On 2/1/11 2:24 PM, Steve Hunt wrote:
> Here are a couple of simple calculations based on EZNEC predictions of
> the impedance at the end of the G5RV ladderline - where the 1:1 balun
> operates:
>
> 40m: Z = 27-j78
> At the 1500W level, I = 7.5A rms and Vpk = 875v
>
> 30m: Z = 37 + j291
> At the 1500W level, I = 6.4A rms and Vpk = 2,655v
>
> Are we sure that the cables being recommended for the bifilar winding
> can withstand those sorts of voltages? I'm not familiar with the US THNN
> cable, but from what I see on Internet pages THNN #12 is rated at 600v.
>
rated at 600V means that it can be used in a 600Vrms system, where it
will have to withstand surges of more than that without breakdown. The
actual breakdown voltage is substantially higher.
In reality, it will stand off more than that. THHN is nylon over PVC
insulation, and at least one catalog says that the nylon thickness is 4
mils over 15 mils (about 0.4mm) of PVC. Typical breakdown strength for
PVC insulation as used in wires has been reported at around
600-900V/mil, with some folks reporting lower, depending the amount of
plasticizers and such. There's also the issue of small cracks, etc.
A safe number for breakdown might be 100V/mil (which leaves a lot of
margin) and still gets you to 1500V breakdown, without considering the 4
mil thick nylon jacket, which is much tougher and probably has a 1000V
breakdown in and of itself.
A chart in a paper by Babrauskas(2005) shows a breakdown at 60 Hz in .4
mm thick PVC of around 30kV.
That paper, by the way, has more information on breakdown and fire in
PVC insulated wire than you could possibly need.
I wouldn't worry about HV breakdown in this application. I would worry
about heating.
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