Science Questions

Why don't conductor rails short out in the snow?

Sun, 8th Feb 2009

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Dave asked:

I was wondering why is it, when you get piles of snow building up on the railway line, you donít get a short-circuit between the conductor rail and the ground beneath it or the track rail that runs alongside. If we were to stand astride these two rails bare-footed it would make an effective short-circuit. Even though our bodies are composed of 2/3 water.


Dave - Thereís a couple of things. The first one is that pure water is quite a good insulator. My girlfriendís dad used to work in a lab with huge amounts of very high voltages and very pure water. If waterís pure it doesnít collect electricity very well. You need some salts in it.

Chris - Why doesnít it?

Dave - Basically, if you conduct electricity in a liquid you want some free ions. You want some positively-charged things, negatively-charged things. If youíve got some salt in there the salt splits up into negative chloride ions and positive sodium ions. The positive ones move to the negative end of the battery and vice-versa. You get a current flowing. Thatís one thing and I think you do lose quite a lot of the power through things shorting out. I think you get some gentle losses through there all the time. If thereís anywhere where it does get a lot of power dumped: if you get a particularly conductive bit of snow or ice itís going to get a lot of power going through it. Itís going to get hot and itís going to melt and move away from the rail. It immediately breaks that short-circuit.

Chris - Is it worth bearing in mind that the voltage on things  like the London Underground is a lot lower than  when you have trains doing long distances when itís much more economically sensible to use high voltages?

Dave - The rails on the ground are about 600V and the overhead lines are about 25,000Vbecause of this effect. If you had a 25,000V on a rail you would get sparks going to the ground. Youíd also kill a lot of people! Itís not actually lethal unless you touch it. I think they probably do lose a lot of power through them but less than youíd expect where if you do get a short itís going to burn out.

Chris - On a foggy day if you go near to a pylon of high-tension cables where higher volts are used you can hear them buzzing as the mist is settling on the insulators supporting the cables. You get the effect youíre describing where the little bit of arcing vaporises the dampness there and that goes away  and then some more settles.

Dave - Yes, if it vapourises itís going to expand the air out.


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David Jones asked the Naked Scientists: Why don't conductor rails on the railways short out when heavy snow connects them not only to the ground beneath but also to the earthed track? Surely there would at least be a massive current loss due to electrolysis of the snow? What do you think? casals, Fri, 6th Feb 2009 Chemistry4me, Fri, 6th Feb 2009

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