Grounding straps for cars


When I was a kid (a long, long time ago), all the cars had a strap hanging down under the frame that dragged on the street. We called them “grounding straps.” The story back then was that if the car got struck by lightning, the strap would ground the car and the occupants would be okay. I didn’t think a strap would conduct electricity, but, what do I know? So, what’s the real scoop?

— Paul

You can still buy a grounding strap for your car if you’ve got 20 bones and a few minutes to spare browsing Amazon. Normally, rubber straps are poor conductors, but the grounding versions are impregnated with a conductive material so free electrons can flow from car to earth with only nominal impedance. The manufacturers claim that static buildup in a car can lead to passenger fatigue and other nonspecific ailments, which sounds like a load of snake oil to me. Lightning isn’t the real reason for the strap, either. During a lightning strike, a car’s metal body acts as an impromptu Faraday cage, transmitting the electricity from the strike along the car’s periphery because of a phenomenon called the “skin effect.” The most useful aspect of the grounding strap would be that it neutralizes the vehicle’s and passengers’ electrical state so that nobody receives a nasty little shock upon disembarking. That same spark has been implicated in explosions and fires at gas stations, sometimes erroneously blamed on cell-phone use. I’ve never been hit by such a shock, but I’ve heard of it. We Alices are naturally good electrical circuits as it is. Great-uncle Marvin Alice was once hit by lightning on the Old Course at St. Andrew’s during a stormy round of golf. To hear him tell it, he insisted on finishing his game (in which he shot a 69) before consenting to medical attention.

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Only one "r" in Faraday, Matt!

Former engineer; San Diego

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