Electric Trains and The Third Rail

[Elektrotechniker]

So we're always told not to touch the third rail, as it's at a very high potential. Makes sense.

But how does return current work in such a system? Let's say there is some load (the motor - we'll simplify and assume DC). One terminal of the motor is connected to the third rail, what's the other end connected to? My friend theorizes ground... but that seems wrong to me. You can't just charge up the Earth through the motor... can you? Could that work? Why not?

It seems to me one of the rails ridden upon must be used for return current - and this would seem to imply that if you were between the train and the generator and you touched said rail, you'd be electrocuted.

 

[triden]

I would assume that one of the other two rails would be ground. Since they are at ground potential, you would not get shocked if you touch it because your body is also at ground potential. All electricity, theoretically does return through the earth. Consider it as a huge bathtub full of electrons. You can pull some out and put some in.

The same principle is in a simple circuit. If the circuit is energized, you can most likely touch the GND side of the circuit without getting a shock. If you were to touch the "hot" side, you would get a zap because there would be a difference in potential between you and the ground you're standing on.

Wikipedia has a good article on it: http://en.wikipedia.org/wiki/Third_rail

"As with overhead wires, the return current on a third-rail system usually flows through one or both running rails, and leakage to ground is not considered serious. Where trains run on rubber tires, as on parts of the Paris Métro, Mexico City Metro and Santiago Metro, as well as on all of the Montréal Métro, live guide bars must be provided to feed the current. The return is affected through the rails of the conventional track between these guide bars (see rubber-tired metro). Another design, with a third rail (current feed, outside the running rails) and fourth rail (current return, half way between the running rails), is used by a few steel-wheel systems. The London Underground is the largest of these, see Fourth Rail."

So it does depend on the type of system in place, but the running rails are often used as returns.

 

[ravenprp]

You are correct in thinking that the return current in an electric train system does not flow through the ground. In fact, the return current flows through the other rail that is not connected to the third rail. This is known as a "return loop" and it completes the circuit for the electricity to flow from the third rail, through the motor, and back to the power source.

Touching the return rail while standing between the train and the generator would not result in electrocution as the return current is not at a high potential like the third rail. However, it is still important to avoid touching any part of the train's electrical system as it can still carry a dangerous amount of electricity.

In addition, electric trains also have safety mechanisms in place to prevent electrocution. For example, the third rail is often covered and only exposed when the train is in the station, and the return rails are also insulated to prevent accidental contact.

The reason we are warned not to touch the third rail is because it carries a much higher voltage than the return rail and can result in a dangerous electrical shock. So while the return current does flow through the other rail, it is not at a high enough potential to cause harm.