Don't mess with 25kV

Discussion in 'International Transport' started by metrocammel, 12 Jun 2009.

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  1. MCR247

    MCR247 Established Member

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    I thought riding on top of trains was common in India, so that people might not have thought it as as out of the ordinary as they would over here?
     
  2. jopsuk

    jopsuk Veteran Member

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    Whilst is common, I think the vast, vast majority of them are sensible enough not to do it on electric trains.
     
  3. MCR247

    MCR247 Established Member

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    true
     
  4. Thewanderer

    Thewanderer Established Member

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    I had seen the video about a week ago (I got a shock when I saw it the 1st time). Was on a course last week (safety related) and the instructer showed us the video also, as good example what happens when you touch the OHLE.

    A good training tool if nothing else.
     
  5. adc82140

    adc82140 Member

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    I can't quite account for the double explosion- I guess the first would be from the effective completion of the circuit by his hand touching the OHLE, but the second?
     
  6. 90019

    90019 Established Member

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    He didn't actually touch it, he just went near enough for it to arc and the current to flow.

    I'm curious as to what the current must have been, but the resistance of a person can be anywhere between approx. 1000Ω and 100000Ω, so using V=IR and ∴ I=V/R, the current will have been somewhere between 0.25A and 25A, although judging by the fact that it killed him and set him on fire, I'd guess it would be about 10A or above.
    P=IV, ~10x25k = 250kW of power.

    I'm very bored :lol:
     
    Last edited: 17 Jun 2009
  7. O L Leigh

    O L Leigh Established Member

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    When something causes the circuit breaker to trip the system will attempt to reset itself just in case it was a very brief short circuit. If it then trips out again the system does not attempt to reset because there is obviously a problem somewhere that requires attention. This is why there were two flashes because the OLE was effectively tripped twice.

    O L Leigh
     
  8. adc82140

    adc82140 Member

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    Ok that explains it- thanks O L.

    I use high tension supplies in my line of work (up to 150kV, best part of 1 Amp), and I've witnessed what happens when a primary transformer gives up the ghost. That's one hell of a bang, and indoors too!! I dread to think what would happen if any part of the anatomy came into contact with the wrong part of one of our generator cabinets.
     
  9. jopsuk

    jopsuk Veteran Member

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    Out of interest, what sort of current do 750V DC trains tend to draw,

    We've got a hot acid bath at work that can have running through it a 40V DC, 100A current. Lovely to work with- even just the bath solution requires thick rubber gauntlets and a rubber apron.
     
  10. Teaboy1

    Teaboy1 Member

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    What sort of current do 750V DC trains tend to draw ?

    Depends on if they are starting, stopping or steady state. The electrical pick up shoe draws current from both directions (upstream & downstream) and because its DC it is simple Ohms law at work. I dont know the power rating of the traction motors but would hazard a guess of some 150 kVA (kW are reserved for AC motors where power factor is considered).
    Therefore at full power each motor is drawing about 200 amp steady state that is!! However at start-up the current for a traction motor can be some 6-8 times full load current, this is unacceptable for the supply transformers and rectifiers, so resistance is added to the rotor circuit at start up to limit the current and progressively switched out as the train (motor) speed increases and the torque (load) decreases. This is what used to happen on the old stuff like the tube trains where you could sense the surges as the resistance was switched out.
    The newer stuff Im not sure about because the may use invertors now and have AC motors on the axles in stead of old fangled traction motors.
    So I would estimate about 1000 amp for a 5 motor commuter train.;);)
     
  11. DaveNewcastle

    DaveNewcastle Established Member Fares Advisor

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    I'm only guessing, but perhaps the largest current draw for any 3rd rail passenger train will have been the Eurostars when running on 3rd rail.
    I'm sorry that I don't have a power figure for them, but for comparison, a Pendolino can easily draw 5MegWatts under high load, which, at 750 V., would correspond to 7000 Amps.
     
  12. chuffchuff

    chuffchuff Member

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    A 4 Car 400 series draws 800A max for traction
    Class 73 1500A traction plus a bit more for both for ETH
     
  13. Teaboy1

    Teaboy1 Member

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    I have to agree that as a training tool, it is compulsory viewing to those in the industry. However we have to respect human life and so must be viewed as 'what will happen' and not as sick-humor ! Also think of those who have to clear up afterwards and family left behind without a wage earner.:cry:
     
    Last edited: 19 Jun 2009
  14. Captain Speaking

    Captain Speaking Established Member

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    Is the OLE 25KV in India?
     
  15. 90019

    90019 Established Member

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    Mostly 25kV AC, with a few areas of 1500V DC.
     
