London Undergroun 4th rail

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.")
EMWTK
Reply to
John Gilmer
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Reply to
Andrew Gabriel
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Basically, the supply rails are earth free (other than bleed resistors to detect earth shorts), so if either rail gets shorted to earth, the full 630VDC is still available to drive the train.
Reply to
Andrew Gabriel
So if you are standing on the ground and touch only one electrified rail, do you get fried?
Beachcomber
Reply to
Beachcomber
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.
Reply to
Andrew Gabriel
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!
Beachcomber
Reply to
Beachcomber
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If the system is properly floating you will not recieve a shock.
Reply to
Alan Taylor
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.
Reply to
Andrew Gabriel
I suspect that a "coke can" would create a "self-clearing" fault.
Reply to
John Gilmer
|> 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. | | I suspect that a "coke can" would create a "self-clearing" fault.
If the coke can were touching poor ground, not necessarily. Sure, if it were a solidly grounded low impedance system, the coke can will join the air pollution. But you wouldn't want such a system in this case so that you avoid tripping on other things that can stop the trains.
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phil-news-nospam

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