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Old 01-13-2014, 05:29 AM
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Train third rails and electrocution

I grew up in a surburban town with a train line through it that was electrified via a third rail. So two sets of tracks, in each direction, and two third rails in the middle, for each direction.

All sorts of bollox was talked when we were kids about how the rail could kill you. Like if you just touched it with your foot you'd be dead. No, you could stand on it and you'd be fine as long as you weren't touching the ground. No, that's not right, you need to touch both electric rails etc etc.

What's the SD here? Would simply placing your foot on the third rail lead to electrocution?
Old 01-13-2014, 05:39 AM
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If you touch the rail and ground (or another rail), you're at risk - I guess if you had very well insulated shoes, you might be OK.

If you were standing on the rail and touching nothing else, there would be nowhere for current to flow, so you'd be OK, but being on the rail implies either:

Stepping on (the other foot is still on the ground - see above)
Jumping on (better not miss, or slip and fall across the rail and touch it and ground or another rail)

Better still: stay off the tracks. Always stay off the tracks, unless you're Superman.
Old 01-13-2014, 05:48 AM
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People die on the El tracks in Chicago fairly regularly. Usually from falling off the platform when they're stupid drunk. As Mangetout says, if you manage to jump onto the third rail so as not to complete a circuit to the ground (as birds, rats, and mice can and do), you'll be OK. But touch a toe down to catch your balance, and you're toast. Kinda literally. No one can help you, or they're toast too.
Old 01-13-2014, 07:40 AM
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What train system was this? A pair of third rails going down the middle (as opposed to by the side) is unusual. If you tell us the location, then it's easy to look up how the particular system in question worked and how to kill yourself on it.

In most third rail systems with which I'm familiar, there's a singe third rail and the current return path is via the running rails. If you touch the third rail and one of the running rails, or you have a low-impedance path to ground (e.g. are standing in a puddle) you're toast.
Old 01-13-2014, 07:50 AM
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It's the merseyrail system in Merseyside, UK. I see that me saying down the middle is confusing - they're down the middle of the complete track bed, which means they are by the side of the rails the train sits on (image here).
Old 01-13-2014, 07:58 AM
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Yep, that's a pretty typical system. There is a single third rail for supplying each track, and it can be on either side of the track, though only one side will be used in a given location. The third rail supplies power and power is returned via the running rail. If you touch both the third rail and one of the running rails simultaneously, you will complete a circuit and it will be unpleasant.
Old 01-13-2014, 08:13 AM
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Quote:
Originally Posted by friedo View Post
If you touch both the third rail and one of the running rails simultaneously, you will complete a circuit and it will be unpleasant.
Only briefly though.
Old 01-13-2014, 09:01 AM
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Are the 2 third rails in phase? What if you touched both, and only both?
Old 01-13-2014, 09:07 AM
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Originally Posted by AaronX View Post
Are the 2 third rails in phase? What if you touched both, and only both?
There's only one third rail in this case, but if there are two parallel tracks then there will be one third rail for each track. They will be connected to the same power source, so if you touched both of them (but nothing else) you will raise the electrical potential of your body but should generally be OK, I think.

The London Tube has an unusual system where they actually do have two third rails: one one the side and one in the middle. The middle rail is at -210V and the side one at +420V, so if you complete a circuit between them, it's 630V of fun. I have no idea why they did it that way, though. Something about not wanting to use the running rails for the electrical load.

ETA: And third rail systems are usually DC, so there's no phase involved. Overheard catenary systems are typically AC, and do require "phase breaks" when moving from one power grid to another.

Last edited by friedo; 01-13-2014 at 09:09 AM.
Old 01-13-2014, 11:25 AM
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Quote:
Originally Posted by Busy Scissors View Post
All sorts of bollox was talked when we were kids about how the rail could kill you. Like if you just touched it with your foot you'd be dead. No, you could stand on it and you'd be fine as long as you weren't touching the ground. No, that's not right, you need to touch both electric rails etc etc.
Where I grew up, it seemed like there was always one kid who could be coaxed into testing these theories.

Well, for a while there was this one kid, anyway.
Old 01-14-2014, 08:11 AM
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nm

Last edited by runningdude; 01-14-2014 at 08:12 AM. Reason: answered already
Old 01-14-2014, 08:55 AM
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Quote:
Originally Posted by friedo View Post
The London Tube has an unusual system where they actually do have two third rails: one one the side and one in the middle. The middle rail is at -210V and the side one at +420V, so if you complete a circuit between them, it's 630V of fun. I have no idea why they did it that way, though. Something about not wanting to use the running rails for the electrical load.
The tunnels are lined with metal - apparently there was some concern that using ground for the return path could cause electrolytyic corrosion.
Old 01-15-2014, 12:25 PM
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I guess you're talking about something like this.

One of my all-time favorite movies.
Old 01-15-2014, 12:33 PM
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"Leftenant Gaaaaaarber...."
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