Make your own HV cable, wasRe: How many STSG's in service?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>



> 
> I also had another idea. for HV cables. After buying (cheap) some Belden 35
> KV test lead wire for the pig leads I studied the construction and came up
> with my own idea for building cheap HV wire. I noticed that the Belden wire
> uses polyethylene as the HV insulation. I found that you can buy
> polyethylene tubing very cheapy at the building supply stores. It is
> typically used for water lines. I think that you can buy this tubing and
> then pass your conventional low voltage wire through the tubing to make your
> own HV cables. Judging from the thickness of the available polyehtylene
> tubing and after looking up the breakdown voltage, I would think that you
> could very easily make 50 KV cables with no problem. These cales could then
> be passed through the flexible aluminum conduit to make a very rugged but
> flexible HV cable.

Excellent ideas, in general.  Here are things to watch out for:

1) PE tubing may not be quality controlled for electrical properties. Off
hand, though, I think you're ok here.

2) The failure mode on HV cabling isn't usually the catastrophic punch
through (at first).  What happens is that you get corona discharges off the
inner conductor, which creates a small track in the dielectric, which then
provides a place for the field to concentrate, etc. This repeats
(particularly with AC) until the "treeing" gets all the way through, at
which time you DO get the catastrophic failure.

The usual solution on real HV wire (not obvious to the eye, by the way) is
to put a semiconductive layer around the current carrying conductor.  This
makes the radius of curvature larger, so corona doesn't start. It doesn't
have to carry any power, so the resistivity can be quite high.

3) If you do use PE (or vinyl) tubing for insulation, having a grounded
overshield is important, because eventually, the dielectric will fail, and
you want the failure to be a short to ground, not a short to your hand.
The flexible metal conduit is a great idea.  Rugged, cheap, and a good
enough conductor (Hmmm.. maybe that's why the NEC allows it to be used as
the safety grounding conductor)

4) Most dielectrics are VERY sensitive to water.  A small amount of water
will radically alter their properties (nylon and silicones are notorious
for this).  You'd want to make sure that your tube in tube scheme has some
way to keep the insides of the tubes dry. Silica gel?