Adding a 4th rail to southern region 3rd rail network to give an effective voltage of 1500V

[ap2048]

I have been reviewing posts re 3rd rail southern region in the UK. I know that power supply limitations mean that trains are often derated eg class 444) and also limit the number of units used at any one time. I was wondering if a second rail in the middle of the tracks could be added at -750V to give a total effective voltage of 1500V similar to what they do on the underground (albeit at 750V)? Is this theoretically possible and if so what are the principle reasons that it hasn't been done? Is it because it is uneconomical/too costly? Thanks

 

[Lockwood]

Wouldn't that still need the power stations to be upgraded to supply double the voltage?

You would either need to upgrade the power supply, or upgrade the power supply, install miles of additional rail and retrofit all the existing stock to use the higher voltage and return current via the 4th rail instead of the running rails

And probably have to tweak some of the signalling as well?

 

[ap2048]

That is what I thought but is it something that could be done gradually say when life expired equipment is replaced? The reason I say this is that it is highly unlikely that the 3rd rail system will be upgraded to OLE anytime soon so this may be an alternative way of providing more power to longer and more power hungry trains.

 

[Watershed]

Safety is already a significant concern with 750V third rail, due to the risk of staff, passengers or trespassers receiving an electrical shock from the unshielded conductor rail. At 750V, a shock would be likely to cause serious but not lethal injuries. At 1500V, the likelihood of fatal injuries is already considerably greater.

For a given voltage, having a 4-rail system is not much safer than 3 rails, as you can still easily bridge the gap between the highest and lowest potentials by tripping and landing across the two conductor rails, for example. Having a fourth rail also makes it a lot easier for such a trip to occur, as the gap for your feet in the foot-foot is much smaller.

The ORR is already very cautious about 750V third rail - it is only legal because it is "grandfathered in" - so there is no way they would entertain any modification causing an increase in risk.

You could only really do it by introducing shielding such that the underside of the suspended, covered rail is live - as is done on many metro systems around the world, and for example on the DLR. Such an upgrade would be highly disruptive, and would never deliver acceptable cost:benefit ratios (bearing in mind the rather minor performance improvements that it would enable). Far better to switch to overhead electrification, if at all.

 

[GRALISTAIR]

Safety is already a significant concern with 750V third rail,

For a given voltage, having a 4-rail system is not much safer than 3 rails, as you can still easily bridge the gap between the highest and lowest potentials by tripping and landing across the two conductor rails, for example. Having a fourth rail also makes it a lot easier for such a trip to occur, as the gap for your feet in the foot-foot is much smaller.

The ORR is already very cautious about 750V third rail - it is only legal because it is "grandfathered in" - so there is no way they would entertain any modification causing an increase in risk.

Agreed. Not going to happen - would never be sanctioned.

 

[Dunfanaghy Rd]

Safety is already a significant concern with 750V third rail, due to the risk of staff, passengers or trespassers receiving an electrical shock from the unshielded conductor rail. At 750V, a shock would be likely to cause serious but not lethal injuries. At 1500V, the likelihood of fatal injuries is already considerably greater.

For a given voltage, having a 4-rail system is not much safer than 3 rails, as you can still easily bridge the gap between the highest and lowest potentials by tripping and landing across the two conductor rails, for example. Having a fourth rail also makes it a lot easier for such a trip to occur, as the gap for your feet in the foot-foot is much smaller.

The ORR is already very cautious about 750V third rail - it is only legal because it is "grandfathered in" - so there is no way they would entertain any modification causing an increase in risk.

You could only really do it by introducing shielding such that the underside of the suspended, covered rail is live - as is done on many metro systems around the world, and for example on the DLR. Such an upgrade would be highly disruptive, and would never deliver acceptable cost:benefit ratios (bearing in mind the rather minor performance improvements that it would enable). Far better to switch to overhead electrification, if at all.

