Technical: Electricity

[Sconner]

Hey all, I have this question in my mind thinking of how electric current travels on OLE and returns via electric bonds through the rails.

So when everything is working fine circuit is complete, trains and rails are safe to touch no risk of death.

But what happens when there is contamination on the railhead, you touch train wheel and theoretically complete the circuit standing on the ground. Death? Or is it something that can't happen?

Cheers

 

[edwin_m]

Hey all, I have this question in my mind thinking of how electric current travels on OLE and returns via electric bonds through the rails.

So when everything is working fine circuit is complete, trains and rails are safe to touch no risk of death.

But what happens when there is contamination on the railhead, you touch train wheel and theoretically complete the circuit standing on the ground. Death? Or is it something that can't happen?

Cheers

Pretty sure it isn't an issue. The current flowing between the wheels and rails is enough to prevent contamination building up.

 

[bahnause]

Hey all, I have this question in my mind thinking of how electric current travels on OLE and returns via electric bonds through the rails.

So when everything is working fine circuit is complete, trains and rails are safe to touch no risk of death.

But what happens when there is contamination on the railhead, you touch train wheel and theoretically complete the circuit standing on the ground. Death? Or is it something that can't happen?

If the contact between the wheel and rail is completely lost (e.g. in the event of a derailment), the circuit is no longer closed. In these cases, there is a real danger when getting off the train. The safest place is therefore in the train. If necessary, the train must be exited with a jump.

 

[Sconner]

Pretty sure it isn't an issue. The current flowing between the wheels and rails is enough to prevent contamination building up.

I'm kinda thinking very unlikely scenario, like layer of insulator on the track.

If the contact between the wheel and rail is completely lost (e.g. in the event of a derailment), the circuit is no longer closed. In these cases, there is a real danger when getting off the train. The safest place is therefore in the train. If necessary, the train must be exited with a jump.

Good point, if train maintains contact with OLE and it has no connection to the tracks it can break circuit.

On the same note I'm guessing that broken track bond would cause issues too if track wasn't grounded well for some reason.

 

[edwin_m]

If the contact between the wheel and rail is completely lost (e.g. in the event of a derailment), the circuit is no longer closed. In these cases, there is a real danger when getting off the train. The safest place is therefore in the train. If necessary, the train must be exited with a jump.

That could only occur if no wheel anywhere in the train was in contact with a rail, but the pantograph was still in contact with the wire and the circuit breaker remained closed. If the wire fell on the roof of the train then I think at 25kV the path to earth via grounded wheels would draw enough current to trip the supply.

 

[John Webb]

Contamination on the railhead is mostly leaves being crushed and is a relatively thin layer. While this contamination can cause problems for braking and for track circuits only working at a small voltage and current, they remain conductive to higher voltages - leaves after all are mostly carbon which is a conductor of sorts! I don't know if anyone has done any work on the electrical resistance of leaf contamination, but I've never heard of a train being halted by loss of power due to such contamination - only loss of traction due to loss of grip.

 

[edwin_m]

Contamination on the railhead is mostly leaves being crushed and is a relatively thin layer. While this contamination can cause problems for braking and for track circuits only working at a small voltage and current, they remain conductive to higher voltages - leaves after all are mostly carbon which is a conductor of sorts! I don't know if anyone has done any work on the electrical resistance of leaf contamination, but I've never heard of a train being halted by loss of power due to such contamination - only loss of traction due to loss of grip.

It does seem to be a non-problem on electrified lines, although these tend to have more and longer trains which would also help in keeping the rails clean. The Track Circuit Actuator, fitted to all UK DMUs but not EMUs, works by circulating a large-ish electric current through the wheel-rail interface, and I think was inspired by the thought that the traction return current did the same thing on electrics.