Thursday, February 7, 2008
News / Chicagoland The return of stray voltage
Posted by Whet Moser on Thu, Feb 7, 2008 at 10:33 AM
Stray voltage is back in the news with reports that 5 people were shocked by walking on a slushy part of Ashland just north of Division. The Reader reported on this phenomenon in 2006:
“This phenomenon is not mysterious,” says Allen Taflove, a professor of electrical engineering at Northwestern University who has studied the hazards of electricity carried by high-voltage transmission lines. “The electrical infrastructure gets old, and insulation around wires degrades. Some of the wires might have been buried decades ago.” He says moisture and salt contribute to the deterioration and help conduct underground currents to the surface. “The wire is usually cased inside a metal conduit. During a storm water may leak into the conduit and travel along the wire. At some point the water may come in contact with a part of the wire where the insulation has deteriorated. The water conducts the electricity from the wire to the conduit and then to whatever is saturated outside the conduit.” Cracks on the surface can carry the current, he says, but if the pavement is thoroughly saturated, they aren’t always necessary. “It turns out that concrete is not always an insulator. Concrete can be a conductor itself, depending on its composition.”
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As a matter of public service: the focus of that story was how it can be especially dangerous for dogs, who (typically) won't be wearing rubber shoes when they hit the electrified spot.
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