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Electric train current return

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chris7153

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Always wondered how electric trains work, they take power from either a live rail or overhead line. However to complete the circuit a return current path is required. I understand this is done via the wheels and running lines but can anyone explain exactly how this works?

If the running lines provide an electrical path back to the substation, they must be electrical insulated from the ground? if this is the case are the running lines not "live" ie carry a risk of electrocution? Also wonder how this current effects the signal track circuits? How are track circuit sections isolated from each other whilst allowing the return current to pass?
 
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petersi

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If you look in the infrastructure forum this has been discussed..Most recently in a thread about Langley junction
 

Laryk

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If the running lines provide an electrical path back to the substation, they must be electrical insulated from the ground?

They do and they are, but as the voltage returning is no more than what it takes to overcome the resistance of the track and / or the traction return wires (few volts at most) they don't require any fancy insulators.

if this is the case are the running lines not "live" ie carry a risk of electrocution?

Technically they are live, and can become live at line voltage if there is a failure of negative return bonding. But as I said, in normal conditions there will be nothing more than a few volts on the track. As harmless as an AA battery.

Also wonder how this current effects the signal track circuits? How are track circuit sections isolated from each other whilst allowing the return current to pass?

Track circuit supply is the opposite to whatever the traction supply is. In DC areas, it will be AC and in AC areas it will be DC.
My work is mainly in DC third rail areas. To separate the track circuit from the traction return we can have the track circuit on the one rail and negative return on the other.
More commonly, we use impedance tanks which is essentially a big inductor. An inductor won't allow AC to flow through it (Track circuit) but will allow DC to flow (Traction return)
 

dgl

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The way to think about it is like this. Think of the wires carrying trays with food. This food represents the current/voltage flowing though the cable.
These trays are passed to the train and the train eats the food and this makes it go it then returns these trays via the rails to be refilled with food and so on there will still be the odd crumbs of food left on these trays but it is so small it does not matter. The energy in the overhead wire or electrified rail (like the food in this example) is used up by the train leaving only a very small voltage/current being passed along the running rails.

The tracks will never be at any real potential above a ground reference (0v) the same way the neutral conductor in house wiring shouldn't be either (there are fault conditions however that would allow significant current to flow in the neutral/return path but it should never happen on a properly designed system)
 
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TheEdge

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Someone has mentioned that the rails would become electrified to line voltage if the return bonding failed.

So, if that did actually happen what would be the outcome for a train in the area and any unfortunate souls on there?
 

cool110

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As with any electrical system the importance is the difference in voltage between the various parts. So while the whole train would become live relative to earth it wouldn't cause any problems for the passengers or crew unless they tried to get off. As for the train's equipment it would have a similar effect to a loss of power.

This is all assuming that there isn't any arcing between the rails and earth.
 
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AM9

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Someone has mentioned that the rails would become electrified to line voltage if the return bonding failed.

So, if that did actually happen what would be the outcome for a train in the area and any unfortunate souls on there?

Those on the train wouldn't feel a thing, except that the train wouldn't be going anywhere. Getting onto the train would be a bit hazardous as the passengers would be bridging the line voltage. The risk of such a fault would be virtually zero as the earth bonds are multiple, and a failure of a single one would probably not even be detectable by passengers.
DC running lines are more hazardous than ac as with higher currents and insulated track, there is more likely to be a rise from a local earth potential. The insulation is necessary to prevent direct current from causing electrolytic corrosion and fault currents in adjacent grounded hardware. These currents are more than 30 times less for the same power draw under ac, and as they are alternating current, corrosion is far less a problem.
 

PermitToTravel

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Someone has mentioned that the rails would become electrified to line voltage if the return bonding failed.

So, if that did actually happen what would be the outcome for a train in the area and any unfortunate souls on there?

It's a bit like how chavs can do pull-ups on the OLE - they don't get barbecued unless they're simultaneously touching something at line voltage and something that isn't
 

pne

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It's a bit like how chavs can do pull-ups on the OLE - they don't get barbecued unless they're simultaneously touching something at line voltage and something that isn't

Or birds sitting on high-voltage power lines.
 

mikeg

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I suppose that the rails don't need to be insulated that well, they'd be well connected to terra firma anyway. Presumably some sort of protective multiple earthing is used?

I assume it's basically a TN-C supply? TN-C supplies are banned in normal installations by the way. Or could it be viewed as TN-C-S (common in domestic installations, banned only for certain industrial and commercial activities)? Is the protective earth then separated on the train?
 
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swt_passenger

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I suppose that the rails don't need to be insulated that well, they'd be well connected to terra firma anyway.

In AC areas, yes. But in DC areas they do attempt to insulate the rails from 'earth' except at the supplying substation, for protection against electrolytic corrosion of structures. This is of course subject to damp and dirty ballast effects.

The difference between AC and DC area's respective earth bonding policy is what leads to the main 'issues' with dual electrified areas.
 