Spot on. I'm still amazed (in a sad sort of way) by the people who still won't stop to think of the bloke working alongside this stuff.
Pat

 

[aleggatta]

Worth noting that electrostars DC equipment is generally rated up to 1000V, 1200V if you’re lucky, but the costs of uprating all the rolling stock high voltage distribution equipment would probably make it unviable also

 

[TSG]

As already mentioned, the likely reaction of the ORR/HSE would be "Go away you maniac". Apart from that, it's difficult to see how once you've developed it, converted all the trains, installed all the extra substation equipment, installed the bonding and rails, redesigned and modified a LOT of the signalling it would be cheaper or better than converting to the standard system of OHLE. Let's not even go there with transition strategy....

 

[Snow1964]

Isn’t part of the Southern region already at more than 750volts.
I’m sure I read that some of the newer substations feed at nearer 850v

The de-rated trains are more to do with cable and transformer sizes limiting the current, or as it’s approx 750v (a constant) the total wattage. Unrestricted some can draw nearer 2MW, which is ok for a single unit, but hefty when 3 units are coupled

 

[Jozhua]

Honestly the easiest interim solution to OLE seems like making sure to order lighter trains! Get some more composite materials in there, so crash-worthyness isn't compromised.

But yeah, like others have said, 3rd rail voltage isn't an incredibly fixed thing, so there's some tolerances in there. Upping the voltage to its higher band can deliver a bit more power.

Possibly another solution would be capacitors/batteries, just to give trains a boost out of stations. Then they can recharge again from regen braking/power supply in coast.

Ultimately though, a rolling OLE programme is needed. Unfortunately, there is a lot of work to do bringing electric traction power to much of the network to reduce carbon emissions and speed up journeys. The south east 3rd rail network will need to be updated at some point though. Impressive that they have done so much with this technology though, ideally they would have OLE now, but investment in this country is terrible so...

 

[Juniper Driver]

Some fo the 444's are working at 100% now...Got on one the other day...

Weymouth to Bournemouth third rail is a bit underpowered but is not really needed to be upgraded...Plus they are adding Substations in places....Noticed at Feltham and Virginia Water today...Sure there are others...

Wouldn't 4th rail be the retun or do you mean another rail on the outside????

 

[swt_passenger]

Isn’t part of the Southern region already at more than 750volts.
I’m sure I read that some of the newer substations feed at nearer 850v

The de-rated trains are more to do with cable and transformer sizes limiting the current, or as it’s approx 750v (a constant) the total wattage. Unrestricted some can draw nearer 2MW, which is ok for a single unit, but hefty when 3 units are coupled

In a discussion back in 2014 it was said there is definitely no part of the SR network that operates at 850 volts nominal. However that doesn’t mean the voltage cannot reach 850 V under no load or regeneration conditions...

 

[grumpyoldman01]

From memory, the former Southern Railway's third-rail network was electrified at 660v but it was raised to 750v in the early 1950s; some lines over which LT trains also ran were left at 660v. However, when the Bournemouth line was electrified, 850v as the line voltage was usually stated in railway publications at the time.

When lines like Bournemouth - Weymouth were electrified, the power supply was designed to cope with the trains service envisaged to justify the project; obviously, there would be some "spare", but it wouldn't be able to cope with major service improvements west of Poole.

There have been higher third rail voltages in the UK - the Manchester - Bury was 1200v - and the Maurienne line in France was at 1500v; these were protected by wooden boards.

 

[furnessvale]

There have been higher third rail voltages in the UK - the Manchester - Bury was 1200v - and the Maurienne line in France was at 1500v; these were protected by wooden boards.

In wet weather we treated the wooden boards on the Bury line with caution, never mind the rails!

 

[GRALISTAIR]

Indeed at very high voltages even the breakdown voltage of air has to be considered and wet air- enough said. So wet wood ........

 

[Journeyman]

How to make a complex and dangerous system even more complex and dangerous.

 

[contrex]

Before the Grouping the South Eastern & Chatham Railway looked at 1500v DC electrification using two outside conductor rails at +750v and -750v but the post-1922 Southern Railway went for the LSWR three-rail system. The Great Northern and City line was originally done with 2 outside conductor rails, I believe the original voltage difference was 575V but I don't know what the + and - split was.