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F A C T   S H E E T

For immediate release: October 1, 2003
Media Contact: Percy Della - 916-654-4989

Hardware to Prevent Sagging of Transmission Lines

The Sagging Line Mitigator is a new class of hardware developed to prevent excessive sagging of transmission lines during hot weather and high usage.

An overheating electrical transmission line sagging into a tree sparked the greatest power failure in the Western United States in 1996. A similar incident is suspected to have caused the recent East Coast blackout.

By using cutting edge materials and a proven concept - SLiM reacts to increasing conductor temperature by decreasing line sag during high temperature operation. The mitigator was developed by Material Integrity Solutions, Inc. of Berkeley with funding of $900K from the California Energy Commission's Public Interest Energy Research (PIER) program. PIER supports energy-related research and development of environmentally safe, affordable and reliable energy services and technologies.

How the technology works

As high temperature increases conductor length and its sag, SLiM changes its geometry (by contracting) to decrease line length. As conductor temperature returns to normal, SLiM returns (by expanding) to its original shape. The device with no motors or electronic controls remains always ready to respond to the next conductor high temperature excursion.

Tests at the Pacific Gas and Electric Company facilities in July 2002 showed that SLiM reduces the conductor sag significantly to eliminate sag problems. The temperature increase on the transmission lines was 130°F, from 80° to 210°F. The test transmission line reduced the sag by 3 feet 8 inches when compared to the control transmission line.


The SLiM hardware can provide solutions to routine utility problems, thus saving million of dollars in economic loses resulting from power failures.

  • The device can increase line capacity in lower voltage transmission lines to expand their thermal sag limits. Such expansions allow the line to operate closer to its thermal capacity.

  • SLiM enables older lines with a sag-limited maximum conductor temperature of 120°F to operate in temperatures exceeding 200°F without compromising line, line tensions or clearances.

  • The device can delay the need for either a new line or considerable line modifications as local growth increases demand.

The SLim device is rugged and designed for a long life.

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For more technical information, contact: David Chambers, Energy Commission, SLiM project manager, (916) 653-7067.

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