Railsigns

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Track circuits are 'traction immune' using a tuned circuit

There are other, simpler, ways of immunising track circuits against traction currents, which are to use AC track circuits in DC electrified areas and AC-immune DC track circuits in AC electrified areas.
 

EM2

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There are other, simpler, ways of immunising track circuits against traction currents, which are to use AC track circuits in DC electrified areas and AC-immune DC track circuits in AC electrified areas.
Even in DC areas, TI circuits are still used. I installed enough of them!
 

mtbox

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It is the "along track conductor" that returns the current back to the substation. This runs along the top of the portal/stanchions, is a single wire and insulated from the structure. Return current from the rails is fed into this wire by so called "red bonds" (they are spray painted red) which are situated quite regularly along the track. These can be very dangerous if they become disconnected from the rail.
 

superkev

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Hmm
add in auto transformers which use the return current to boost the supply voltage and things can get complicated.
 

contrex

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The way to think about it is like this. Think of the wires carrying trays with food. This food represents the current/voltage flowing though the cable.
These trays are passed to the train and the train eats the food and this makes it go it then returns these trays via the rails to be refilled with food and so on there will still be the odd crumbs of food left on these trays but it is so small it does not matter. The energy in the overhead wire or electrified rail (like the food in this example) is used up by the train leaving only a very small voltage/current being passed along the running rails.

This is actually nonsense.
 

edwin_m

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There are other, simpler, ways of immunising track circuits against traction currents, which are to use AC track circuits in DC electrified areas and AC-immune DC track circuits in AC electrified areas.

That was the traditional method but no longer used on most new schemes, mainly because traction inverters to drive AC motors on modern electric trains can have some (very unlikely) failure modes that could produce the same AC frequency in the traction return and cause the train to disappear from the signalling system. There is also an issue with transformer inrush which can produce a DC current in an AC return rail for a short period when a pantograph is raised or goes through a neutral section. Therefore where track circuits are used at all they are likely to be more complicated ones which have more complicated electrical signals so are less likely to fail wrong side in the presence of interference.
 

MarkyT

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That was the traditional method but no longer used on most new schemes, mainly because traction inverters to drive AC motors on modern electric trains can have some (very unlikely) failure modes that could produce the same AC frequency in the traction return and cause the train to disappear from the signalling system. There is also an issue with transformer inrush which can produce a DC current in an AC return rail for a short period when a pantograph is raised or goes through a neutral section. Therefore where track circuits are used at all they are likely to be more complicated ones which have more complicated electrical signals so are less likely to fail wrong side in the presence of interference.

Much better to use axle counters today. No insulated rail joints or impedance bonds required. No need to worry about the effects of ballast contamination, e.g. sea spray. Reduced trackside equipment. Many fewer compatibility issues with traction power supplies and possible interference signals in traction return current.
 

edwin_m

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Much better to use axle counters today. No insulated rail joints or impedance bonds required. No need to worry about the effects of ballast contamination, e.g. sea spray. Reduced trackside equipment. Many fewer compatibility issues with traction power supplies and possible interference signals in traction return current.

Yes axle counters are usually used. The main reason to install new track circuits seems to be when there are joints in platforms or other places trains might stop, as there is a risk of the axle counter head getting confused and leaving the section as occupied when the train has gone.
 

Bald Rick

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Much better to use axle counters today. No insulated rail joints or impedance bonds required. No need to worry about the effects of ballast contamination, e.g. sea spray. Reduced trackside equipment. Many fewer compatibility issues with traction power supplies and possible interference signals in traction return current.

But no good in very complex, heavily trafficked areas, as the evaluators can't cope.
 

MarkyT

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Yes axle counters are usually used. The main reason to install new track circuits seems to be when there are joints in platforms or other places trains might stop, as there is a risk of the axle counter head getting confused and leaving the section as occupied when the train has gone.

But no good in very complex, heavily trafficked areas, as the evaluators can't cope.

Yes the main problem seems to be where wheels are likely to stop right over a sensor. That is often the case with divided track sections in long platforms at major stations, provided to help signallers manage joining, splitting and sharing operations. It is for that reason that, whilst the vast majority of the recently resignalled Nottingham area is covered by axle counters, the platforms at Nottingham station itself have retained track circuits.

Station platforms are also divided into smaller train detection sections on Thameslink. That is to allow signals to be placed part way along the 12 car platforms, one of many measures designed to allow trains to follow each other as closely as possible. Some of the signals between stations are also less than a train length apart so there's a high probability that wheels will be stopped regularly over section boundaries throughout the core. Whilst the mis-count problem afflicting axle counters always results in a 'right side' failure, where the section remains falsely occupied hence holding signals at red, the time-consuming procedures to reset the system safely would result in unacceptable operational delay on such a critical route, so Thameslink have taken the decision to retain track circuits throughout the re-equipped central core, even though the signalling contractor involved, Siemens, is one of the main proponents and suppliers of axle counter systems, both in UK and their native Germany.
 
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