Future of Third Rail

[stuu]

It is certainly most likely that the systems remain at 750V, but converison to DC overhead line is more likely than conversion to 25kV, because the chance of conversion to 25kV is zero.

How does that work? Are there any costs which are lower for DC overhead compared to AC?

 

[zwk500]

How does that work? Are there any costs which are lower for DC overhead compared to AC?

Having DC and AC systems mixed is a total nightmare due to the different current return patterns. Also Lower voltage needs less clearance.

 

[stuu]

Having DC and AC systems mixed is a total nightmare due to the different current return patterns. Also Lower voltage needs less clearance.

But why would you do that? Apart from at boundaries obviously. You couldn't just swap 750V DC for 3kV DC OHLE without doing anything else. Also most of the clearance requirement is for the mechanical movement of the wire, that far exceeds the clearance because of voltage - 25kV will arc about 30mm in clear air

 

[zwk500]

But why would you do that? Apart from at boundaries obviously. You couldn't just swap 750V DC for 3kV DC OHLE without doing anything else. Also most of the clearance requirement is for the mechanical movement of the wire, that far exceeds the clearance because of voltage - 25kV will arc about 30mm in clear air

Swapping 750v for 3KV is a relatively simple process though, because you can install equipment switchable between 750V and 3KV (it's a factor of 4) on a rolling programme and then install the OLE when you need to - see the French 1.5KV to 9KV conversion programme. Also Belgian and Italian 3KV trains regularly run onto the 1.5KV networks in Netherlands and France without a problem
Agree with you that it'd still need a lot of work and it's pretty marginal for clearance between 3KV DC and 25KV AC electrically (although the tensions are different).

 

[HSTEd]

How does that work? Are there any costs which are lower for DC overhead compared to AC?

Converting from an AC to DC electrification system is a complete mess, because the earthing arrangements are fundamentally incompatible.

In DC you want to isolate everything near the traction loop to avoid galvanic corrosion from current leakage, whereas with AC you want to earth everything to prevent dangerous touch voltages in various fault conditions.

You can't really do both.
DC to AC conversion will require a lengthy period of non-electrified operation whilst all the earthing work takes place.

You couldn't just swap 750V DC for 3kV DC OHLE without doing anything else.

You could have 750V DC third rail and 3kV DC OLE installed simultaneously if you wanted.
The switch between them could be gradual and in any case just, involves installing the OLE and turning it on when you want, and turning the third rail off when you want.

Also most of the clearance requirement is for the mechanical movement of the wire, that far exceeds the clearance because of voltage - 25kV will arc about 30mm in clear air

This isn't really true, the ENE TSI has imposed 25kV clearances far in excess of 30mm! I believe the standard minmum is now 270mm unless you want to go through an onerous risk assessment process designed primarily to prevent people making common use of it.

Also "clean air" is the best possible case for air resisting electrical discharges!

EDIT:
If you dig through this mess, it appears that (on 1520mm gauge broad gauge railways) the "normal" clearance for 1.5-4kV electrification systems between live and earthed parts of the overhead line will be 200mm, versus 350mm for 25kV. Minimum allowed is 150mm/300mm.
Obviously different for 1435mm, but I'm struggling to find the standard for standard gauge (badum tish!)

 

[A0]

Converting from an AC to DC electrification system is a complete mess, because the earthing arrangements are fundamentally incompatible.

In DC you want to isolate everything near the traction loop to avoid galvanic corrosion from current leakage, whereas with AC you want to earth everything to prevent dangerous touch voltages in various fault conditions.

You can't really do both.
DC to AC conversion will require a lengthy period of non-electrified operation whilst all the earthing work takes place.

Serious question - how did it work in the Manchester area where there was 1,500v DC OHL and 25kv OHL in close proximity ?

And I assume the conversion between DC OHL and AC OHL isn't as significant, because I seem to recall the conversion of the Glossop line, for example, was done reasonably quickly.

 

[stuu]

This isn't really true, the ENE TSI has imposed 25kV clearances far in excess of 30mm! I believe the standard minmum is now 270mm unless you want to go through an onerous risk assessment process designed primarily to prevent people making common use of it.

Also "clean air" is the best possible case for air resisting electrical discharges!

