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Author Topic: Why don't electrified rails short-out in the rain and snow?  (Read 9550 times)

Offline casals

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Hi there!

Does anyone know why conductor rails on the railways don't short out after the recent heavy snowfall which was deep enough to connect them to the ground beneath as well as the earthed track itself? Wouldn't there at least be a large draw of current as the snow is turned into hydrogen & oxygen by electrolysis?
« Last Edit: 05/02/2009 12:31:03 by chris »


 

Offline Pumblechook

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The conductivity of snow (being more or less pure water) is very low so very little conduction or electrolysis takes places.  I would imagine any current flow is small wrt to the huge currents that the trains take. 
 

Offline casals

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Thanks for info. Does something have to be added to pure water to allow it to electrolyse effectively?
 

Offline Chemistry4me

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Water soluble electrolytes, e.g, strong acids and strong bases, Li+, K+, Ba2+, Ca2+, Na+, and Mg2+. Sodium and lithium are most commonly used, as they form inexpensive, soluble salts.
 
 

Offline techmind

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I have heard of DC/third-rail electric trains continuing to run even when the track is flooded and underwater. Not recommended!

Bear in mind that such trains operate at around 650V, and a train will draw something like 3000-5000amps at maximum acceleration ... you'd need an extremely low-resistance 'short' to have much effect anyway.
 

lyner

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I have heard of DC/third-rail electric trains continuing to run even when the track is flooded and underwater. Not recommended!

Bear in mind that such trains operate at around 650V, and a train will draw something like 3000-5000amps at maximum acceleration ... you'd need an extremely low-resistance 'short' to have much effect anyway.

Can you give a reference to this? Bearing in mind that floodwater may be highly impure and the resistance between a length of live rail and Earth would be pretty low, I can't see how this can be true except in some very special circumstances (perhaps a short length of under water rail).


Edit / Added
I looked up conductivity of non pure water.
Seawater is about 5S/m and Drinking water is up to 0.05S/m. Dirty water will be something in between - if it were 0.1S/m, a length of 1km of rail would have a resistance something like 0.01ohms.
(Assume the spacing is 0.1m and the effective width of the rail is 0.1m and doing a crude sum.)
For 650V, this gives 65000A.

Even if I'm a couple orders of magnitude out, you will still get around 1000A.
« Last Edit: 05/02/2009 10:53:13 by sophiecentaur »
 

Offline lightarrow

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I have heard of DC/third-rail electric trains continuing to run even when the track is flooded and underwater. Not recommended!

Bear in mind that such trains operate at around 650V, and a train will draw something like 3000-5000amps at maximum acceleration ... you'd need an extremely low-resistance 'short' to have much effect anyway.

Can you give a reference to this? Bearing in mind that floodwater may be highly impure and the resistance between a length of live rail and Earth would be pretty low, I can't see how this can be true except in some very special circumstances (perhaps a short length of under water rail).


Edit / Added
I looked up conductivity of non pure water.
Seawater is about 5S/m and Drinking water is up to 0.05S/m. Dirty water will be something in between - if it were 0.1S/m, a length of 1km of rail would have a resistance something like 0.01ohms.
(Assume the spacing is 0.1m and the effective width of the rail is 0.1m and doing a crude sum.)
For 650V, this gives 65000A.

Even if I'm a couple orders of magnitude out, you will still get around 1000A.
Maybe they count on the fact that it's very difficult to have the rails immersed for such a great lenght.
Using 0.05 S/m as conductivity (in wiki it's reported as max conductivity for drinking water) and 0.5 m as effective width, we get 325 A for a lenght of 50 m.
(I know, it's very difficult to make predictions at this level).
 

lyner

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Why don't electrified rails short-out in the rain and snow?
« Reply #7 on: 05/02/2009 13:17:46 »
We've certainly shown, with our various estimations, that a short length 'dipping' under water would not constitute a load which would trip the system. It would be an added expense, though!

The original question involved snow / ice.  The lower conductivity of relatively pure water could certainly reduce the current considerably as long as there were no 'impurities' next to the ground.. Many km of snow cover would be expected, compared with relatively short lengths of flooding, but it may be that power dissipation could melt the snow in contact with the rail and create a void around the rail, reducing the load. A passing train would clear the snow when it wouldn't affect the water level of a flood.
All these practical situations have so many factors. Engineering is such whizzo fun.
It is a shame that they have to use the third rail system, mostly, in the UK. There is such a lot of existing  third rail infrastructure that I don't think they'll ever change.
 

Offline Pumblechook

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Why don't electrified rails short-out in the rain and snow?
« Reply #8 on: 05/02/2009 16:12:53 »
Snow is 'fluffed up' as well and will conduct less than compacted snow and much less than melted snow.  Not sure there is much 3rd rail in UK.  London Underground and mainline trains are mainly 4 rail I think (?).  A -210 V rail (outside rail) and +420 rail (centre rail).  Not sure whether any current flows in the running rails??  and the live rails are well spaced.   I did try some from the freezer and real snow and got 11 Mohms between two short prods about 2 cm apart. 
 

Offline Bored chemist

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Why don't electrified rails short-out in the rain and snow?
« Reply #9 on: 05/02/2009 17:08:48 »
I melted some snow the other day and measured the conductivity. It was about a third of the conductivity of the tap water where I am (the instrumentation isn't really calibrated- I just wondered how pure is "as pure as the driven snow)).

