# Electric Train Power Supply Circuit

#### howartthou

Joined Apr 18, 2009
111
Hi

In Australia, as in most countries I expect, the power to electric trains comes from an overhead power line that feeds power through a spring loaded arm on the roof of the train.

So thats one side of the current, in through the spring loaded arm, but where is the other side?

If power on the arm is the positive side are the railway tracks the negative side?

If so, does current travel back along the railway track to the power source in order to complete the circuit?

Also, how does the circuit connect from the spring loaded arm to the railway track without electrifying the whole shell of the train which is made of metal?

If not, where is the negative side of the circuit and how does it connect to the original power source to complete the ciruit?

#### mik3

Joined Feb 4, 2008
4,843
I am not a train technician but I can say that the return path is the railway. The spring loaded arm can be isolated from the metallic locomotive (ie mounted on a palstic base) and connect it with a wire to the trains electric system. If the metallic locomotive frame is connected to the negative side of the supply which is also grounded there is no problem of electrification.

#### howartthou

Joined Apr 18, 2009
111
Thanks Mik3, so do you uhink the current runs back down or up the railway track to the power supply?

Is so it would have a long way to travel to get there I suppose...

#### BillB3857

Joined Feb 28, 2009
2,569
It wouldn't have any further distance to travel through the rails than it would travel through the overhead lines.

#### mik3

Joined Feb 4, 2008
4,843
Thanks Mik3, so do you uhink the current runs back down or up the railway track to the power supply?

Is so it would have a long way to travel to get there I suppose...
I am not sure, but I believe the current provided to the train is AC and it is rectified (where they need DC) on the train. Therefore, if the current is AC, it oscillates back and forth in the track.

#### Duane P Wetick

Joined Apr 23, 2009
440
See the French TGV rail speed record on U-tube (357 mph). They have a camera on the brush as the overhead sweeps back and forth.

Cheers, DPW [ Everything has limitations...and I hate limitations.]

Last edited:

#### howartthou

Joined Apr 18, 2009
111
See the French TGV rail speed record on U-tube (357 mph). They have a camera on the brush as the overhead sweeps back and forth.
Thats quite amazing, its a wonder that train isn't airborne at that speed...

I am not sure, but I believe the current provided to the train is AC and it is rectified (where they need DC) on the train. Therefore, if the current is AC, it oscillates back and forth in the track.
Yeah, that makes sense that the current would go up and down to and from the power source if its A/C.

What I am not clear about is that if the tracks are overloaded with electrons looking for a home wouldn't you get electrified if you lay accross the tracks making contact with both sides?

Or is it like holding a live wire but somehow not being part of the circuit?

#### BillB3857

Joined Feb 28, 2009
2,569
Both tracks would be at ground potential. Power from the overhead.