[es373]

If there is 3rd rail with OHLE on the same section of track, I wonder if OHLE could induce a voltage in to the third rail?

[John55]

Originally Posted by HSTEd

Tube type trains have separate pickup shoes for each bogie, this means they do not have to worry about how the current gets back to the negative rail.

Main line trains have a 750V bus (with some exceptions like the Class 313s), if you are going to return the current to the fourth rail rather than the running rails you will need a second bus line at the fourth rail voltage which must be able to handle the same currents as the normal one.

The only way to ensure that you do not add more mass to the train is to cut the current in each bus in half by doubling the voltage between the two rails compared to the normal one between the third rail and running rails.

I assume you wrote this when very tired as it is nonsense. Trains may or may not have a power/return bus but all have a positive feed to the power equipment from the 3rd rail and a return back to the running rails or the 4th rail. In the case of a 4th rail return the cable carrying the return current goes to the appropriate shoe, in the case of a 3rd rail supply the cable goes to the axle of the wheelsets via a contact usually in the axlebox. There is no need for any change in voltage or change in current rating when switching between 3rd and 4th rail operation as demonstrated in my example of the changeover at Birkenhead Park between the LMS and the Mersey Railway.

One could design a system using +750V and -750V as the SER proposed in the 1920s but it has nothing to do with the difference between 3rd and 4th rail design.

[es373]

Running a current through an axle box? Are you sure you are not tired as this would cause catastrophic damage to the axle bearing thus causing a derailment or seized wheelset.

I think the cable you are referring to is the axle box earth fault detection where if current is sensed going through the box then there should be a warning in the cab or the HSCB/VCB will open.

[John55]

Originally Posted by torquewrench

Running a current through an axle box? Are you sure you are not tired as this would cause catastrophic damage to the axle bearing thus causing a derailment or seized wheelset.

I think the cable you are referring to is the axle box earth fault detection where if current is sensed going through the box then there should be a warning in the cab or the HSCB/VCB will open.

Not through the bearings but the last I heard the normal transfer of return current was via a wiper/brush on the end of the axle therefore mechanically in the axlebox. This will not of course apply to an inside frame bogie! There are lots of ways this could be done and may well be becoming obsolete.

[hedpe]

313's have an axle brush box for ground where the running rails are 0v. There is no direct physical contact between the train and the 4th rail that exists between moorgate and drayton park.

[HSTEd]

Originally Posted by John55

I assume you wrote this when very tired as it is nonsense. Trains may or may not have a power/return bus but all have a positive feed to the power equipment from the 3rd rail and a return back to the running rails or the 4th rail. In the case of a 4th rail return the cable carrying the return current goes to the appropriate shoe, in the case of a 3rd rail supply the cable goes to the axle of the wheelsets via a contact usually in the axlebox. There is no need for any change in voltage or change in current rating when switching between 3rd and 4th rail operation as demonstrated in my example of the changeover at Birkenhead Park between the LMS and the Mersey Railway.

And where will the fourth rail pickup shoes be?
If you don't want to allow current leakage to the running rails when that bogie does not have a pickup shoe in contact with the 4th rail, you will need some sort of bus along the train connected to the fourth rail pickup shoes. (Doing so would largely undermine the entire purpose of the fourth rail in the first place.)

As this bus carries the same current as the third rail pickup bus it must be of similar size.

Therefore the only way you can convert a train to fourth rail operation without requiring a net addition of bus lines to the train is to halve the current flowing through the traction package.

The only way to achieve this without reducing available power is to double the voltage between the 3rd rail and 4th rail compared to between the 3rd rail and running rail in a normal system.
This means +750V and -750V.

Converting a main line train to 4th rail is an engineering nightmare.

[Dstock7080]

LU trains now only have power bus lines within a motor car linking shoes/bogies and to just 1 adjacent trailer car for the supply to compressors/MA etc.
Power bus lines between motors cars no longer exist, except on the LTMuseums' '38 Stock.

[John55]

Originally Posted by HSTEd

And where will the fourth rail pickup shoes be?

Usually under the front bogie.

[/QUOTE]
If you don't want to allow current leakage to the running rails when that bogie does not have a pickup shoe in contact with the 4th rail, you will need some sort of bus along the train connected to the fourth rail pickup shoes. (Doing so would largely undermine the entire purpose of the fourth rail in the first place.)[/QUOTE]


The return path to the centre (4th rail) pickup shoe is insulated from the vehicle chassis and wheels so if the shoe looses contact and there is no bus then the train looses power until the shoe regains contact. Exactly the same as for the 3rd rail shoe at pointwork or other gaps.

[/QUOTE]
As this bus carries the same current as the third rail pickup bus it must be of similar size.

Therefore the only way you can convert a train to fourth rail operation without requiring a net addition of bus lines to the train is to halve the current flowing through the traction package.[/QUOTE]

The current flowing in a circuit is the same at all points in the circuit. If a current of 1000A is required to drive a train from a 750V power source then 1000A flows from the 3rd rail shoe to the power equipment and 1000A flows back to the 3rd rail or the 4th rail. How the return current gets to the return path doesn’t affect the current demand or the voltage required. The only difference between 3rd and 4th rail operation is the termination of the return cable on the shoe or the axle/wheel and the isolation of the return side of the current path from the vehicle chassis..

