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The 220-V lines in neighborhoods running from the transformers to the
homes are indeed insulated wires, but often the insulation is in bad
shape. There was a time that we did not have good UV-resistant
insulation. The insulation became brittle and cracked away. I see m=
any
areas where the wires are more bare than insulated because most of th=
e
insulation has cracked and fallen away.
In my town the street lights have the worst insulation. Street light=
s
were strung with insulated-separated wires having poor-quality
insulation. Almost all the insulation is gone. The village is in no
hurry to replace it because the separation allows everything to keep
working.
In residential areas the high-voltage line running from pole to pole =
(at
the very top) is approximately 9000 volts (depends on the neighborhoo=
d)
and is not insulated. In single-phase areas this lone wire is well
separated from other wires and is the highest wire from ground level.
In three-phase areas the high-voltage wires are three parallel 9000V
lines along the tops of the poles. They are separated from each othe=
r
by about two feet. It is indeed difficult for a bird or other animal=
to
get across these.
The most common cause of local power outages in my village is loss of=
a
fuse on a power pole because of a squirrel. The squirrels treat the
high-voltage wires as highways. (I think they think we put the wires
there for their benefit.) Squirrels don't have a problem when they
*jump* from tree to wire or from wire to tree or from wire to wire. =
The
problem is when they decide to use the transformer as the entrance ra=
mp
onto their highway. Their hind legs are on the transformer (grounded=
)
and their bodies are long enough to span the HV insulator and they pu=
t
their front paws on the 9000-volt wire that feeds the transformer. T=
his
fries the squirrel and blows the fuse for that transformer. This kno=
cks
out power to all the homes supplied by that transformer. Power is ou=
t
for about an hour because the power company has to send a person to
replace the fuse. The person who comes to replace the fuse also has =
to
ascertain why the fuse blew before inserting a new one. The first th=
ing
they do is look around on the ground for a freshly fried squirrel. T=
he
squirrel is often found pretty close to the base of the pole, but can=
be
10 yards away and can be hard to find if the area has brush or a lot =
of
foliage.
When a fuse blows in my neighborhood or on my campus, I usually go ou=
t
and find the squirrel before the power company gets here. That way w=
e
get power restored a lot more quickly because the power-company perso=
n
does not have to spend time looking for the fault. I watch for the
truck to arrive, and I go out and say, "There is the blown fuse, and
there is the fried squirrel."
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu
-----Original Message-----
=46rom: Carl E. Mungan [mailto:mungan@USNA.EDU]=20
Sent: Wednesday, February 26, 2003 8:45 AM
To: PHYS-L@lists.nau.edu
Subject: Re: Power Lines
Okay, time for what's probably a really dumb question. Why would a bi=
rd
touching two lines get electrocuted: Isn't there any plastic insulati=
on
on ordinary electrical power lines? (I'm not talking about the
ultra-high long-distance high-voltage lines, just what one might find=
in
a neighborhood.) Carl
--
Carl E. Mungan, Asst. Prof. of Physics 410-293-6680 (O) -3729 (F)
U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
mungan@usna.edu http://physics.usna.edu/physics/faculty/mungan/