Change third rail to bottom contact

[EM2]

How difficult would it be?
A new design of ceramic pot (or at least bracket) and new shoes for the stock. Got to be more economical than stripping it all out and going to OHLE?

 

[IanXC]

How difficult would it be?
A new design of ceramic pot (or at least bracket) and new shoes for the stock. Got to be more economical than stripping it all out and going to OHLE?

The driving issue for conversion to OHLE seems to be the cost of renewing life expired equipment, rather than the reliability issues of top contact. With that in mind I don't see why bottom contact would be on the agenda.

 

[asylumxl]

The driving issue for conversion to OHLE seems to be the cost of renewing life expired equipment, rather than the reliability issues of top contact. With that in mind I don't see why bottom contact would be on the agenda.

Probably as it's considered safer than top contact.

In reply to the original poster, perhaps it would be good to look at former overground services which were converted to DLR, seeing as such a conversion was done there.

 

[30907]

In reply to the original poster, perhaps it would be good to look at former overground services which were converted to DLR, seeing as such a conversion was done there.

Meaning bits of Stratford-North Woolwich? IIRC there was a lengthy closure involved (though that wasn't only for the change of power supply), not to mention new trains. At least with OHLE conversions you can continue to use 3rd rail up to changeover date.

And isn't one reason for changing over that 25kV is more energy-efficient than 750V (my Physics O-level is so rusty I can't attempt to explain).

 

[JGR]

How difficult would it be?
A new design of ceramic pot (or at least bracket) and new shoes for the stock. Got to be more economical than stripping it all out and going to OHLE?

You'd still have higher costs than OHLE due to the need to run the system at a low voltage, which implies the need for more substations, each of which needs more kit in it.
If you're going to replace all the contact rails and all the rolling stock shoe gear you might as well replace it with OHLE.

Also, how could you do a rolling replacement of top-contact with bottom-contact rails/rolling stock? Or do a bi-mode type switch-over between the two?
It's be a nightmare to implement it on anything other than an entirely segregated route such as the DLR.

 

[jopsuk]

How difficult would it be?
A new design of ceramic pot (or at least bracket) and new shoes for the stock. Got to be more economical than stripping it all out and going to OHLE?

The rails used are usually completely different. You couldn't have stock with both sorts of shoe. You'd gain none of the long term advantages of AC.

As for the DLR conversion, the entire line was ripped up and replaced.

 

[455driver]

How difficult would it be?

Extremely!
What do you do during the transition, you cannot have both types of shoe on a train.

A new design of ceramic pot (or at least bracket) and new shoes for the stock. Got to be more economical than stripping it all out and going to OHLE?

The whole 'economic' point is that a 750DC system needs a substation every 3 miles (or less in busy areas) whereas a 25KV overhead system needs a substation every 30 miles or so, high voltage AC is also more economical to 'transport' (hence the national grid being at 115KV) but a lot of losses occur low voltage DC systems. That is where the saving is, not on the pickup method.

Shall I mention what happens to the water that will naturally congregate on the bottom of the rail when the temperature drops below zero degrees?

 

[edwin_m]

The bottom contact shoes have to project outwards from the bogies to fit underneath the conductor rails, so would be outside the loading gauge. Hence various existing platforms, structures, signals etc would need modifying to make room for the shoes.

 

[HSTEd]

Extremely!
What do you do during the transition, you cannot have both types of shoe on a train.

Does modern stock not have retractable shoes now?
Although changeover would be fun.

 

[455driver]

No they dont, neither does any of the old stock except dual voltage stock.

 

[59CosG95]

And isn't one reason for changing over that 25kV is more energy-efficient than 750V (my Physics O-level is so rusty I can't attempt to explain).

It IS more energy efficient because the high voltage (25kV) has a low, constantly changing current at AC, so less energy is lost as heat through the cables. 750V DC has a higher current, and so energy loss as heat from the 3rd rail is greater. Also because they are higher up and have a smaller surface area, OHLE cables are less susceptible to ice and snow, whereas 3rd rail...just take a crowded SWT service to Waterloo in the snow to find out . This knowledge comes from being in the midst of A2 Physics, with transformers looming on the horizon...