EDIT:
If you dig through this mess, it appears that (on 1520mm gauge broad gauge railways) the "normal" clearance for 1.5-4kV electrification systems between live and earthed parts of the overhead line will be 200mm, versus 350mm for 25kV. Minimum allowed is 150mm/300mm.
Obviously different for 1435mm, but I'm struggling to find the standard for standard gauge (badum tish!)

Electrical clearance and mechanical clearance are not the same thing - mechanical clearance (wires vibrating up and down) is much larger than that needed to avoid arcs.

I'm struggling to think of a reason why railway gauge would have any impact on electrical clearance.

 

[AM9]

Electrical clearance and mechanical clearance are not the same thing - mechanical clearance (wires vibrating up and down) is much larger than that needed to avoid arcs.

Agreed and doscriminating between the two is even more relevant in the matter of limited clearance overbridges where common practice is to anchor OLE positions with supports actally mounted on the bridge itself, rather than supporting wiring independent of structures. Thus the envelope of high voltage conductor movement is very small so the air gap becomes the most significant issue.

I'm struggling to think of a reason why railway gauge would have any impact on electrical clearance.

Apart from Finland, all the other countries where 1520mm is still predominant were part of the old Soviet sphere. I doubt that those nations are concerned about refining the requirements for minimum OLE clearances. I'm surprised that @HSTEd went to the bother of noting that without recognising which railways would be affected.

 

[zwk500]

FWIW, belgium recently converted 66km of a 3KV DC OLE line to 25KV AC OLE with a LOT of pre-work, some nifty new equipment design and a 3 week closure:

A major Belgian rail link re-electrified using a technology never seen before in Europe

In just 3 weeks of traffic interruption, a 66km section of the "Brussels-Luxembourg" rail link will be switched from 3kV DC to 25kV AC...

www.railway-international.com

I'd love to see the reaction of the good folk of Surrey and Hampshire if you closed the main line to London for 3 weeks!

 

[AM9]

FWIW, belgium recently converted 66km of a 3KV DC OLE line to 25KV AC OLE with a LOT of pre-work, some nifty new equipment design and a 3 week closure:

A major Belgian rail link re-electrified using a technology never seen before in Europe

In just 3 weeks of traffic interruption, a 66km section of the "Brussels-Luxembourg" rail link will be switched from 3kV DC to 25kV AC...

www.railway-international.com

I'd love to see the reaction of the good folk of Surrey and Hampshire if you closed the main line to London for 3 weeks!

Well they've managed to close most of Waterloo today!

More seriously, cvonverting from medium voltage (ac or DC) to high voltage ac is easy to prepare for, - I remember when the 6.25kV on the GEML was converted to 25kV, the prep was mainly im[proving a few clearance on overbridges and replacing what were the original 1500VDC insulators with new 25kV types. I think at the chageover, the feed points that were to be kept were upgraded (in power capability) and after switchover, some redundant ones were dismantled.

 

[A0]

FWIW, belgium recently converted 66km of a 3KV DC OLE line to 25KV AC OLE with a LOT of pre-work, some nifty new equipment design and a 3 week closure:

A major Belgian rail link re-electrified using a technology never seen before in Europe

In just 3 weeks of traffic interruption, a 66km section of the "Brussels-Luxembourg" rail link will be switched from 3kV DC to 25kV AC...

www.railway-international.com

I'd love to see the reaction of the good folk of Surrey and Hampshire if you closed the main line to London for 3 weeks!

Though I guess the fact they were replacing the catenary (which the article says was life expired) probably meant a 3 week closure was the easier way to do it.

I recognise that things have changed a bit in the last 50 years but BR switched the MSJAR in a much shorter time - the last 1500 v train running on 30th April 1971 and the first 25kv one on 3rd May 1971 - no doubt there had been other works taking place in the run up to that switch over but it does suggest if it's a change from one system to another both using OHL it can be done more quickly.

I'm sure a move from 3rd rail DC to OHL AC wouldn't be as simple.

Perhaps the answer is to move from 750 v DC 3rd rail to 1,500 or 3,000 V DC OHL as an interim step to then move to 25kv AC....... (yes, I am joking).

== Doublepost prevention - post automatically merged: 13 Apr 2023 ==

Well they've managed to close most of Waterloo today!

That is a signalling failure - not exactly a planned outage.

 

[AM9]

That is a signalling failure - not exactly a planned outage.

That was in jest.

