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Re: [TowerTalk] Rain Induced Voltage

To: Dennis OConnor <>,
Subject: Re: [TowerTalk] Rain Induced Voltage
From: Jim Lux <>
Date: Fri, 01 Dec 2006 10:36:38 -0800
List-post: <>
At 09:35 AM 12/1/2006, Dennis OConnor wrote:

>On the comment about the static voltage rising to high levels and 
>possibly damaging the coax... I have not seen that, but I routinely 
>see the voltages rise high enough to arc over at the PL-259... THis 
>is most pronounced during thunderstorms nearby, and conversely 
>during fair, dry weather when the wind blows actively...

The voltage to arc over a PL-259 is probably about 4-5 kV. (the free 
air distance from center pin to shell is probably about 0.15 inches * 
70kV/inch = 10.5 kV.. however, that's for an idealized uniform 
field.  Arcing over a surface is typically about 1/3 the free air 
distance (why HV insulators have ribs) and the corners on the edges 
of the barrel also contribute.

RG-8 and it's ilk (solid dielectric) are regularly used as a HV cable 
for 20-30kV.  The voltage rating on coax (5kV RMS in the case of the 
0.405 inch cables, typically) is based on RMS numbers (so you pick up 
an easy factor of 1.4 for DC) and with a substantial margin to cover 
manufacturing tolerances, etc.

The HV limit will be with corona inception on the center conductor 
where the E field is highest.  But, from a bulk breakdown standpoint, 
you might expect breakdown around 40-50 kV (assuming typical RG-8 
dimensions and PE insulation with 18 kV/mm breakdown).

For HV use, RG-8 style coaxes are used with modified PL-259s with a 
very much extended dielectric and something like a banana plug at the 
end, so that the creepage distance is several inches.

Jim, W6RMK 


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