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 many
areas where the wires are more bare than insulated because most of the
insulation has cracked and fallen away.
In my town the street lights have the worst insulation. Street lights
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 neighborhood)
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 other
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 ramp
onto their highway. Their hind legs are on the transformer (grounded)
and their bodies are long enough to span the HV insulator and they put
their front paws on the 9000-volt wire that feeds the transformer. This
fries the squirrel and blows the fuse for that transformer. This knocks
out power to all the homes supplied by that transformer. Power is out
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 thing
they do is look around on the ground for a freshly fried squirrel. The
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 out
and find the squirrel before the power company gets here. That way we
get power restored a lot more quickly because the power-company person
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-----
From: Carl E. Mungan [mailto:mungan@USNA.EDU]
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 bird
touching two lines get electrocuted: Isn't there any plastic insulation
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/