RAILWAY LOCO EARTH RETURN - EXPLAIN PLEASE 3

[harrisj]

Electric locos pick up high voltage from an overhead catenary. The return path is through the wheels to the rails which complete the circuit. At least, that's what I understand.

But if the wheels are isolated from the rail momentarily (eg leaves on the line!), the circuit is broken and the wheels are at the same potential as the overhead wire. Well that's OK, but the wheels have to be isolated from the frame of the loco - otherwise the whole loco (and the train itself) is at the supply potential. This would mean that opening the doors of a carriage would not make your day.

But how do you isolate the wheels from the axles, the axles form the gearbox, traction motors etc? - while maintaining a very high level of mechanical integrity.

In the UK I believe that 15 kV is the standard voltage, and that 25 kV exists elsewhere. So the voltages are not trivial, but folks don't seem to get frazzled - so how is it done?

I'll drive to work until someone explains the solution!

 

[Barry1961]

I do not know how the locos are wired but it is common in overhead cranes and trolleys to have a supply that is isolated from ground. If this is true the only way you could get shocked is if you were the path of least resistance between the wheel(s) and the rail(s) while the loco motor was under power, moving or trying to move.

Please do let this be an excuse not to walk to work.

Barry1961

 

[bubb375]

Harris-

I always though that the overhead lines provided both positive and negative connections. Thus the electric busses (such as the ones in Seattle and Dayton) can have rubber tires. The overhead wires provide power to electric motors which drive the wheels.

If what you are saying is true, how do you provide power to the electric motors? Wouldn't the casing of the electric motor be charged?

I guess they just do things differently here in the USA.

Brian

 

[MintJulep]

Harris,

You are pretty much right on, except that great pains are taken to keep the wheels and the frame at the same potential. Big problems with bearings if you don't.

It is unlikely that ALL of the wheels will be isolated from the rail simultaneously.

 

[unclesyd]

I have rode many a streetcar with a DC motor. The track was one leg of the circut (ground) and the single trolley was the other. The streetcars used a series wound motor that would run away and the only way to stop it was to run to the rear of the car and disconnect the trolley. They ran until around 1953. The generator station AC/DC and switching was about 1/2 mile from my home and we could tell on our radios everytime they switched. They had to shield the building copper plate.

 

[Skogsgurra]

It is very simple.

The overhead line (the catenaria) feeds current to the contacting device (the pantograph) and from there to the controller/motor. From the motor, the current goes to the axles and through the wheels to the rails and from there back to the feeding transformer (if AC) or rectifier/generator (if DC).

There are many wheels on a locomotive and the weight is heavy. So there is no risk at all that there should be any isolation between locomotive and the rails. Not even if there is ice on the rails since the pressure melts the ice away - or crushes it.

The only real problem is to get the current from the motor to the wheel axles without destroying the bearings. The problem is solved by using slip rings and brushes to by-pass the bearings. Sometimes isolated bearings are used to minimise the current through the bearings. SKF Isocoat bearings are on example.

There is no voltage between locomotive and ground. At least not lethal. Bring a flash-light lamp (3 V) next time and try to make it light up by connecting between locomotive frame and ground. It will not.

And now Harrisj: Buy a railway ticket and impress your neighbours.

 

[harrisj]

Thanks Skogsgurra for an interesting and informative response - I've bought the rail ticket as you suggest.

I agree that it is extremely unlikely that wheels don't provide a return path. Also understand the bearing issue. But unusual freak things do happen and I would still be concerned that (maybe due to track defect?) the return path could be momentarily incomplete. How much of the locomotive rises to the supply potential? Or is all the running gear isolated from the frame?

To other contributors, yes, trolleys, trams and many other urban rail systems have two conductors - but single overhead conductor with rail return is de rigeur for almost all modern high speed trains including Washington - New York - Boston which is 25 kV. I also discovered that Penn RR had 11kV overhead from 1915 - so it's not new and it's not a European phenomenon.

 

[notnats]

Harris,
You seem to be obsessed with the possibility of the wheels losing contact with the track. Both the wheels and the rail heads are hardened because of the very high contact forces caused by the mass of the locomotive. These forces will cause any insulating material such as leaves, plastic, dead bodies etc. to extrude out of the way. If the first wheel does not make contact, one of the following wheels will. Even a high resistance contact will quickly arc if necessary and burn a conductive path for itself.

As Skogsgurra points out, there are many wheels, not only on the loco but also on the carriages, all of which I believe are electrically connected via the couplers (or at least, not isolated).

Having said that, there was an accident here a few years ago on the main line about 50km from Sydney. A vintage steam train on a weekend excursion laid so much sand on the track while trying to stop wheel slippage on a steep grade that it actually formed a layer of glass on the track. This prevented the low voltage track signalling current from flowing. The train disappeared from the signalling system and an electric commuter train ran into the back of it with some loss of life.

However, nobody was electrocuted.

Oh, and I can't believe that Bubb375 can grow up and become an engineer without stumbling across the concept of electrified railways. Or maybe it's just that I grew up in a city that has had electric trains since the 1920's (and had electric trams from the 1800's through to the 1960's).

Jeff