The Electrified Car Hop?

09 December 2007

Question

As a university student, I worked on a research project involving a state highway department. Part of our mandatory safety training involved discussion of high tension (23kV) power transmission lines. The safety trainer told us that if a power line fell across our car, it was not safe to stay inside because the tires -and vehicle - would catch fire. He then told us that we had to open the car door and jump out, being careful to keep our feet together, and had to hop away from the car, still with our feet right together, due to the voltage gradient present in the earth. It made for a funny mental picture, but it also made some sense due to the high resistance of the soil and therefore the potential (sorry) of developing a current in the body (up one leg and down the other) if the feet were different distances from the voltage source. I've often wondered, since, if this was really true. Do you agree that this is plausible?

Answer

This question was answered by Dr Richard McMann from Cambridge University's department of electrical engineering.

The questioner asks for safety advice about what to do if a 23,000V power line falls across your car. The advice given was to get out quickly as first the tyres and then the whole vehicle would set on fire. At 23,000 volts the vehicle tyres will effectively insulate the vehicle and it acts as a so-called faraday cage. You're perfectly safe inside. Your tyres will not on fire in the time it takes a circuit to disconnect. I do wonder though if there's a mistake in the question. In power transmission, high tension will be more like 230,000 volts. In this case the stories a bit different and the current would jump to earth. This effect was shown recently on a popular television programme, where one of the two long suffering assistants was volunteered to be subjected, in a car of course, to a strike of about a million Volts. After 10 seconds of such shocks, the car started and the assistant escaped unscathed. So, if a power line was to fall on our vehicle, assuming we were to survive any mechanical damage, such as broken glass as power lines are pretty heavy, the thing to do is to be well advised to stay put safely inside our Faraday cage until any arching has stopped. In fact, circuit breakers should disconnect permanently in a second or so. So what do we do then? One of my students suggested driving away to dislodge the cable. This seems a good tactic and if we can get free, we can get out safely. If you can't do that, you've got a choice of jumping out or waiting for the rescue services. You can't be absolutely sure that the cable is definitely dead.Finally, your chances of experiencing this life changing event are very, very small. However, if you're of a nervous disposition, avoid parking under power lines.

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