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London Undergroun 4th rail

Discussion in 'Electrical Engineering' started by John Gilmer, Aug 19, 2004.

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  1. John Gilmer

    John Gilmer Guest

    In the London "Underground" there is a rail that runs right between the two
    running rails. (There is you regular "3rd" rail also.

    So: what's the story with the London system. Why do they do it the way
    they do and why does NYC do it the way it does?

    (I asked this question on the NYC. Transit group. One guy said that the
    center rail was -125 vdc in the tubes and is ground potential on the above
    ground portions of the line. The 3rd rail is positive and provides the
    rest of the total 600+ volts needed to operate the train. The reason for
    the 4th rail was said to be certain unstated problems with "leakage.")

  2. 4 Rails?
  3. Beachcomber

    Beachcomber Guest

    So if you are standing on the ground and touch only one electrified
    rail, do you get fried?

  4. So if you are standing on the ground and touch only one electrified
    rail, do you get fried?[/QUOTE]

    I believe so -- the bleed resistor current is probably still
    significant in terms of electrocution as I suspect the expected
    leakage in such a system would be significant in comparison
    to electrocution currents too.
  5. Beachcomber

    Beachcomber Guest

    Just an opinion here... but could anyone have chosen a more
    complicated system? I would have liked to have been there in the room
    when some person said, "OK, we'll do it this way"...

    One rail at +420, another rail at -210, at least double the
    maintenance on shoe replacement, stocking additional inventory of
    spare parts, increased hazards and obstructions to the yard
    maintenance crews, increased hazards to the public should they
    accidently come in contact with a rail, a requirment for a complex
    ground fault detection system, more materials required per
    construction mile... Wow!

  6. Alan Taylor

    Alan Taylor Guest

    If the system is properly floating you will not recieve a shock.
  7. It's not fully floating and you can recieve a lethal shock.

    It's designed so something like a coke can blowing around in
    the tunnel which gets wedged under one of the live rails and
    grounds it does not take that line out of service. The change
    in current flow in the biasing resistors will indicate there
    is a fault which needs seeing to, but this does not shut the
    system down and it can wait until the line closes over night
    for a maintenance crew to find the fault and fix it.
  8. John Gilmer

    John Gilmer Guest

    I suspect that a "coke can" would create a "self-clearing" fault.
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