So freshly melted snow is a worse conductor than tap water.

Much more importantly, ice is a near-perfect insulator, so the question doesn't really arise.
It's only liquid water that conducts.
 

Offline LeeE

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Why don't electrified rails short-out in the rain and snow?
« Reply #10 on: 05/02/2009 17:29:50 »
No trains of any kind would normally be run over submerged rails for number of safety reasons.  Firstly, if the rails are submerged it is not possible to see if they have buckled or fractured (drivers can spot buckled rails at quite distance and small fractures, before they're actually bad enough to cause an accident, can sometimes be heard by the driver, if he doesn't have to filter out the water splashing as well, although they're are usually spotted by Permanent Way teams or test trains, neither of which would work with submerged rails.).  Secondly, if the rails cannot be seen, neither can the ground that supports them and which may have been washed away or unstabilised by the water.  Thirdly, the point motors and their feedback sensors will be under water and cannot be guaranteed to be working or correctly indicating the state of the point.
 

Offline Pumblechook

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Why don't electrified rails short-out in the rain and snow?
« Reply #11 on: 05/02/2009 17:42:27 »
May be better to use a higher voltage and allow electrolyis to take place...with water anyway.    Digital Ohmeters do not apply much voltage.  Think when I tried with thin rod electrodes in tap water there was very little current flow with 30V applied...no bubbles.  It shot up with some table salt in it and there was vigorous bubbling. 

There may also be more flow with warmer water??   

 

lyner

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Why don't electrified rails short-out in the rain and snow?
« Reply #12 on: 05/02/2009 18:47:16 »
BC
Quote
I just wondered how pure is "as pure as the driven snow)).
Steer clear of the yellow variety!
 

lyner

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Why don't electrified rails short-out in the rain and snow?
« Reply #13 on: 05/02/2009 18:53:17 »
Snow is 'fluffed up' as well and will conduct less than compacted snow and much less than melted snow.  Not sure there is much 3rd rail in UK.  London Underground and mainline trains are mainly 4 rail I think (?).  A -210 V rail (outside rail) and +420 rail (centre rail).  Not sure whether any current flows in the running rails??  and the live rails are well spaced.   
Don't you go South very often, then? There are millions and millions of passenger miles traveled on one powered rail by Southerners. None of that new fangled overhead stuff or multiple rails down here. The Thameslink trains (or whatever they call them now) are dual standard, with shoes underneath and pantographs overhead.
 

lyner

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Why don't electrified rails short-out in the rain and snow?
« Reply #14 on: 05/02/2009 18:55:26 »
LeeE
Cheers for the practically informed contributions! Now we'll all shut up and retire to our theories and calculators!
 

Offline Pumblechook

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Why don't electrified rails short-out in the rain and snow?
« Reply #15 on: 05/02/2009 20:29:09 »
Spohie...

HA HA.. and you think 4 rail is 'new fangled'.   Have you ever been on London Underground?

I thought both LU and Southern Trains were 4 rail.  LU is certainly 4 rail. 

Just looked it up and sections of mainline trains are indeed 4 rail because they are shared with LU. 
« Last Edit: 05/02/2009 21:10:58 by Pumblechook »
 

Offline techmind

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Why don't electrified rails short-out in the rain and snow?
« Reply #16 on: 07/02/2009 18:00:29 »
HA HA.. and you think 4 rail is 'new fangled'.   Have you ever been on London Underground?

I thought both LU and Southern Trains were 4 rail.  LU is certainly 4 rail. 

Just looked it up and sections of mainline trains are indeed 4 rail because they are shared with LU. 

London Underground is 4-rail, with the conductor-rails nominally "floating" (well, relatively high impedance) relative to the traction rails. This enables a fault, in the form a a short between the power and conductor rails, to be "detected" remotely while the system still remains operational.

Most of the southern regions trains (out of London Bridge, Victoria and Waterloo) are third-rail systems.
The only 4-rail BR line I'm aware of is the anomaly that is shared with the Bakerloo line between Queens Park and Harrow and Wealdstone. In this case the central rail is strapped with big cables to the traction rails at frequent intervals. This enables the underground trains to get power (but obviously doesn't have the fault-awareness and fault-tolerance of the true LUL 4-rail system.
 

Offline techmind

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Why don't electrified rails short-out in the rain and snow?
« Reply #17 on: 07/02/2009 18:04:40 »
I have heard of DC/third-rail electric trains continuing to run even when the track is flooded and underwater. Not recommended!

Bear in mind that such trains operate at around 650V, and a train will draw something like 3000-5000amps at maximum acceleration ... you'd need an extremely low-resistance 'short' to have much effect anyway.

Can you give a reference to this? Bearing in mind that floodwater may be highly impure and the resistance between a length of live rail and Earth would be pretty low, I can't see how this can be true except in some very special circumstances (perhaps a short length of under water rail).

I think it might have been Northfields (Ealing) on the Piccadilly Line, which is in a deep cutting, after a sudden flash-flooding incident quite some years ago. If my memory is correct, then we would have seen/heard first- or second-hand as we have family friends who live very nearby.
« Last Edit: 07/02/2009 18:06:43 by techmind »
 

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Why don't electrified rails short-out in the rain and snow?
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