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Why Are Power Lines So Deadly? Aren't they insulated?

By Daniel Engber
Posted Thursday, July 28, 2005, at 6:40 PM ET
Danger: High voltage Click image to expand.

Four adult Boy Scout leaders were electrocuted on Monday as they attempted to pitch a large tent at this year's National Scout Jamboree. The disaster happened when the tent's central pole bumped into an overhead power line. Don't power lines have insulation?

No, they don't—at least the ones that run aboveground. Most of the hundreds of thousands of miles of high-voltage transmission lines in this country are made solely of metal—either aluminum or aluminum wrapped around a steel core. Adding a layer of insulation to every line would be pricey and has been deemed unnecessary given how high the lines are off the ground. (Underground lines are insulated, both for the safety of the walkers above and to protect the lines from shovels and the like.)

The biggest overhead power lines can carry more than 700,000 volts, but deadly accidents occur with smaller lines that carry just a few hundred volts. The federal government estimates that overhead lines cause 128 work-related electrocutions every year. (Work-related injuries are more common than non-occupational ones.) A worker will get shocked if he—or his crane, ladder, or other piece of equipment—bumps up against a cable while touching the ground. This allows current to flow out of the power line, through the victim, and into the earth.

Electricity will stray from a power line only if it has a direct path to the ground. If you hang from a power line with both feet in the air, you won't get shocked—that's why birds can sit on a line with no insulation. (Birds do get zapped when they touch two lines at the same time or one line and the grounded wooden pole that supports it; power companies try to prevent bird deaths by increasing the space between the wires.)

The air around a power line isn't a good conductor, but very high voltages do create a significant electrical field. For large-scale transmission lines, this field can have a radius of a foot or more. That means electricity could arc out of the wire to any crane or pole that gets close enough, even if it never makes contact.

Most power companies warn workers to stay 10 feet away from power lines and up to 25 feet away from the highest-voltage lines. Even regular folks trimming trees near a power line need to take care—wood isn't as conductive as metal, but a stray branch can still transmit a deadly shock down the trunk. If workers have to be within 10 feet of a line, the power company can "de-energize" it (turn it off). They might also run a ground wire from the cable to the street below. If there is an unexpected surge, the ground wire will siphon the electricity down before it reaches the part of the line that's getting repaired.

Next question?

Explainer thanks Anne Mayberry of the Electrical Safety Foundation International, Jim Owen of the Edison Electric Institute, John Palmer of Knott Laboratory, and reader Stuart Cleland for asking the question.

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Daniel Engber is an associate editor at Slate. He can be reached at .
Photograph of power lines by John G. Mabanglo/AFP.
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