Question of the Week

The Electrified Car Hop

Sun, 9th Dec 2007

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Jeff Crowell asked:

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?


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

High Voltage Warning SignThe 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. Youíre perfectly safe inside. Your tyres will not on fire in the time it takes a circuit to disconnect. I do wonder though if thereís 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 canít do that, youíve got a choice of jumping out or waiting for the rescue services.  You canít be absolutely sure that the cable is definitely dead.

Finally, your chances of experiencing this life changing event are very, very small. However, if youíre of a nervous disposition, avoid parking under power lines.


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I think it's plausible. Rdn00b, Tue, 4th Dec 2007

While teaching a 4th grade class a worker from our local power company came in for a presentaion. Tht question was asked and his response was that you should stay in the vehicle unless there was impending doom...<fire,etc> he then said you should stand in the car, facing outward and jump as far as you can but be sure to land on both feet at the same time. so, I feel there must be something to this indeed.He did not continue to tell us to do the bunny hop to get away though. Broca, Tue, 4th Dec 2007

My guess it that the strategy outlined in the question of the day is not necessary.  I think that the same "skin effect" that keeps you relatively safe from lightning (100 million - 1 billion volts) would keep you safe from the powerline until help arrives.  Still, if you felt you had to leave the car, I'm betting that a 23 kV line is not a threat if placed on dry earth. J_Schattel, Wed, 5th Dec 2007

I suspect that the reason for landing on both feet has more to do with the temptation to have one foot trail the other when you leave the car (or even just that the trailing foot might get inadvertently caught in the car), so by mandating that one should land with both feet together, it guarantees that both feet have left the car before either foot lands on the ground. another_someone, Thu, 6th Dec 2007

This is the same advice as being caught in a field during a thunderstorm. You should keep your feet together so the electricity can't form a circuit between them. When cows die due to being struck by lightning, it is because their feet are not together. Actually, I think I read this on this website, in the true/false quiz. I am unsure about the car not protecting you, though, as a car is, I believe, safe in a thunderstorm, and I am sure there is more voltage in a flash of lightning than a power line. On the other hand, a lightning flash is brief, whereas the power line would be on until someone yells "Hey, there's a guy in a car frying over here! Turn the power off!" So maybe the car isn't safe- I dunno, really. rhade, Fri, 7th Dec 2007

I remember somebody telling me a about a 'leaky' pylon were cows were found to be dying but humans were not. It was caused by the cows front legs being at a greater distance from their back legs.

A car is like a Faraday cage and is a safe place during a lightning storm. I would hope the power line would have some sort of fuse/breaker on it somewhere and would trip. turnipsock, Sat, 8th Dec 2007

It is it possible to communicate through the ground by developing a voltage gradient.  Imagine two metal spikes in the ground 5 metres apart and two more also 5 m apart say 20 metres away.  .......The 4 spikes form the corners of a rectangle 5 x 20 meters.  Apply a high level signal to one pair and you will receive a proportion at the second pair.   I supposed you could work what the proportion will be by drawing it out as 4 points connected by resistors which have ohmic values proportional to the distance between the spikes.  Pumblechook, Sat, 8th Dec 2007

turnipsock is correct, the safest thing to do would be to stay in your car (faraday cage). If for any reason you have to exit the car then yes, jump with both feet together, but remember, what part of the car are you going to touch to open the door?

It must be a plastic handle, and you should not be holding any part of the car when initiating the jump. once you have landed, do not walk away...shuffle along, making sure both feet are always on the ground.

from the news, Sat, 8th Dec 2007

If you are exiting the car then opening the door is not a problem. The issue is if you open the door whilst providing a route to earth i.e. if you are standing on the ground when you touch the car.

A Faraday cage works fine with you standing inside it, even touching the sides, so long as you don't connect it to anything using yourself as a wire!

Chris chris, Sun, 9th Dec 2007

A week or two late...

what would be the effect if the car was a  Reliant Robin?, Fri, 14th Dec 2007

Tyres are coductive(carbon content) Prolonged contact will cause them to burn, causing contact to ground. Thus jump clear.Don't touch anything. Get away untill electricity is disconnected. John Kahts, Tue, 1st Jan 2008

You deserve to be electrocuted for having the audacity to be seen "driving" in such a sh!te car. chris, Mon, 7th Jan 2008

I would be ok because I wouldn't be seen dead in a Reliant Robin. lyner, Thu, 10th Jan 2008

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