NLL third rail to OHLE electrification

[Joseph T]

How come the North and West London Line's are OHLE from South Acton and Shepherd's Bush onwards despite the NLL being third rail throughout prior to becoming part of the Overground?

 

[RJ]

How come the North and West London Line's are OHLE from South Acton and Shepherd's Bush onwards despite the NLL being third rail throughout prior to becoming part of the Overground?

It's also an important freight route which has had OHLE for years. Not all electric locomotives have third rail capabilities.

The NLL has dual voltage EMUs as parts of it don't have a third rail.

 

[swt_passenger]

Even before the Overground took off the Silverlink Metro 313s switched to OHLE where it was already available, IIRC because the DC system was on its last legs and current draw was restricted. Then when the TfL sponsored NLL four tracking extension was being planned, and the diversion into Stratford High Level was going to use OHLE anyway, and I think at one stage there was also a presumption that Eurostar would continue to use North Pole depot, so with all things taken together it made sense to standardise on OHLE on the majority of the NLL.

 

[randyrippley]

Essentially much of the NLL was converted to OHLE to allow electrified container trains to run from the WCML via Willesden to the East Anglian ports without needing to change locomotives. Previously the NLL transit needed a diesel

 

[LNW-GW Joint]

The remaining oddity is why 3rd rail continues into Euston despite all tracks being wired.
It must make life very complicated on that final mile with mixed electrical systems.

 

[etr221]

My thoughts are (1) into Euston is dual system, and (since the WCML ac electrification in the 1960s) everything is set up for that - unless/until major changes (to what?) come along that will easier if single system, is there much/any benefit in changing? and (2) there is still objection (or at least relectance) to trains changing system on the move - which makes extending the overhead to (at least) South Hampstead desirable, if not necessary. Are clearances in the New (dc) Line Tunnels such to make this easy, or is it in the difficult category? If the later, is there a suitable space/length for on the move changing at the top of Camden bank? How do these difficulties, of facilitating trains changing system, compare to the benefit of making Euston single system? I would guess they exceed it - at least until there is change in propect to alter the balance....

 

[Irascible]

Even before the Overground took off the Silverlink Metro 313s switched to OHLE where it was already available, IIRC because the DC system was on its last legs and current draw was restricted. Then when the TfL sponsored NLL four tracking extension was being planned, and the diversion into Stratford High Level was going to use OHLE anyway, and I think at one stage there was also a presumption that Eurostar would continue to use North Pole depot, so with all things taken together it made sense to standardise on OHLE on the majority of the NLL.

IIRC ( it's been a long time since I took the NLL regularily & that was before Silverlink! ) the OHLE was turned off on the weekends, just for some extra complication. Fairly sure none of the platforms the freight line west of Dalston ran through were open back then, but it's been a long time.

Living in Dalston got me stuck on a fair number of trains failing to switch input system. Oddly don't remember any at Drayton Park...

 

[Sad Sprinter]

The remaining oddity is why 3rd rail continues into Euston despite all tracks being wired.
It must make life very complicated on that final mile with mixed electrical systems.

I thought it was converted for Regional Eurostar?

 

[janahan]

I thought it was converted for Regional Eurostar?

Not sure if you miss quoted, but I am certain that regional eurostar never planned to come into Euston!

 

[stuu]

The remaining oddity is why 3rd rail continues into Euston despite all tracks being wired.
It must make life very complicated on that final mile with mixed electrical systems.

Presumably because of the preference for doing it whilst stopped, which would mean stopping on the slows next to carriage sidings which would affect timekeeping

 

[MarkyT]

ISTR there were some lengthy stretches of dual electrification when the OHLE was first installed for freight on the NLL. This is undesirable beyond necessary short sections for changeover as there's a conflict in the earthing standards between the two systems. In DC only areas, the rails are not earthed, to encourage the large return current to stay in the rails en route back to the substation instead of taking all kinds of alternative paths through the ground and any metallic objects in the vicinity, which are at risk of damage by a form of corrosion caused by the current. By contrast, in AC areas, the traction return rail is earthed and bonded to every metallic object nearby, including electrification stanchions, signal gantries and bridges for safety with the high voltage used and to avoid the risk of dangerously high 'touch potential' that staff and passengers might otherwise be exposed to.

In a dual electrified area, the safety concerns must take precedence, so comprehensive bonding is provided, giving massive opportunity for the DC current to stray and cause damage to structural metalwork and underground utilities. The AC return current is much less of a problem if it strays as there isn't the same corrosion problem with AC and the current is so much smaller due to the high supply voltage. Stray current immediately became a problem on the NLL which concentrated minds on removing as much of the 3rd rail as possible except for unavoidable short changeover sections. For the DC lines running into Euston from Camden I believe some mitigations have been applied such as parallel reinforcing conductors, and while the fairly limited service and small trains involved don't produce the high levels of DC return current in rails south of the river, there must still be some effect and long term it makes sense to decommission the 3rd rail and have the Watford trains switch to overhead for the final run into the terminus.

