Re: Birds DON'T perch on high-voltage wires

Ron Verrall wrote:
> "Mike McDowall" <m.a.mcdowall@xxxxxx> wrote in message
> news:fn18h1h9feq5bdbjmh0q7h0ab7m1bu45gr@xxxxxxxxxx
> The voltage on the steel-tower distribution network ranges from about
> 100 kV to over 250 kV. You will notice that sometimes the wires come
> in clusters of four - spaced about 10 inches apart. Sometimes they
> come in pairs - again spaced about 10 inches apart. These four (or
> two) wires are at the same voltage. The reason that they go to the
> trouble and expense of making these spaced groups is to cut down the
> corona and thus save power (which is worth money). The corona is
> strongly dependent on the maximum electric field strength, and the
> maximum field strength occurs right at the surface of the wire. A
> large-diameter wire has a smaller field strength than does a
> small-diameter wire. (Remember the small dimples of pigeon poop.) Thus,
> the electrical authority would like to use wires with an
> enormous diameter. Since this would be ridiculously expensive, they
> do the next best thing and run several wires separated by a few
> inches. This, too,has the effect of reducing the maximum electric
> field strength. The procedure must be very expensive, so the return
> must be real and quite appreciable. From the bits of information
> I've picked up, a double wire implies a voltage of about 150 kV, and
> a quadruple wire implies approximately 250 kV. Anything less than
> about 100 kV doesn't merit the expense of running a double wire. So,
> this is how I get a rough idea of the voltages. Any more details on
> this would be greatly appreciated.
> Cheers,
> Ron
> Victoria, BC

That is my recollection from doing Applied Physics in the 1960s, too. So a
bird on a wire would certainly have a corona discharge current passing
through its body, probably mostly from feet to sharp beak. There would be a
current through its legs due to the fact that even an isolated object has a
capacitance to ground. But I think, or rather guess, that the capacitance
would be quite small (of the order of a few pF) and at only 50Hz , or even
the USA's 60Hz, this current would be even smaller than the corona current.

We have all seen photos of people, usually delighted children, charged up to
a few kV DC so that their fine hair stands upright on end. Some may know
that this effect is even used to "spray" a fur onto a conductive surface
coated in glue. But I don't think that this effect survives changing the DC
to AC even at only 50-60Hz. So I would not expect a bird on an AC HT wire to
have all its feathers erected into the sort of fluff-ball that we see when
they are trying hard to keep warm.

Mike.
--
If reply address = connectfee, add an r because it is free not fee.


.

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