polyethylene and not a foam type. I have never heard of HV RG-8 cable having a foil as part of it shield. The shield is usually a heavy, tight weave braded naked copper. Some higher class cables have silver plated braid.

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Re: RG-8 Coax Cable Failing Under High Voltage

Post by Maxwell_Epstein »

I was able to get some time to build a new cable based around the 30kV silicone wire and have seen great success!

I think I just purchased some rather low quality RG8 as it failed pretty dramatically with holes burnt all the way through it. Additionally, its shield does have foil as a part of it which I guess is nonstandard.
PXL_20230411_184213018.MP.jpg
PXL_20230411_184520425.jpg
The new cable that I made has worked perfectly at and a little bit above 30kV in preliminary testing without any load.
PXL_20230411_213846192.jpg
I'm swapping around a few vacuum flanges at the moment, so it might be a few days before I begin testing with plasma. In the meantime, I am being very tentative with high voltage and am just beginning to experiment with my setup. Despite that, I have now definitively completed one third of a full fusor system (nearly two thirds when considering that almost all of my vacuum system is in place too)!! This is monumental. I hope to begin accelerating in my progress towards fusion and see it as an achievable goal to be making neutrons this summer!

Thank you all!
Max E.
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Re: RG-8 Coax Cable Failing Under High Voltage

Post by Liam David »

That's a pretty catastrophic cable failure. It could be a low-quality cable or perhaps a fluke manufacturing defect. The foil is ok and I suspect it would actually help in field uniformity over the braiding. I meant that if there was only foil and no braiding, then that would definitely be a low-quality cable. A quick check would be to see how centered in the dielectric the center strands are.

Great to hear you had success with the new cable! Do be careful.
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Re: RG-8 Coax Cable Failing Under High Voltage

Post by John Futter »

That is not Mil spec RG8/U
RG8/U has solid polyethylene dielectric no foil and a woven copper shield
we use RG8/U at work to 70kV to polarisz wein filter electrodes and these sit at this voltage 24/7 365 / for the last 19 years I've been at work.

Buy quality coax from a good maker ie Suhner, Alpha, Belden

better spec'd coax's of this size include RG213, RG214
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Re: RG-8 Coax Cable Failing Under High Voltage

Post by Finn Hammer »

A couple pictures of proper RG8.
Notice full copper braid shield
20230412_170259.jpg
Notice solid polyethylene dielectric.
20230412_170318.jpg
Your RG8 has foam dielectric, which is unsuitable for high voltage

Cheers, Finn Hammer
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Re: RG-8 Coax Cable Failing Under High Voltage

Post by Rex Allers »

Yes, my first thoughts were either the shield was not cut back far enough or the dielectric insulation was foam -- which would not be actual RG8/U. Could be RG8/something-else.

Others have pointed out that it looks like you had foam dielectric and Finn's pictures show the cable you should have with RG8/U. No foil though. Main importance is hard (not squishable) dielectric insulation between the center conductor and the shield. It should be white or maybe slightly yellowish and somewhat translucent. You should be able to see the center conductor dimly through the insulation.

I don't know anything about the connection needed for your Hippotronics supply.

For possible reference of some help, here is a link to a pdf about cables that I made for Glassman supplies.
http://www.xertech.net/pub/Glassman%20HV%20cable.pdf

The first couple pages cover some general stuff that came from this forum.

I'm sure I have used good quality RG8/U up to 50 kV.
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Re: RG-8 Coax Cable Failing Under High Voltage

Post by Maxwell_Epstein »

Thank you for the information and for that PDF! I will certainly refer back to it when I am building new cables! I definitely went too cheap for my RG8 because although it advertised itself as RG8/U it clearly isn't. My new unshielded wire has been working great, though, and I've been able to push it to 40kV which is well above where I plan to go for fusion. There's always lots more to learn, and I'm very lucky to be able to learn from the best here on this forum!

Max E.
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Re: RG-8 Coax Cable Failing Under High Voltage

Post by Bob Reite »

I was going to comment that while foam dielectric cable is great for RF purposes the breakdown voltage is a lot lower. Only solid dielectric RG/8 should be used for high voltage purposes. I also have run solid RG/8 up to 60 KV with no issues.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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