Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: neilep on 07/10/2008 11:39:45
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Dearest Academic Mentors Of Me,
See these train tracks ?
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Nice eh ?...being delivered Tuesday !
Train tracks are my all time favourite medium for conventional trains to travel on !..but say I was touching the track and then at the same time a few miles down it was struck by lightning ..would I be ok ?..or would I be Shish-kebab ?...How far would the surge travel down the track ?
Train your thoughts my way will ewe ?
Thanks
neil
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mwah mwah mwah mwah
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I hope you looked over your shoulder before you too that photo.
You would have, essentially, a two wire transmission line with resistive loading across it. If the sleepers were dry concrete and the ballast was dry, too (keeping the shunt resistance low), the pulse could travel a long way and be detected hundreds of miles away, I should imagine. It would travel at a bit above 2c/3, due to the effective refractive index of the stone /sand below and air above the line.
I believe telegraph signaling was sometimes done using the rails when the telegraph poles had fallen over.
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Check out this lightningerific story, Neil!
http://www.sptimes.com/2006/07/10/Hillsborough/Lightning_survivor__c.shtml
As the train finally moved across the tracks, Galusha said the lightning bolt hit the train's engine. It instantly heated the track beneath to a glowing bright orange.
He saw a spark jump from the track to the bumper of the pickup and from the pickup's trailer hitch ball to his car.
Then it jumped to the Chevrolet Impala behind him, knocking the woman inside out of her car.
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Railway tracks are split into 'blocks' that are insulated from each other as part of the 'Track Circuit' system of detecting whether a train is occupying a block of track. Invented in 1872, and widely introduced thereafter (although some railway companies were notorious for failing to implement the system and suffered some very bad accidents as a consequnce), it relies upon insulated blocks of track, with insulated rails within each block - the presence of a train in a block closing the circuit between the rails in that block.
The significance of this to your lightning question is that lightning protection has to be built into both the Track Circuit system and also into the electric supply systems for third rail and catenary power pick-up, both for operational safety and to prevent damage.
So, a surge from lightning strike to a rail should not travel any further than the limits of the block that it strikes and should not damage any of the equipment that's attached to the track or rolling stock that happens to be occupying it.
However, if a train is straddling blocks at the time of the strike it would probably form the path of least resistance between the blocks.
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The lightning protection system is interesting - I didn't know it existed. My idea was just theoretical. I bet they didn't have them in the wild west (western region?) scenario of neil's picture - the rails are still bolted together.
However, with or without a train on the track, a pulse may still be launched. If a lot of current flows across the track through the train then this will launch a pulse which will also travel down the line. A pulse can either be caused by a voltage pulse (E field) across the lines or a current (H field).
As you say, the Current can follow the low resistance path but some of the Energy propagates down the line rather than being dissipated in the resistance across it.
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We have to look at the lightning traveling in two ways either between the two rails if there is no train in the vicinity short circiting the path via its axles or collectively between the lines and the ground.
In the electrically powered system both rails are effectively grounded to provided a return path for the overhead power supply and the lightning can only produce a short lived pulse until traveling at 2/3c it reaches the next grounding point.
On a simple line across a desert or prairie it could well travel a long way before it dissipates due to the poor dielectric properties of the interface between the lines and the ground.
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interface between the lines and the ground.
I was assuming that the two lines would be propagating in balanced mode. Does your remark assume the unbalanced mode?
Of course, both modes would propagate but I would think the balanced mode would have fewer losses.
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Actually, I doubt that the rails themselves are very often hit by lightning - they're pretty close to the ground and there are often fences or telegraph/phone wires running alongside the lines, not to mention trees and bushes etc. It could well be different in deserts though, where the blocks could be extremely long, un-electrified and where the rails might well be the highest point in the local area. But then you wouldn't really need any protection systems for those stretches of track because the only thing to protect would be the train, which would have it's own lightning protection system built into it's design.
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What would be launching a pulse would be the current flow. Current would flow over a big area of ground and this could easily induce a current pulse in the lines - as they are so extended. The lines would act like a huge antenna which would carry energy away.
It struck me (sorry for the pun) that where the lightning path is non-vertical, there would be a significant amount of current induced directly into the lines by the component of the current in the arc parallel with the lines. In this case, I guess, the propagation mode would be unbalanced for longer wavelengths associated with the pulse.
I guess we need some factual input about actual cases of lightning strike.
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Hi sophiecentaur - can you explain the basis of the pulse travelling at 2c/3, and how you arrive at that figure please?
Chris
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That's an interesting observation sophiecentaur - I hadn't thought about induced current in the rails.
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Chris
The velocity of a wave along a transmission line is affected by the dielectric constant of the medium around / within it. In a coaxial TV down lead, the dielectric is polythene foam and the speed is about 2c/3. The dielectric constants of glass and plastic are fairly similar. A wave travels faster (almost c) along an 'open wire' feeder (two parallel wires) but the rails are right next to the ground, which may contain more or less anything but could be much like sand / glass. So half of the surrounding medium would be air and half would be the ground. The result would be a bit higher than for 'glass' alone. If the ground is damp, of course, there would be much more loss because the dielectric constant becomes a complex number and the resistive component is low enough to absorb the wave.
I might mention that the dielectric constant can vary a lot with frequency.
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I think the magnetic steel rail and the water in the ground would have an effect on the propagation velocity.
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The effect of the steel would not be great, I think. The majority of the current flowing on the lines would be on the outside of the conductors (skin depth) so the external magnetic field (transverse electric and magnetic - TEM mode) will not actually pass through the steel of the rails. Whatever the direction of the fields or currents which start the wave, it soon settles down into a conventional wave along the line.
Water makes things much more difficult. A good, wet bit of ground could behave almost as well as a metal Earth and could support an unbalanced mode for low RF frequencies (same as a microstrip printed circuit line). 'Poor' ground would cause very high loss. It's too hard, guv. That's why I bottled out and went for a dry situation.
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In the UK the answer to the original question is that no, you wouldn't be OK, because you would be trespassing and this forum doesn't aprove of such behaviour.
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Everybody's answers have not gone unnoticed..
THANK YOU all for these fascinating reads.
hugs you all