power lines in the trees

Les Schaffer schaffer at optonline.net
Sun Aug 17 11:35:29 MDT 2003

if you read thru some of the articles on transmission lines over at
http://tdworld.com/, one of the running themes for several years is
the continual pushing of the current transmission line system to its
design limits and beyond.

one article from sometime late 2002,


discussed the design of these high tension lines. the limits often are
in terms of how much a line will sag as it heats up -- and if you
recall basic Ohm's law stuff which tells you that heat generation in a
power line goes as the square of the current you push down the wires
(I^2 R)-- we are justified in being suspiciuous of FirstEnergy (story
below) and how hard they were driving their lines on Thursday

   Finding "Hidden Capacity" In Transmission Lines

   By Blake Forbes, Public Service Company of New Mexico, and Dale
   Bradshaw and Fisher Campbell, Tennes

   Transmission & Distribution World, Sep 1, 2002

   As the transmission infrastructure ages -- and as permitting and
   constructing new facilities become increasingly difficult --
   pressure to increase the use of existing facilities is
   mounting. Finding available capacity within current systems has
   emerged as an issue of prime importance as we are required to move
   more and more power over existing rights-of-way (R/Ws).

   The maximum line loading is subject to its "rating." While any one
   of several variables can govern the rating of a circuit, perhaps
   the most common limiting factor is the National Electrical Safety
   Code (NESC)-imposed minimum clearance.

   As utilities load their transmission lines to higher levels, the
   conductors heat up, elongate and sag. The real limiting factor, and
   hence the basis for a line's rating, is often the ability of the
   utility to maintain a safe clearance between energized conductors
   and the ground, trees, vehicles and other objects directly below
   the line, as set forth in regulations such as the NESC.

check out other feature articles at the tdworld.com site on politics,
economics and physics of electric transmission lines.

les schaffer


Ohio May Be Source Of Failure, NERC Says

By Kenneth Bredemeier
Washington Post Staff Writer
Sunday, August 17, 2003; Page A14

Three electrical transmission lines failed in Ohio on Thursday
afternoon, possibly after one of them came in contact with a tree, in
a fast-paced series of events that investigators said yesterday they
are now "fairly certain" triggered the biggest blackout in
U.S. history.


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