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

08 February 2009

Question

I was wondering why is it, when you get piles of snow building up on the railway line, you don’t 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.

Answer

Dave - There's a couple of things. The first one is that pure water is quite a good insulator. My girlfriend's dad used to work in a lab with huge amounts of very high voltages and very pure water. If water's pure it doesn't collect electricity very well. You need some salts in it.

Chris - Why doesn't 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 you've 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. That's 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 there's anywhere where it does get a lot of power dumped: if you get a particularly conductive bit of snow or ice it's going to get a lot of power going through it. It's going to get hot and it's 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 it's 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. You'd also kill a lot of people! It's not actually lethal unless you touch it. I think they probably do lose a lot of power through them but less than you'd expect where if you do get a short it's 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 you're 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 it's going to expand the air out.

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