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Home / Articles / Opinion / The Straight Dope /  Flying Sparks
The Straight Dope

Flying Sparks

Can car static kill you?

By Cecil Adams
Posted // April 2,2012 -

In winter, I’ve gotten big shocks from static electricity when getting out of my car, and once saw a video where a crewman touching a race car during a pit stop was thrown back several feet, I presume from static charge built up as the car circled the track. This has gotten me wondering: Have there been instances of injury or death from static discharge? —Mike

Yes, many—and if you’re not careful, it could happen to you.

For static electricity, basically you need a giant capacitor—something with a positive charge on one side, a negative charge on the other and a gap in the middle. When you twist around getting out of your car, friction between your clothing and the seat generates a substantial voltage difference between the car body and you, with the cloth and other insulators acting as the gap. When you brush against the car on exiting, an electric arc jumps the tiny space between you and the sheet metal, and blammo, static shock.

Under normal circumstances the shock is harmless. Static charge can be measured in millijoules (mJ). You typically need at least 1 mJ to generate a shock you can feel, 10 to 30 mJ to make you flinch and 1,350 mJ to kill you. Shuffling across a carpet can generate from 10 to 25 mJ, just 1 or 2 percent of a lethal jolt. You might generate more in a car, but even assuming maximum human-body capacitance and low winter humidity (high humidity lets the charge leak away), you could maybe get zapped with about 300 mJ—a shock you won’t soon forget but still not fatal.

That doesn’t mean there’s nothing to worry about. An electric arc is a spark. Sparks are used in auto engines to ignite fuel. Fuel is poured into racing cars during pit stops. Racing cars build up static charge while circling the track … you see where this is headed.

Straight Dope readers may recall our column about the chains gasoline trucks used to drag behind them to prevent static buildup. They don’t do that anymore, not because the risk was nonexistent but because research showed the chain didn’t accomplish jack, and the real static danger came from the sloshing of the tanker’s contents anyway. Now electrical grounding during pump-out and other measures are used to prevent bad things from happening. Likewise, grounding strips prevent static arcing when race cars come in for pit stops.

Having seen that video of the pit crewman being knocked on his butt, you may say: Those strips don’t always work. Maybe not, but that footage wasn’t proof. The vehicle in question was a Formula 1 car equipped with what’s called a kinetic energy-recovery system. This captures in a battery or capacitor some of the energy normally lost to braking, then uses it in a burst of up to 60 kilowatts to power an electric motor when the driver wants to torque out. While it’s not clear what went wrong, evidently the stored-up juice went through the unfortunate crewman rather than into the motor.

Fine, you say, I’ll just stay away from the Formula 1 cars and live a life of comfort and ease. Don’t be so sure. A 1977 study, evidently conducted by the kind of nonsqueamish research team I should have talked to about boiling those frogs a few weeks back, found plain old static shock can cause injury or death in dogs fitted with pacemakers. Another danger arises from the startle reaction to a shock, which can cause you to lose your grip, fall or otherwise put yourself or others in harm’s way.

Getting back to vehicles, you’ll want to watch out for helicopters. Static discharge can be a problem in any aircraft due to friction with dust and water while in flight, and several fires and explosions have been attributed to static discharge during fueling. But helicopters can be especially hazardous because of those big spinning blades. Charges of 60,000 volts have been measured in craft hovering above red clay dust, and 200,000 volts above loosepacked snow.

Just getting in and out of a car won’t generate that kind of voltage, but one study found the resulting charge frequently exceeded 10,000 volts and once (with the test subject dressed in nylon) reached 21,000. Toll collectors and motorcycle cops have been known to get 5,000-volt shocks from drivers or their vehicles. No one knows for certain how many fires have been started at filling stations due to static electricity, but industry statistics suggest it may be in the hundreds per decade.

So, to avoid all this you figure you’ll shun civilization and live in the woods? You’re not safe there, either. The biggest static discharge most of us will ever see is the dramatic display known as a lightning bolt. A typical strike delivers 500 megajoules, or 370,000 times the lethal level, as demonstrated by the roughly 100 annual U.S. lightning deaths—probably, but not certainly, none of which will be yours.

Send questions to Cecil via StraightDope.com or write him c/o Chicago Reader, 11 E. Illinois, Chicago 60611. Subscribe to the Straight Dope podcast at the iTunes Store.

 
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