Electric Shock in a running train

A good explanation can be found below on the principle

http://en.wikipedia.org/wiki/Faraday_cage

But see here we are dealing with dynamic flow of charges through high tension wire and not the static electrical field other electrostatic discharges? Please clear this Shouldn't we will get a shock? Is the faraday cage principle still apply? Acco. to me Due to continuous flow of charges the charges wont get enough time to distribute outside the compartment to make the electric field zero inside and so we might get an electric shock.

The structure of the train andeverything toucing it is the same potential as the return running rail on which it runs. The return running rail is bonded to the lineside equipment supplying the power.

The 25kV feed is conneccted to various traction transformers and other equipment, and is not presented to the structure of the train; if it were to happen then a circuit protective device at the lineside equipment supplying the 25kV would operate to disconnect it as a fault.

http://en.wikipedia.org/wiki/List_of_current_systems_for_electric_rail_traction

The train might be a Faraday cage; it might not. The matter is of no consequence.

I got the point, thanks.

You are right. Most deaths happening in the world by this phenomenon

If the line falls on the metal roof, charge goes straight to ground through the train, no shock to passengers. ( Train is ground)

If the line falls, but is hanging outside a window, (no contact with train), and passenger grabs line with his hand, charge still goes to ground through train, but goes through passenger first. Passenger gets shocked and most likely killed. (Passenger is ground, providing link between line and train)

Electricity will take easiest and fastest path to ground.

"Electricity will take easiest and fastest path to ground."

And if you become the ONLY path, you are by default, the easiest path. We are essentially bags of water with a few electrolytes, so we tend to be fairly decent temporary* conductors. But not as good as most metals.

*I say temporary because we have a relatively high Positive Coefficient of Resistance; we are essentially PTC resistors. As we conduct, we cook and in doing so, we eventually carbonize, which increases our resistance until eventually the surrounding environment dielectric becomes lower than our remains. We are of course long gone by then.

Shouldn`t the current be disastrous though even 1% (as it flows to ground through shortest possible path),but definitely some portion of it must flow through our body and i guess even a tiny fraction OF THAT HIGH TENSION WIRE IS SUFFICIENT for instant death.

Also there is continuous flow of charge through metal body wall , so though no matter if touched from inside we must get electric shock.

Is the flow of current from transmission lines to ground through metal body ,is the only protection against such hazards??

The metal of the train is probably aluminum, even if it's not, it's a better conductor than the human body. There is no good reason for the electricity to leave the path it's already on, go through a passenger, and return to it's original path.

If a person is boarding the train in the same scenario and has one foot on the ground and has his hand on the handrail of the train getting ready to board, he, in all likelyhood would receive a shock.

I don't know if it's the only protection.

You ain't kidding about the carbonization. I worked for a power company one summer. We got sent out to repaint a switch (I think around 30 kV, but it was long ago?) where two kids had climbed up to do whatever. By the time they got the power off, we had to use putty knives to chisel some of the remains off before we could prime and paint.

Where's PWSlack when you need him and his 24V lamp

It is important to note that electricity takes more then the easiest path back to the source, it takes EVERY path back to the source. The size of the current flow in each path is dependent upon the resistance (for a DC circuit) and the impedance (for an AC circuit) of the full circuit.

So... given the example, why will someone in the car get shocked or not get shocked during the described failure event? As long as the individual is at one "impedance" or "resistance" value relative to the point that he contacts the metal of the train car, no current will flow though his body. But... say he touched a second point of the car, by moving his feet or by touching a support member that is now at a different resistance value, current will now flow across that circuit.

This is why a bird sitting on a high tension cable with many thousand of amps flowing through it at several thousand volts does not get killed. If two birds, several feet apart on the same conductor touch wings... zzzztttt. Burnt Bird!

This effect is called "Step and Touch Voltage"' and is a well known hazard in the utility field. EUSA, the "Electrical Utility Safety Association', offers a good course on how to deal with this issue in a fault condition in a HV situation. I have taken this course. I believe the material may be on-line and is worth a read.

Here is a Wiki link on this issue for more reading...

http://en.wikipedia.org/wiki/Earth_potential_rise

Thanx for a healthy discussion.Seems the idea is cleared.

Thanks guys,

I figured someone more knowledgeable than me would come along and clarify.