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Re: RG-58 core as high voltage wire? (fwd)



---------- Forwarded message ----------
Date: Thu, 15 Jan 1998 09:12:45 -0800
From: Jim Lux <jimlux-at-earthlink-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: RG-58 core as high voltage wire? (fwd)

> ---------- Forwarded message ----------
> Date: Wed, 14 Jan 1998 22:03:19 -0500
> From: "Alfred C. Erpel" <aerpel-at-op-dot-net>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: RG-58 core as high voltage wire?
> 
> Hello All,
> 
>     If you strip off the outer insulation, and braid immediately under
it,
> from a standard RG-58 coax cable, you're left with an inner core of 20 ga
> wire surrounded by .042 thickness of plastic insulation. I don't know
what
> type of plastic this is,

it is Polyethylene

 but most plastics have a dielectric strength of
> around 400-500volts /.001 inch. So it seems to me that this remaining
core
> would be good for up to 16,000 volts DC or maybe 8000 volts AC. This
cores
> insulation appears to be very consistently concentric on all the cables I
> have seen, which would be important if you wanted to use this for high
> voltage.

Actually, you'd be be better off leaving the shield on and grounded. It
makes the field across the dielectric more uniform, as well as conferring a
safety advantage: if you grab the wire, and the voltage is too high on the
center, it will arc to the shield, and not your hand.

>     Can anyone comment as to the composition of this core insulation and
its
> dielectric strength and suitability for use in high voltage applications?

Actually, RG-8 or RG-213 is more popular: it has a higher breakdown voltage
because it is bigger, and the center conductor is #13, which increases the
radius, reducing the corona, and it carries more current.

I note that RG-213 is only rated at 5000 WVRMS. This is to allow for a
potential VSWR mismatch in a transmitting application. I've used it up to
40 kV DC without breaking down.

You can also make a nice HV connector by drilling out the standard "UHF"
connectors (PL259, SO239 series) and extending the center conductor a few
inches (in dielectric, of course).

>     This seems like an ideal material to use to wind chokes on ferrite
> cores. And right now it is easy to scavenge 1000's of feet of this stuff
as
> old coax networks are being torn out of business', and being replaced by
> twisted pair.

As to scavenging: I would be concerned about the inevitable crunches and
kinks in the cable. A small ding in the side won't affect it much for a
network (at least not that the users would notice), but would radically
reduce the breakdown voltage. The other concern has to do with it having
been run around a corner and then having gotten warm, letting the center
conductor move through the dielectric towards the side.

BTW, There are several kinds of RG-58 (A,C, etc.) Some have a solid center
conductor, others are stranded.

Overall, except that if it is free (can't beat that price...), why not get
real HV wire, which will have less corona, etc. It's about 0.20/ft for 20
kV wire at the surplus places, even cheaper if you can use 2-3 foot lengths
(surplus 2nd anode lead from TV's, rated at 40 kV).  Of course, I have
piles of RG-6 and RG-8 around which I use, too, especially when I want the
shielding/safety ground.

More at http://home.earthlink-dot-net/~jimlux/hvwire.htm  
 (soon to change to http://home.earthlink-dot-net/~jimlux/hv/hvwire.htm, in a
few days, as I reorganize my web site).