Ice has weighty effect on power lines

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Crews make repairs to poles and distribution lines damaged during a winter storm near Tecumseh in November, 2009.

Lightning, thunderstorms and tornadoes aren’t the only threats to the electrical infrastructure. In winter, ice and wind, even wind alone, can be just as dangerous and destructive to power lines, leading to a disruption in service.

Each drop of freezing rain adds weight to power lines and their structures. It also adds more pressure and strain.

The following infographic explains the stress ice places on power lines and infrastructure. Another phenomenon to consider is “galloping,” which sends power lines sailing in a dangerous way.
ice on power lines

Designing for bad weather

OPPD uses guidelines in the National Electrical Safety Code (NESC) and various weather condition scenarios to determine the design criteria for ice load on overhead power lines.

Distribution lines are designed to handle up to 1/2-inch of ice and 40-mph winds. New OPPD transmission lines are designed to handle 1 1/4 inches of ice with no wind, or winds of up to 90 miles-per-hour (equivalent to a weak EF1 tornado) with no ice.

Galloping

ice on power linesAs ice accumulates on power lines, it forms a teardrop shape.

When wind blows, wires can start to move up and down in an oscillating motion. In essence, the wires encased in ice act like an aerodynamic airplane wing. This is known as “galloping.”

Galloping can cause wires to eventually touch, resulting in a fault or subsequent power outage. The increased movement can also cause cross-arms to break, bringing lines to the ground.

To reduce galloping and its dangers, you may see twisted wire or metal pieces attached to certain power lines. This video shows you how that works.

Laura King-Homan

About Laura King-Homan

Laura King-Homan is the managing editor of The Wire and a communications specialist at the Omaha Public Power District. She has nearly 20 years of print journalism and design experience, including the Omaha World-Herald.

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