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.
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.
As 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.