London Undergroun 4th rail

and

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

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.

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

If the system is properly floating you will not recieve a shock.

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.

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

|> 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.