OK, I should have used Google before responding. I guess I have heard of the term and it is used to determine if the tower line might be subject to a cascade failure when a wire is broken.
You can get a copy of ASCE Manual of Practice 74 which talks about it. The second edition is currently the latest but the third edition is currently at the printing company and should be available soon. I am on the committee that revised the manual and you might be able to talk to some knowledgeable people at the ASCE/SEI ETS conference in Ft. Worth on Nov 8, 2009.
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I have been called "A storehouse of worthless information" many times.