[/QUOTE]The only way to achieve this without reducing available power is to double the voltage between the 3rd rail and 4th rail compared to between the 3rd rail and running rail in a normal system.
This means +750V and -750V.[/QUOTE]

The class 503s ran for 18 years (from 1938 to 1956) changing from 3rd rail operation on the former Wirral Railway (electrified on the 3rd rail) and then during the station stop at Birkenhead Park switched to 4th rail operation so they could operate on the Mersey Railway with no loss of performance. Both railways operated at nominal 600V.

[/QUOTE]Converting a main line train to 4th rail is an engineering nightmare.[/QUOTE]

The Mersey Railway managed to convert their trains in 1938 to operate off both 3rd and 4th rail systems with no difficulty. Seems odd if it is/was an “engineering nightmare”.

[apk55]

The LU system is a nominally floating system with the power supply isolated from ground or only connected via resistors. I gather it is possible to short either conductor rail to ground or running rails without tripping the power supply (although a alarm would be tripped). A emergency shorting bar has to go between the two conductor rails.

Regulations for underground trains prohibited the use of train power bus bars, requiring each motor coach picking up its own supply normally from shoes at each end. This means that cars can be easily gapped when negotiating complex junctions although this is normal not a problem as there are several motor coaches in each train.

Where inter running with third rail trains is required there is a long section gap (with no conductor rails) at least a motor coach in length between the systems and the supply goes from floating to one side grounded as the centre conductor rail is hard connected via high current cables to the running rails.
For the lines into Moorgate which were converted from 4 rail to 3 rail operation the old centre conductor rail was left in place but also bonded to the running rails to improve the return current path. It is the return voltage drop that is the problem with combining a high voltage AC system with low voltage DC systems. Impose more than a few volts drop on the rails and a AC train transformer nearby will be driven into saturation, draw excessive current and trip out. I presume where dual electrified sections occur they make the return path very low resistance and use very frequent sub stations. Therefore I would not see any problem in dual electrification on LU 4 rail systems with high voltage AC (although the signalling system would need considerable work to immunise for both systems as at lest one of the running rails would be needed as a return current path).

[Nym]

Originally Posted by torquewrench

If there is 3rd rail with OHLE on the same section of track, I wonder if OHLE could induce a voltage in to the third rail?

Yes.

It would work much as the way as LU does at the moment (referred to in another post) with AC distribution feeding the DC network.

Although it would be dreadfully inefficent and anoying to be using a 1 phase supply, if we had two running lines it gets better as one could use a Scott T Transformer to feed the two running lines (Changes 2 phase to 3 phase), then taking current from both of these through a reverse set up to feed into standard 3 phase rectifiers for the DC circuits, or just use 1 phase rectifiers, but they're horrid!

[es373]

Thought so.

Unless something could be brought up with earths and isolates either of the traction supplies as a train passes through the section.

Way too costly I would imagine, something for the RailWorks boys to have a play about with I think!

[Jan]

It seem that such a system is actually in use between Farringdon and City Thameslink, but it wouldn't be viable on any larger scale.

[joeykins82]

Originally Posted by apk55

... I would not see any problem in dual electrification on LU 4 rail systems with high voltage AC (although the signalling system would need considerable work to immunise for both systems as at lest one of the running rails would be needed as a return current path).

Thanks apk55, great explanation! I wonder if this might be considered as an option for the shared running sections at some point down the line? I guess it'll depend on just how "considerable" the signalling work would be.

[Kneedown]

Originally Posted by ole man

The reason that they don't share the same routes for very long is that DC current is highly corrosive to metallic structures, hence why above i said that all new OHLE masts on the NLL have plastic insulated washers.
This also applies to signal gantry's has also stated above, lineside phones if base is metallic.
This problem has existed since the beginning of using DC current, and the only solution is to have insulation between any DC current and any metallic structure that is close by.

I'm having a bit of trouble getting my head around this one.
Anyone out there with a better knowledge of physics than i enlighten me as to how dc current causes corrosion to adjacent, unconnected structures, and ac doesn't?

[Nym]

Originally Posted by Kneedown

I'm having a bit of trouble getting my head around this one.
Anyone out there with a better knowledge of physics than i enlighten me as to how dc current causes corrosion to adjacent, unconnected structures, and ac doesn't?

From what I've read of the research done at Manchester (A Floor, UMIST Main Building) it's mainly to do with a constant current path causing a type of electrolosis as the electrons all flow in one direction, as does corrosion, as there is no net current flow in AC systems this doesn't happen.

There are papers on this but you need to be a subscriber to sci-verse to be able to read them, I get access through my IET membership so I'll have a quick check to see if there are any decent titles worth *cough* referencing in a subsiquent post for you.

EDIT:

Some free stuff found via Google Scholar...

This paper has some nice diagrams and explanation...
http://www.energy-cie.ro/archives/2010/n1-1-17.pdf

And this is another slant, but more general background.
http://www.jrtr.net/jrtr16/pdf/f48_technology.pdf

A bit more searching (that I might do myself later) could show up something, the IEEE has a lot on IEEExplore, but I've let my IEEE Membership lapse so I don't have access to that any more.