 

[plugwash]

A way to improve safety might be for someone to create a design for bottom-contact third rail as well as the accompanying shoe, that is fully compatible with top-contact third rail.
That way you could replace top-contact third rail as it is life expired, and use the safer bottom-contact third rail for sensible extensions (such as the third rail diesel islands)

Now of course, making such a design that is both practical to use and safe is likely to be much harder than I made it sound above!

I think the big issue would be the transitions of the shoe on and off the rail for pointwork, level crossings etc. A top-contact shoe runs up a ramp onto the rail, while a bottom contact shoe runs down a ramp onto the rail.

 

[zwk500]

Though I guess the fact they were replacing the catenary (which the article says was life expired) probably meant a 3 week closure was the easier way to do it.

I recognise that things have changed a bit in the last 50 years but BR switched the MSJAR in a much shorter time - the last 1500 v train running on 30th April 1971 and the first 25kv one on 3rd May 1971 - no doubt there had been other works taking place in the run up to that switch over but it does suggest if it's a change from one system to another both using OHL it can be done more quickly.

I'm sure a move from 3rd rail DC to OHL AC wouldn't be as simple.

If anything should 3rd rail to OLE be easier as you don't have to de-energise the 3rd rail until the OLE is ready?

Interestingly 66km is 41miles, which is the same distance as Basingstoke to Southampton, and it's approximately another 40 miles to Weymouth.

 

[A0]

If anything should 3rd rail to OLE be easier as you don't have to de-energise the 3rd rail until the OLE is ready?

Interestingly 66km is 41miles, which is the same distance as Basingstoke to Southampton, and it's approximately another 40 miles to Weymouth.

Surely the challenge would be 3rd rail DC to OHL AC because of the changes to earthing etc ?

3rd rail DC to OHL DC ought to be fairly straightforward as things like the signalling earthing would presumably need minimal changes ?

 

[zwk500]

Surely the challenge would be 3rd rail DC to OHL AC because of the changes to earthing etc ?

3rd rail DC to OHL DC ought to be fairly straightforward as things like the signalling earthing would presumably need minimal changes ?

Yes but I don't see why 3rd rail DC to OHL AC is any more difficult than OHL DC to OHL AC.

 

[plugwash]

Surely the challenge would be 3rd rail DC to OHL AC because of the changes to earthing etc ?

I presume they would just have to live with sub-optimal earthing during the transition, just as they live with it on areas that run both systems.

 

[zwk500]

I presume they would just have to live with sub-optimal earthing during the transition, just as they live with it on areas that run both systems.

Or fit 80km capable batteries and convert 60km at a time by removing all the 3rd rail and DC then installing AC OLE in night shifts and weekends

 

[HSTEd]

Apart from Finland, all the other countries where 1520mm is still predominant were part of the old Soviet sphere. I doubt that those nations are concerned about refining the requirements for minimum OLE clearances. I'm surprised that @HSTEd went to the bother of noting that without recognising which railways would be affected.

Because I don't have the chart for standard gauge, so I gave the 1520mm one to give an idea of the values in question!

 

[Trainbike46]

Or fit 80km capable batteries and convert 60km at a time by removing all the 3rd rail and DC then installing AC OLE in night shifts and weekends

still seems like a lot of expense for what, on most routes, wouldn't be that many benefits

Then again, for routes with heavy freight traffic it is maybe worth it (electric spine 2.0 anyone?)

 

[zwk500]

still seems like a lot of expense for what, on most routes, wouldn't be that many benefits

Then again, for routes with heavy freight traffic it is maybe worth it (electric spine 2.0 anyone?)

Oh I agree.

 

[HSTEd]

I presume they would just have to live with sub-optimal earthing during the transition, just as they live with it on areas that run both systems.

Given the toughening of the stance of the railway inspectorate on third rail and on reduced 25kV clearances, they are not going to tolerate anything other than fully specced 25kV compatible earthing on a 25kV installation, even temporarily.

And fully specced 25kV earthing being in place for months is going to cause the mother of all stray current corrosion issues.

You'll be lucky if you don't destroy the wiring gantries before the switch over even occurs!

In areas with both systems they either have (expensive) switches on the earthing points and use only one system at a time, or the two systems are adjacent railways that are kept electrically separate as much as possible.

== Doublepost prevention - post automatically merged: 13 Apr 2023 ==

Yes but I don't see why 3rd rail DC to OHL AC is any more difficult than OHL DC to OHL AC.

In the modern environment, as in Belgium, it will require weeks to months of closure.

