Re: Birds DON'T perch on high-voltage wires
- From: Richard Brooks <richardbrooks@xxxxxxxxxxxxx>
- Date: Tue, 30 Aug 2005 23:52:13 +0100
Mike Coon wrote:
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.
There has been at least one field that has strip lights stuck into the ground in a field under a pylon. If you run between the rows of lighte they glow as you pass.
If they glow it means you're going to die some time in the future! ;-)
Richard. .
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