It’s more expensive to make the cars that way. Usually, you have the pairs of wheels connected by a metal axle, which is simple and cheap, but if the wheel rails carried the current, that’d be a short. You can, however, use the two wheel rails together as one leg of the circuit, and a third rail as the other leg. This is pretty common in the States.
I don’t know absolute answers to your other questions, but if you fell onto the tracks in such a way that you were touching the two current rails, or a hot rail and a ground (like the other rails-- I don’t know whether the Tube uses two hots or a hot and a neutral), you would probably be electrocuted. If you manage to jump onto a hot rail without contacting a ground, you’d probably be safe.
In most places the wheels/rails are one side and an overhead trolley/cable the other. Having a third rail is not common practice because it is dangerous but it is used in some underground trains because it saves overhead space and the tunnel can be smaller. The trains in the London system are tiny and the tunnels just as tiny.
But the question in the OP is why do you need two electrified rails? I do not know the answer. It could be that the two rails provide the juice and the wheels have no part in the circuit but I doubt it. Another possible explanation is that there are two types of trains working with differnt voltages, old ones with one voltage and really old ones with a different voltage.
Having two electrified rails makes it easier to get electrocuted when pissing on the tracks. Maybe that’s the only reason they put it there.
Quite a few, I should think! If you are living and commuting there, you will soon get bored with all the “incidents” on the line, and wish they would designate a special "dedicated terminal - suicide for the committing of!
So the circuit is completed with the two electrified rails. However,
In this case, the circuit is completed with the single conductor and ground. My guess is that it was easier to do this than to retrofit the entire Underground system to the Railtrack standard.
And yes, if you fell across the electrified rails or between one rail and ground, you would be electrocuted. If you could manage to jump onto one rail and stay away from ground, you’d probably be safe.
Not sure (I’m an American). From your description it sounds like the wheels ride on a seperate set of tracks from the power. The power is coupled to the trains through sliding contacts, and this is used to turn the motors which drive the train. There’s a motor speed control circuit on the train which the driver uses to control the train. Pretty simple really.
Not sure, but I imagine the voltage probably isn’t too terribly high, but it has enough current behind it to move a train. That’s a lot of power.
Electricity can kill you in a couple of ways. First it can interrupt your heartbeat, and once you get your heart out of whack it often won’t go back into whack all by itself. The other way it kills you is by cooking you. If you take a lamp cord and cut the lamp off so all you have is a cord that can plug into the outlet with bare wires on the end (we call these suicide cords, they are dangerous), wrap the bare wires around 2 nails, and put one nail on each end of a hot dog it will cook it in a fairly short amount of time. For all of you junior science whizzes out there, don’t plug it in until the nails are in the hot dog and your hands are off of the hot dog. The science guys on tv usually have this setup bolted to a piece of wood for safety.
No problem. Birds do this with high tension wires all the time. Just don’t get close enough to ground that the electricity can arc over. The higher the voltage the farther it can arc.
By the way, I don’t know if they do this, but if they power the tracks from isolation transformers you can piss on either track and not get electrocuted. The only problem with isolated systems is that mother nature likes to ground it for you, so if you don’t take measures to keep it isolated then it won’t stay isolated for very long (which is why they probably don’t do it this way, too expensive to maintain).
Most third rail systems I have seen seem to be designed to make accidental electrocution very unlikely, although it is easy to do it on purpose. The third rail is outside the track and the live face is on the side away from the track, so you have to stand or lie on the track and touch the outer face.
handy: London Underground (no LTA here!) offer a range of “Mind The Gap” merchandise. It’s actually what the pre-recorded announcements say at stations where, due to curved platforms, there is a gap between the door and the platform. The rail is underneath the train at that point.
The London Transport Museum offers their merchandise online too – rather than link to that site directly (which I suspect would be in contravention of the rules here) you can access it via their main page at http://www.ltmuseum.co.uk/.
excellent answers, everybody. Thank you. esp. engineer_comp_geek. you win the free travel card and the once in a life-time (and last in a life-time) chance to demonstrate falling onto the tracks and getting fried Where can i read up on the electric arc bit ?
Chronos, yes that makes sense. It would be a short, i should’ve thought of that :smack: (yay, i finally get to use the smack smiley)
Crusoe, headshok thanks for the links.
Hari, what you say is interesting. But if the outside face were live and the entire track were metallic, i would presume the inside face is also live. Unless there’s an insulation between the two sides of the same track, which doesn’t make sense.
from Headshok’s link:
Number of traction substations 115
Substations output voltage 630v DC
Nominal track voltage 630v DC
On sections served exclusively by London Underground trains, this is
Centre Conductor : -210v
Outside Conductor : +420v
So how does DC voltage kill me ? I did a search for AC vs DC deaths and all i got was the electric chair history and why AC is more dangerous, but not actually how DC would kill someone as against AC.
ok so i’m clear on 1, 4, 5, 6. and most of 2 and 3. so i’ll just narrow down 2 and 3.
2. What is the current rating ?