  16. moonrakerz

    moonrakerz Member

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    Not really - the presence, or otherwise of a Faraday Cage arrangement is partially irrelevent. The main reason that the passengers didn't get "juiced up" is that they were at the same potential as the rest of the train and that they were not providing a path to earth for the current. Rather like birds sitting on HV power lines.
    A passenger standing on the platform with a hand on the train door handle could have been at risk, but this would depend on many other factors.

    There is a lot of misleading information being floated around on this subject. (Including Wiki !) If a car is struck by lightning, it doesn't stay charged at some huge voltage, so where does the energy go - as usual, it finds a path to earth - through the tyres.
    No ! you all shout, the tyres are made of rubber. Surprisingly tyres are pretty lousy insulators. Tyres have a lot more metal in them than they do rubber they also have a lot of carbon compounds - both are excellent electrical conductors. Apart from which, if the voltage has managed to jump several thousand feet from the clouds to your car does anyone really think that the final few inches between your car and the ground is going to stop it ?
     
  17. O L Leigh

    O L Leigh Established Member

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    Not so. If anything, it sounds like you've bought the urban myth.

    I will concede that the science is a little complicated and that I don't entirely understand it myself, but the facts remain that being inside a vehicle is one of the safest places to be during a thunderstorm. The body of your car (or a train for that matter) may not be a perfect Faraday Cage, but it obeys a number of the principles common to true Faraday Cages which will protect you from shorts in the OLE or even lightning strikes. The reason is because, like a Faraday Cage, the charge will be conducted around the outside surface of the metal structure rather than any differences in potential.

    Lightning strikes are very complicated phenomena. Cars and other vehicles may be used as conduits to earth, but the electricity building up in a storm cloud will always take the shortest and easiest path to earth. Damage to road vehicles struck by lightning strongly suggests that the path taken by the electricity is from the cloud to some point on the roof of the vehicle, then down through the metal parts of the vehicle to a point low enough to the ground for it to then arc out from the vehicle directly to earth. Tyres may indeed be lousy insulators, but the voltages in lightning are sufficiently large for the juice to jump many hundreds (or even thousands) of feet through often moisture-laden air, so how hard is it to jump those last few inches.

    Unfortunately, most often the juice arcs to earth from a point where the evidence of such a jump is not immediately obvious (e.g. the floor of the car). However, there is evidence of lightning strikes on road vehicle wheels. Now I will admit to being a layman, but I can't understand how lightning would miss a vehicle roof and strike a wheel in preference to jumping those last couple of inches directly to earth. To my mind, the most obvious explanation is that these marks are the "exit wounds" showing where the lightning jumped out of the car to earth.

    Wiki may not be entirely trustworthy in regard to a number of things and I would never suggest that anyone takes their word as Gospel, but wider reading shows that on this topic they are at least essentially correct.

    O L Leigh
     
  18. moonrakerz

    moonrakerz Member

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    'fraid not !

    I wouldn't dispute either of those statements !

    You have fallen into the usual trap, I'm afraid; a "charge" isn't "conducted" anywhere. A charge is a potential difference between two points, commonly called a voltage. If there is a conducting path between the two points a current will flow until the potential difference is equalised. People often incorrectly say that a voltage passes through something - it doesn't, a current does. The voltage is mainly irrelevant.
    That is why a voltage of 15,000 volts in an old CRT TV probably won't kill you where as 240 volts from a mains socket probably will. The 15000 volt potential, in this case, is not capable of producing a potentially fatal current.

    As you correctly say, a car is not a perfect Faraday Cage (it doesn't have to be in this case), but that misses the point. The main reason for being safe in a car during a thunderstorm is the fact that if the car is struck, your body will be at the same voltage as the rest of the car and will not be providing a current path to earth. If you have your arm out of the window poking a metal coin into a parking meter - you will be "juiced" !
    O L Leigh[/QUOTE]

    To say that Wiki is "essentially correct" is essentially incorrect. Just about its first statement is that car tyres are an insulator - this is patently NOT correct.

    To return to the original subject - that film clip should be shown in every school in the UK - but it won't be because some expert will say it will cause "psychological harm" - it would save lives though !
     
  19. Teaboy1

    Teaboy1 Member

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    I concur about the 'charge' bit, charge as such is a flow of electrons (coulombs ??) while voltage is a PD (potential difference) or a differential of charge. The exact definitions I have long forgotten from uni but the other point about showing kids this film, I disagree. Certainly not to be shown to those under 15, I would not want my 2 to see that happen, give 'em nightmares!! Maybe show to mature kids at secondary school just before the holidays but thats all.