And that line has been being prepared for 25kV conversion for years

 

[Ken H]

Given the toughening of the stance of the railway inspectorate on third rail and on reduced 25kV clearances, they are not going to tolerate anything other than fully specced 25kV compatible earthing on a 25kV installation, even temporarily.

And fully specced 25kV earthing being in place for months is going to cause the mother of all stray current corrosion issues.

You'll be lucky if you don't destroy the wiring gantries before the switch over even occurs!

In areas with both systems they either have (expensive) switches on the earthing points and use only one system at a time, or the two systems are adjacent railways that are kept electrically separate as much as possible.

== Doublepost prevention - post automatically merged: 13 Apr 2023 ==


In the modern environment, as in Belgium, it will require weeks to months of closure.

And that line has been being prepared for 25kV conversion for years

How is this managed at places like Mitre Bridge and where the North London Line changes between 25Kv and DC (Willesden isnt it?), and Drayton Park, and on Thameslink?

 

[stuu]

Because I don't have the chart for standard gauge, so I gave the 1520mm one to give an idea of the values in question!

Why does rail gauge matter? How does the width of the track have any bearing on the electrical clearances?

 

[HSTEd]

Why does rail gauge matter? How does the width of the track have any bearing on the electrical clearances?

The setting of TSI clearences is a political matter, and for political reasons track gauge does matter.

A political decision was made by the ORR and DfT to impose compliance with continental standards. Continental standards are defined by a political process.

 

[AM9]

The setting of TSI clearences is a political matter, and for political reasons track gauge does matter.

A political decision was made by the ORR and DfT to impose compliance with continental standards. Continental standards are defined by a political process.

I suspect the reason why nothing has been done to change 1520mm electrical clearances whereas 1435mm electrical clearances have been upgraded clearances is more to do with the countries involved. For example, most advanced countries on the world have standard gauge track on their main railway network. Their administrations are generally receptive of safety recommendations on their modern railways. In my post #98 above I alluded to the countries that have 1520mm as their principal system's gauge, and apart from Finland, and Mongolia, those countries are all ex-members of the USSR, and as Russia had an extensive railway system in 1520mm gauge, their puppet states were 'encouraged' to follow their practice. Those countries are:
Armenia, Azerbaijan, Belarus, Estonia, Georgia, Kazakhstan, Kyrgyzstan, Latvia, Lithuania, moldova, Mongolia, Tajikistan, Turkmenistan, Ukraine and Uzbekistan. I would imagine that if there's no domestic drive to improve their railways, - there's also little interest in agreeing a tightening of electrical clearances for safety reasons.

 

[61653 HTAFC]

The setting of TSI clearences is a political matter, and for political reasons track gauge does matter.

A political decision was made by the ORR and DfT to impose compliance with continental standards. Continental standards are defined by a political process.

That's not answering the question asked though. Surely the electricity doesn’t know or care how far apart the running rails are.
The clearance standards used in a country that uses a different track gauge might well be different, but not because the track gauge is different.

 

[HSTEd]

The clearance standards used in a country that uses a different track gauge might well be different, but not because the track gauge is different.

The negotiation processes that produced the standards included a carve-out by track gauge, thus reducing the political cost necessary to obtain the agreement from all parties necessary to introduce the standard.

Not everyone in Europe is as keen on harmonisation of these standards as the ORR and DfT. It is likely that this was necessary to get the likes of Estonia and/or Finland to come aboard and agree. The track gauge is just a tool used to differentiate railway systems in different places without appearing to do so.

 

[zwk500]

That's not answering the question asked though. Surely the electricity doesn’t know or care how far apart the running rails are.
The clearance standards used in a country that uses a different track gauge might well be different, but not because the track gauge is different.

A slightly wider gauge would need slightly bigger gaps from the centreline though. AFAICT the table @HSTEd pointed to was in the section talking about pantograph clearances so I don't know if that's where any variation comes from.

 

[RobShipway]

If you where going to change from 3rd rail to AC OHLE eventually, could it be done in stages over 10 - 15 years as follows:

1) Install DC OHLE
2) Leave DC OHLR running for 5 years
3) Replace DC OHLE with AC OHLE

The issue I see that, is as I have stated before you will need a locomotive or train to be able to operate AC OHLE, DC OHLE and third rail unless you use as others have stated Electric/diesel engine or train that can be later converted to be electric/battery.