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


"Mike Coon" <mjcoon@@connectfee.co.uk> wrote in message
news:df2in7$b3o$1@xxxxxxxxxxxxxxxxxxxxxxx
> 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.

My initial reaction was that the current due to the small capacitance would
be quite negligible. However, if you assume 10 pF, 50 Hz, and 250,000 V,
you get a current of 0.8 milliamps. This would, indeed, give you a tingle.
However, I agree that the corona current would be much larger. Also, I
believe that these very-high-voltage lines are DC, not AC, and so for them
the capacitance discussion is a non-starter.

>
> 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.

The small demonstration Van de Graaff generators put out about 100,000 V,
which is appreciably smaller than some of the high-voltage lines. I suspect
the bird would, indeed, puff up into a ball of feathers. I wonder if the
bird can sense this as it gets close to the wire.

Thanks for your comments, Mike.

Regards, Ron
Victoria, BC


.

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