 

[Peter Mugridge]

In a dual electrified area, the safety concerns must take precedence, so comprehensive bonding is provided, giving massive opportunity for the DC current to stray and cause damage to structural metalwork and underground utilities. The AC return current is much less of a problem if it strays as there isn't the same corrosion problem with AC and the current is so much smaller due to the high supply voltage. Stray current immediately became a problem on the NLL which concentrated minds on removing as much of the 3rd rail as possible except for unavoidable short changeover sections. For the DC lines running into Euston from Camden I believe some mitigations have been applied such as parallel reinforcing conductors, and while the fairly limited service and small trains involved don't produce the high levels of DC return current in rails south of the river, there must still be some effect and long term it makes sense to decommission the 3rd rail and have the Watford trains switch to overhead for the final run into the terminus.

What is the normal level of current ( i.e. amps ) in both these systems?

 

[Steve Harris]

ISTR there were some lengthy stretches of dual electrification when the OHLE was first installed for freight on the NLL. This is undesirable beyond necessary short sections for changeover as there's a conflict in the earthing standards between the two systems. In DC only areas, the rails are not earthed, to encourage the large return current to stay in the rails en route back to the substation instead of taking all kinds of alternative paths through the ground and any metallic objects in the vicinity, which are at risk of damage by a form of corrosion caused by the current. By contrast, in AC areas, the traction return rail is earthed and bonded to every metallic object nearby, including electrification stanchions, signal gantries and bridges for safety with the high voltage used and to avoid the risk of dangerously high 'touch potential' that staff and passengers might otherwise be exposed to.

In a dual electrified area, the safety concerns must take precedence, so comprehensive bonding is provided, giving massive opportunity for the DC current to stray and cause damage to structural metalwork and underground utilities. The AC return current is much less of a problem if it strays as there isn't the same corrosion problem with AC and the current is so much smaller due to the high supply voltage. Stray current immediately became a problem on the NLL which concentrated minds on removing as much of the 3rd rail as possible except for unavoidable short changeover sections. For the DC lines running into Euston from Camden I believe some mitigations have been applied such as parallel reinforcing conductors, and while the fairly limited service and small trains involved don't produce the high levels of DC return current in rails south of the river, there must still be some effect and long term it makes sense to decommission the 3rd rail and have the Watford trains switch to overhead for the final run into the terminus.

Exactly this ^

As the whole NLL was originally+ third rail from Richmond to North Woolwich, then (as stated up thread) parts gained OHLE to let freight get between the WCML (Primrose Hill) and the GEML (Stratford High Level). When Class 313's* got transferred to run the NLL services some parts of the 3rd rail system was ripped out because of the above !

*313's being dual voltage units, were as previously pure 3rd rail units had been used.

+ OK, probably not since time internal, but certainly when I travelled the line in the early 1980's.

 

[MarkyT]

What is the normal level of current ( i.e. amps ) in both these systems?

I can't find any info online about the installed power of the class 710 units but let's assume they're rated at about 2000kW. To deliver that power at 750VDC a current of 2666A is required. The same power on 25kVAC would draw around 80A.

 

[Bikeman78]

I can't find any info online about the installed power of the class 710 units but let's assume they're rated at about 2000kW. To deliver that power at 750VDC a current of 2666A is required. The same power on 25kVAC would draw around 80A.

If I recall correctly, a typical four car unit such as a CIG, CEP or VEP drew about 2000A when pulling away.

 

[GRALISTAIR]

If I recall correctly, a typical four car unit such as a CIG, CEP or VEP drew about 2000A when pulling away.

That was my “round numbers” memory too. A lot of current. I^2 R losses large.

 

[Peter Mugridge]

I can't find any info online about the installed power of the class 710 units but let's assume they're rated at about 2000kW. To deliver that power at 750VDC a current of 2666A is required. The same power on 25kVAC would draw around 80A.

If I recall correctly, a typical four car unit such as a CIG, CEP or VEP drew about 2000A when pulling away.

Thank you both.

 

[Richard Scott]

If I recall correctly, a typical four car unit such as a CIG, CEP or VEP drew about 2000A when pulling away.

I think maximum draw is around 4000amps or just over for 3rd rail and around 200amps for 25kV overheads? Means 12 car CIGs etc would be limited when accelerating? Know if too many trains left a London Terminus at once the power sometimes tripped due to excessive current draw so 3rd rail quite limited. Know a 2 car EPB wasn't ever going to cause these sorts of problems on NLL!

 

[Ken H]

When cl 313 replaced cl 501, drivers found that on dual electrifies parts, the 313's accelerated better in AC mode. So using AC allowed them to keep time better. Quite within the rules.

More 25kv was added and I think only the furthest west section is 3rd rail.

Found this old thread

North London Line

The North London line use to be third rail throughout when was the third rail partially replaced by overhead wires?

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