 

[ianhr]

Some of us will remember the ex-L&YR electrification on the Manchester-Bury line (i.e. before Metrolink). This was unusual for the UK in using SIDE contact 3rd rail at 1200V DC. The conductor rail was enclosed by wooden shuttering, presumably for safety reasons, and 1200V was the highest voltage that the Board of Trade would allow at the time (1913) for 3rd rail installations.This had some advantages over the 'London Standard' system with 600-750V DC top contact but still less electrically efficient than 25kV AC overhead. The higher the voltage the lower the amps and so at higher voltages cables, switchgear etc can be lighter. Bottom contact or protected top contact 3rd rail does seem to be more common in other countries e.g. France (although I think most systems there have been converted to overhead) and North and South America, in the latter case British firms installed most of it.

 

[swt_passenger]

The manufacturer of the DLR style bottom contact reckons its max speed is about 30 mph.
Apart from the practicalities of trying to change over piecemeal, it basically could not work at mainline speeds.

 

[jimm]

It's rather more complicated than just a new ceramic pot. This picture of the Oslo metro gives a pretty good idea of what kind of set-up you need to support bottom contact conductor rails. What you can see is actually a continuous cover, for safety and to guard against snow, with the conductor rail inside.

http://www.urbanrail.net/eu/no/oslo/Oslo_017.jpg

The Hamburg S-bahn has side-contact 1,200-volt DC with more modern safety covers than the wooden boarding described above on the Manchester-Bury line but again rather more complex than the Southern Railway-style arrangement.

http://www.railway-technology.com/projects/hamburg-s-bahn/hamburg-s-bahn2.html

 

[HSTEd]

They do use bottom contact rules in North America at mainline speeds however.
But OLE is probably the better bet.

 

[IKBrunel]

Current = Power/Voltage

Say you have a 3 car electrostar drawing about 750kW
at 750V thats a current of ~1000 Amps
at 25kV thats current of 30 Amps

So for the same train running at the same speed/acceleration on 750V you need approximately 30 times as much conductor cross sectional area as you would for 25kV.

If the conductor rail is steel then you need even more cross section, as it has a higher resistivity than copper.

ok you have the extra cost of catenary supports, but combine this with the extra number of substations needed @ 750V...

I think also the high current draw places significant constraints on the size and number of trains that can be operated on a given section, restricting available paths south of London

Basically new third rail kit doesn't make sense other than for light rail systems with lower power requirements.

 

[edwin_m]

Current = Power/Voltage

Say you have a 3 car electrostar drawing about 750kW
at 750V thats a current of ~1000 Amps
at 25kV thats current of 30 Amps

So for the same train running at the same speed/acceleration on 750V you need approximately 30 times as much conductor cross sectional area as you would for 25kV.

If the conductor rail is steel then you need even more cross section, as it has a higher resistivity than copper.

ok you have the extra cost of catenary supports, but combine this with the extra number of substations needed @ 750V...

I think also the high current draw places significant constraints on the size and number of trains that can be operated on a given section, restricting available paths south of London

Basically new third rail kit doesn't make sense other than for light rail systems with lower power requirements.

It's actually worse than that because power lost in a conductor is the SQUARE of the current multiplied by the resistance. So even though the conductor rail is much fatter than an overhead wire it is still losing more power as heat over the same length. The lost power means that the voltage at the train is much less than line voltage when it is a long way from a substation.

Voltage drop in turn makes regenerative braking less efficient. It also means that short circuit currents are not much more than the normal operating current, which puts a limit on the maximum spacing of substations to ensure that power will still be shut off reliably in the event of a short.

While a bottom contact third rail may be a better conductor it has a smaller cross section than a top contact third rail, so it might actually make all these problems worse. In engineering terms the third rail is fine for DLR-type systems but a bit of a pig's ear for main line railways, and if there is money available to improve things then OLE conversion seems the best idea.

 

[Murph]

The thing I have to ask about this is: What problem does it solve?

It's pretty clear that there would be quite significant disruption and expense involved in converting from one type of 3rd rail to another. I'd guess there would be years where not all units could run on all lines (ignoring the current AC vs DC situation, as this would not change that), individual lines taken out of service for extended periods, inevitable teething troubles after each conversion giving intermittent issues for months, etc.

So, what is the perceived benefit gained from going through all that disruption? Safety? Bottom contact 3rd rail is still quite dangerous, needing to be treated with much the same care, and the rails would likely be much higher and therefore a bigger trip hazard. Just how many serious incidents would it prevent? I'm guessing that the number of incidents caused by the current 3rd rail setup is so small that it's really not worth the disruption.

I can see reasonable justification for wanting to convert some lines to OHLE in the medium to long term, but I just can't honestly see any justification for changing the style of 3rd rail.

As far as snow and ice goes, there are solutions to deal with that on the current infrastructure, we just need to make sure that they are available and used.