    If you have your arm out of the window poking a metal coin into a parking meter - you will be "juiced" !
    O L Leigh[/QUOTE]

    ...how unlucky would that be? Better to have stayed in bed that morning I would say !!:lol::lol:
     
  20. O L Leigh

    O L Leigh Established Member

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    OK so I got my terminology wrong, but the science is still correct. The reason why being in a car is safe is because the electricity/current is directed away from the occupant as it follows the easiest route to earth and not to do with being at the same potential.

    Safety guidance for waiting out storms in a car is to keep your hands in your lap and not touch the steering wheel or any other component that runs into the safety cell to prevent yourself from becoming part of the conducting path to earth. Surely if you were at the same potential as the car this precaution shouldn't be required.

    O L Leigh
     
  21. nicknackpaddywack

    nicknackpaddywack New Member

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    It only takes 50mA to effect your heart in such a way it could stop it.
     
  22. G8

    G8 Member

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    An open-air flashover of 400 mm is not possible with only 25 kV AC at ordinary atmospheric pressure and temperature.You're right that rod-to-rod (point-to-point) and rod-to-plane gaps are the most critical gap configurations.However,you messed the figures a bit up.In these cases the average breakdown field of 1 kv per mm will require 35-40 mm of gap distance before 'bang' occurs.The critical field varies but very little with weather conditions.Definitely ,it is not as low as 0.1 kv/mm (or 1 kv/cm).
    Less than 400 mm of axial lenght of insulators is more than sufficient to prevent 25 kV catenary wire arcing during normal OLE operation.However,significant overvoltage surges and transients might happen during thunderstorms and lightning.This is a real danger and can gives a possibility of breaking much longer air gaps than just ~4cm.Althought I don't know of any case for somebody approaching to 40 cm from 25 kV railway power line being electrocuted.
     
  23. TDK

    TDK Established Member

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    You will find the gap between the contact wire and the train in the UK is a lot smaller than in India so anyone climbing on a 170 at New Street would get juiced immediately so the BTP would only have to cart them away in a bin bag.
     
  24. 37401

    37401 Established Member

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    on all UK trains you will notice an orange line, under the wires going past that point means you will fry.
     
  25. 90019

    90019 Established Member

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    Depends where you are.
    The height of the OHLE varies by quite a lot, although from what I've seen, it tends to be fairly low at stations.
     
  26. Aussie_Rail

    Aussie_Rail Member

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    the height between the train and the OH varies so much on the Melbourne network, bridges, the City loop and Flinders Street can change the height a lot. From being at a height that a tool can 'train surf' to a low that you couldn't even lye under it without getting zapped.

    Why then even want to go up there beats me.
     
  27. Bill EWS

    Bill EWS Member

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    You are correct about the 'Fizz'. I heard it manty times during my driving career and it is avery unpleasant sound. It was experienced mostly on the north London lines in and around Stratford where the wires were really quite low and in heavy rain the wire would arc onto the wagons, especially freightliner containers. There would be this eerie fizzling sound that filled the air, followed by the flash. I have seen the electricity go down the sides of the wagons. That fizzling sound is probably the last thing anyone getting that close remembers.

    As a driver I have seen people do some really stupid things but getting on top of a wagon or coach with 25K of elecrtricity above them has to be one of the worst. It really does make you wonder about some people. Standing at the cab door on a 37 is as close as you want to get.

    I remember making comment at the north end of the Bletchley flyover of how close the wire was when at the cab door of an 800 hydraulic and they never appeared on those duties again. You could almost have streched your arm up and touched the wire.

    As a rule of thumb we were told not to get closer than nine feet to the wire.
     
  28. schenker

    schenker Member

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    That's crazy, never seen anything like that before!
     
  29. mbonwick

    mbonwick Established Member

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    You can see a good example of this at Lancaster. If you stand at the London end of the southbound platfor and look towards where the footbridge goes over the North bound platform as a Pendo comes in, you can see the panto compress down.
     
  30. DaveNewcastle

    DaveNewcastle Established Member Fares Advisor

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    and rises significantly at level crossings (and down again at the other side). This can often be at high speeds. So rapid movement of the pantograph.

    Yes. I was VERY FIRMLY taught that the 9 foot rule was indisputable. It was supported by the (apocryphal) story of a worker who lost a leg by climbing just a little less that 9ft closer to the conductor. Every one seems to nod knowingly about this poor worker - (though I'm not persuaded that anyone actually knew them).

    I've also found that one of the fastest ways of getting senior station staff to appear at any point on their station is to move a structure (even a non conductive fibreglass ladder) within about 9 meters of a live conductor!
     
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