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Re: [TCML] Primary Support Materials

Hi Gary,

Unfortunately, PVC tends to track more quickly than many other polymers in the presence of a corona discharge. Corona is typically generated at "triple points" - where a conductor, air, and insulator all come together. The E-field is enhanced in these regions, and if allowed to create corona, the insulation system will ultimately fail. In PVC, the combination of UV, ozone, and heat in a corona discharge cause some of the PVC to break down, liberating carbon and hydrogen chloride. The latter combines with water vapor in the air, condensing onto the nearby surface of the PVC (and anything else in the vicinity).

Partially-conductive chloride ions on the surface lead to development and accelerating growth of carbonized tracks. Once begun, these evolve into branching, black, Lichtenberg-like figures that culminate in the complete failure of the insulator. However, IF you can prevent initial corona formation or arcing near the surface, PVC can be an excellent HV insulator. It's just not a very forgiving one.

Polymers that don't contain halogens are considerably less affected by small amounts of corona. Common solid polymers with much better corona and tracking resistance include polyethylene (LDPE, HDPE, non-conductive UHMW), Plexiglas (PMMA), Polycarbonate (PC), and glass-epoxy. Flexible polymers with excellent corona and tracking resistance include silicone rubber and EDPM rubber.

Here's an example of tracking that I encountered within a high energy spark gap (poor design/material selection on my part). Although initial tracking began on the surface of the white PVC spacer, it progressed along the surface of the nearby polycarbonate as an HCl film was deposited onto its surface. The PC surface was roughened from sanding away a previous set of tracking patterns - it didn't fix the problem. Replacing the spacer with one made from PMMA completely eliminated the problem.


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Gary Lau wrote:
It's not clear that folks are in agreement on whether PVC is prone to
surface tracking.  Here's my thoughts.

If PVC were inherently prone to surface tracking, we'd all be in trouble, as
pretty much all of our secondaries are wound on it, right?  Surely a
secondary coil has a far higher voltage gradient than a primary support.

I think Ralph's account of a PVC antenna support succumbing to carbon
tracking is not surprising, given that it occurred when it was covered with

FWIW, I had carbon tracking on some (dry) Lexan primary supports - I had too
many unused primary turns adjacent to a grounded strike ring.  Anything will
surface track, then carbon track, if it's carbon-based.

That said, it's hard not to notice the discoloration of white PVC near a
spark gap.  If that brown stuff is more conductive than PVC (don't know),
perhaps there is a higher risk of surface tracking on PVC near the gap,
although I've never heard this reported.  But secondary coils are
sufficiently shielded or distant to not be affected.  I'm guessing the same
is true for primary supports as well.

Regards, Gary Lau

On Tue, Aug 24, 2010 at 9:35 AM, David Rieben<drieben@xxxxxxxxxxx>  wrote:

Hi Greg, all,

Yes, I certainly agree that PVC is farily prone to
tracking, especially when used in key insulative
components of a fair sized VTTC (personal ex-
perience with this). I've also discovered that it
does not fair too well over the long term as the
containment tube for a multiple "RQ" style sta-
tionary spark gap, as white PVC tends to de-
velop a yellowish-brown tinge, which is brittle
and tends to be somewhat conductive, in the pre-
sence of the strong UV radiation from the SG.
However, I have had absolutely no trouble along
these lines using it (gray electrical grade PVC) as
primary support rails in my large coil, which is now
about half way into it's 7th year of existance. And
I don't think that it's really THAT ugly ;^))

David Rieben

----- Original Message ----- From: "G Hunter"<dogbrain_39560@xxxxxxxxx>

To: "Tesla Coil Mailing List"<tesla@xxxxxxxxxx>
Sent: Monday, August 23, 2010 10:12 PM
Subject: Re: [TCML] Primary Support Materials

  I'll second that motion.  I've been using PVC pipe and zip ties for my
primary supports since the 1990's.  I've never noticed any particular
propensity for PVC to break down and carbon track--not any worse than any
other plastic.  This is the quickest, cheapest way for the non-machinist to
build a primary support.  If you require an appearance-grade coil, do
something else.  The PVC pipe method isn't pretty.


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