 

[gordonthemoron]

Berlin S-bahn uses bottom contact third rail and has a top speed of about 80 km/h I think

 

[jopsuk]

as an aside, an oddity is that with AC going (substantially) thicker doesn't, at high voltages, result in lower resitance. I don't quite understand it, but I think the layman explanation is that with AC the current is in the "skin" of the metal. So for very high voltage long distance you get less voltage drop with DC.

 

[Chris125]

The RSSB conducted a study into the issue of 3rd rail electrification few years back - this included looking at bottom-contact (page 26), and came to the following conclusion:

While the winter performance advantages are recognised, the
practicalities of installing a bottom contact conductor rail system
on a gauge constrained railway will be challenging and appears
impractical with standard electrification equipment. In addition, to
this there are concerns about the bottom contact system working
at line speeds above 60mph and hence this proposal was not
developed further.

Chris

 

[NSEFAN]

as an aside, an oddity is that with AC going (substantially) thicker doesn't, at high voltages, result in lower resitance. I don't quite understand it, but I think the layman explanation is that with AC the current is in the "skin" of the metal. So for very high voltage long distance you get less voltage drop with DC.

It's called the skin effect, and you are right about the current being in the skin of the metal. At AC the current can only pass within a certain depth into the material, so you have to increase the surface area of the conductor in order to reduce the resistance.

Doing some rough calculations for electrical steel at 50Hz, the "skin depth" is only about 5mm.

 

[HSTEd]

This is why the Furrer and Frey (and probably other manufacturers) make the 25kV aluminium contact rails hollow, since the metal in the middle doesn't really improve the conduction of electricity but still costs the same as all the other metal.

 

[IKBrunel]

Also one reason why HVDC is increasingly being used for long distance electricity transmission. The other reason is to decouple national frequency control. The new UK-France interconnect in the chunnel would be HVDC. Germany are planning new north-south HVDC lines, probably on pylons most of the way. I think safety is more challenging with DC though, as its more prone to arcing for a given voltage.

 

[JGR]

as an aside, an oddity is that with AC going (substantially) thicker doesn't, at high voltages, result in lower resitance. I don't quite understand it, but I think the layman explanation is that with AC the current is in the "skin" of the metal. So for very high voltage long distance you get less voltage drop with DC.

This is true, though the effect is not very pronounced at low frequencies such as 50Hz used in mains, OHLE, etc. In most cases the cost/losses of a DC inverter at the far end outweigh the savings.

Also one reason why HVDC is increasingly being used for long distance electricity transmission. The other reason is to decouple national frequency control. The new UK-France interconnect in the chunnel would be HVDC. Germany are planning new north-south HVDC lines, probably on pylons most of the way. I think safety is more challenging with DC though, as its more prone to arcing for a given voltage.

If memory serves, DC isn't any more prone to arcing than AC, but when an arc/short does start, it can be more difficult to cut it off due to the lack of zero crossings (though this depends on a number of things). For a given RMS voltage, AC is generally more prone to arcing as the peak voltage is higher.
Grid synchronisation is a non-trivial issue. Once a grid starts to reach a certain geographic size (around a wavelength at grid frequency, if I recall) you start to get issues with standing waves, etc. which encourage the use of separate AC grids connected by HVDC or similar links.

 

[jopsuk]

One wavelength is just shy of 6000km for 50Hz.

AC has the advantage of being "easy" to step up and down the voltage compared to DC

 

[edwin_m]

One wavelength is just shy of 6000km for 50Hz.

Is that in vacuum? From what I remember it would be less in copper.

 

[JGR]

Is that in vacuum? From what I remember it would be less in copper.

It would be about the same in a free-standing copper wire (ie. not insulated, in air).
It'd decrease if the permittivity or permeability were to go up, which it would do (fairly dramatically) in say a ferrous conductor or an insulated/underground cable.
At any rate, a grid is not going to be laid out as a straight transmission line from one end to the other.

(I'm having to go back to my uni notes and data book now. I've been meaning to refresh my memory on all that stuff for a while now )

 

[HSTEd]

You would probably be more worried about a half or quarter wavelength than a full wavelength.
But any significantly large fraction of a wavelength is going to cause some.... interesting problems.

 

[L&Y Robert]

Was a centre live rail ever used anywhere (like my old Hornby Dublo!)? I've often wondered why third rail electrification was always 'outside third', never 'centre third'. Of course I know about "Outside third, centre fourth" on parts of the Underground, and it crossed my mind that centre fourth seems to work OK there, so why not 'centre third' as the live rail?