HV wire

To: "'Tesla List'" <tesla-at-pupman-dot-com>
Subject: HV wire
From: Tesla List <tesla-at-stic-dot-net>
Date: Sun, 5 Apr 1998 09:02:22 -0500
Approved: tesla-at-stic-dot-net

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From:  Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent:  Saturday, April 04, 1998 12:55 PM
To:  Tesla List
Subject:  Re: HV wire


> 
> ----------
> From:  David Sharpe [SMTP:sccr4us-at-erols-dot-com]
> Sent:  Friday, April 03, 1998 4:57 PM
> To:  Chip Atkinson
> Subject:  HV wire
> 
> Chip, ALL
> 
> You can go to Lowe's (or other hardware store) and pick up
> PE tubing of various dimensional sizes for less than 25 cent
> a foot.  Vinyl tubing is also available.  I've sleeved
> wire with one layer of PE and one layer of vinyl and will
> successfully hold off 20kV (corona yes, power arc NO) off of
> a 20kV, 1.5kVA nameplate PT.

This brings up an important problem with using tubing as insulation. If the
inner conductor is small in diameter (i.e. radius is < 1inch/70 kV), there
will be corona in the air around the conductor. The corona will produce
copious ozone, which will attack the tubing. Also once the breakdown
starts, it can create pinholes through the  insulating layer, which once
there is a path through to something that can carry current, will support
an arc, which will "rapidly" enlarge the defect.

Making corona free wire, which means arc free in practice, is not a trivial
process, as you have to make sure that there aren't any gas inclusions in
the solid insulation around the wire. The inclusions (bubbles) create field
concentrations, which then cause partial breakdown, which then results in
total failure.

The fix is either: really good encapsulation (i.e. in a vacuum tank to
remove any bubbles), or, make the conductor large in diameter to reduce the
field stress. Commercial HV wire does this by surrounding the copper
current carrying conductor with a semiconductive layer (carbon loaded
silicone, or similar) which makes the effective diameter of the conductor
large. Then the outer insulation insulates from the semiconductive layer to
outside.

If you are using tubing, try filling the tubing with oil (or even water) to
eliminate the air space around the wire. Use bare wire, or you'll get an
air gap inside the wire's insulation that will result in breakdown.

More at http://home.earthlink-dot-net/~jimlux/hv/corona.htm and
http://home.earthlink-dot-net/~jimlux/hv/hvwire.htm