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Re: HV xray cable revisited

Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>

Hi Terry,
I understand the conductive layer technology for corona losses and the dielectric layer is around the conductor and then the main insulation followed by the jacket. But, as I understand it, only the outer jacket was removed so I don't think it will be a problem. The oddity is the cable arcing to the outer jacket before he removed it (which must have been observed from the ends where he striped back the outer insulation initially). I can only assume the cable insulation damaged or insufficiently rated. Regardless, if he's got the main insulation on the cable and routed through the pvc, it should be fine (that doesn't mean go grab it hot, but it should be arcing to anything). 

Take care,
Bart

Tesla list wrote:


Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

Hi,
It is common for HV DC cables, like used in X-ray, to have conductive layers inside them in what looks like "insulation". These layers are used to increase the conductor's apparent radius to prevent breakdown (corona leakage) inside the cable's insulation. I would never go cutting the layers off and expecting it to work!! One should normally use the cable with the outer grounded jacket well grounded just like they meant it to be used.
The capacitance between the conductor and the shield "might" be a problem in a given system.
Even a perfect insulator can still carry heavy AC displacement currents (at Tesla coil frequencies) and "glow" when near grounded objects... If they are not designed for it, they can easily fail. Thus, a DC X-Ray cable might fry under AC... Really good cables for such things are teflon or polypropylene. Not much else works... Note that insulators too have a breakdown voltage just like air. Although much higher, there is still a limit to how much RF voltage you can have on a radius. Thus is why HV cables tend to be very "special"...
Best just to not really too much on insulation in our sport. Even a small coil can happily punch though a 1/4 inch thick sheet of glass.... 

Cheers,

        Terry


At 06:34 PM 1/4/2006, you wrote:
Yes Mike, arcing is a problem. Hard to say why it is however without seeing it. The outer jacket/shield should have been stripped back several inches beyond an ability to arc. If it was still arcing, then that is odd. Anyway, stripping the entire jacket off is fine to (a lot of work though). 

Take care,
Bart

Tesla list wrote:


Original poster: "MIKE HARDY" <MHARDY@xxxxxxxxxx>

So you don't think the arcing of the black coating to everything robs any
power? Or in your case to grounded shield.
----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Sunday, January 01, 2006 6:27 PM
Subject: Re: HV xray cable revisited


> Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>
>
> Bart,
>
> That's funny that you say that. Actually, my experience
> with x-ray power cable is to leave it on the ground. May-
> be I'm "messing up" doing it that way, but I've never had any
> problems using x-ray cable in this fashion. I simply strip off
> enough of the outer sheathing and insulation along with the
> semiconductive carbon tape for proper standoff of the 14 kV to 17 kV
> from the inner 3 conductors to the outer
> grounded braiding and leave the rest of the cable intact. I
> solder all 3 of the inner condcutors together on the end as one.
> X-ray cables are really overdesigned for the usual sub-
> 20 kV voltages used to feed  Tesla primary circuits since they are
> generally rated around 75 kVDC. The thin outer rubber
> sheathing seems to be fairly durable, too, and effectively protects
> the grounded mesh sheathing (yes, I do ground the outer-
> mesh, just beneath the outer rubber sheathing and I have NOT
> witnessed any significant Blumlein affects).
> David Rieben
>
>
>
> ----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
> To: <tesla@xxxxxxxxxx>
> Sent: Sunday, January 01, 2006 10:33 AM
> Subject: Re: HV xray cable revisited
>
>
> >Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>
> >Hi Mike,
> >For a quick reply, "get the cable off the ground and your problems
> >are over". I never allow hv feeds to be tossed along the ground.
> >Take care,
> >Bart
> >
> >Tesla list wrote:
> >
> >>Original poster: "MIKE HARDY" <MHARDY@xxxxxxxxxx>
> >>
> >>Some time back I had a discussion on the list about stripping off
> >>the black conductive coating that remains after the shield is
> >>removed, for 8" or so on either end of my HV xray cable. It seems
> >>this black coating still arcs if it is in contact to ground, or the
> >>other xfmr lead (14.4 kV). Is this simply distributive capacitannce
> >>causing coupling due to it's use on AC? Do you think this robs alot
> >>of energy? Is it dangerous if layed on damp ground? I find running
> >>the leads thru pvc to the TC cumbersome, and time consumming, when
> >>I want to drag it outside for short runs. Since I want to run my
> >>coil tonight for new years a quick reply would be greatly apreciated.
> >>
> >>HAPPY NEW YEAR!
> >>Mike
> >>
> >>
> >>
> >>