Neutral sections - how long are they?

[DarloRich]

A question that came to mind while travelling on the WCML last night. I was in the Pendolino coach where the loud bang whenever you enter or leave a neutral section is very noticeable. At one we seemed to be travelling quite slowly and the gap between the two bangs was quite noticeably longer than normal! A couple of questions spring to mind:

• Is there a fixed length to a neutral section?
• Is that bang the circuit breakers opening and closing?
• Has anything ever got stuck in a neutral section

 

[pdeaves]

Has anything ever got stuck in a neutral section

Yes. I can't quote any specific incident but know it has happened.

 

[62484GlenLyon]

I don't know about a train getting stranded between the magnets in a neutral section, though it must surely have happened occasionally over the years. I do recall an incident where at train failed to reset after passing through the neutral section. In the first weeks of the St.Pancras to Bedford electric service in the summer of 1983, at about 18.30 on a Friday evening, a 317 unit didn't reset after going through the neutral section at Sandridge. The train that was sent in behind also failed to reset. With fingers, and everything else, crossed another trains was sent in which luckily did reset and the disruption was then quickly sorted out.

 

[driver9000]

Some seem to be longer than others but the actual dead section of wire is 15-20ft from memory.

Yes, the bangs you hear is the VCB opening and closing.

Yes, electric trains have stopped in Neutral Sections but with a bit of luck it can roll out using gravity or driven out in the case of a train with multiple pantographs.

 

[matchmaker]

Some seem to be longer than others but the actual dead section of wire is 15-20ft from memory.

Yes, the bangs you hear is the VCB opening and closing.

Yes, electric trains have stopped in Neutral Sections but with a bit of luck it can roll out using gravity or driven out in the case of a train with multiple pantographs.

If neither of the above apply, it is possible for the neutral section to be energised in order for the stranded train to be driven out, I believe. It's certainly mentioned in a 1960 booklet "A.C Electrified Lines - Working Instructions".

 

[Tomnick]

Has anything ever got stuck in a neutral section

It would be a shame not to mention the driver who, the story goes, was asked by the signaller to stop adjacent to a train on the opposite line that appeared to have stopped mid-section, to find out what was wrong. He duly did so, and leant out to ask the other driver whether he had a problem. "Yes", came the reply as he pointed up to the neutral section above them both, "the same problem as you..."

 

[Taunton]

Some seem to be longer than others but the actual dead section of wire is 15-20ft from memory.

If neither of the above apply, it is possible for the neutral section to be energised in order for the stranded train to be driven out, I believe. It's certainly mentioned in a 1960 booklet "A.C Electrified Lines - Working Instructions".

Those were the old big 25kV neutral sections installed at that time, maybe a coach or two long overall, which had multiple "snowshoes" shaped insulators, and did indeed have arrangements to supply power to the intermediate lengths if a train got stuck in them. The much shorter single unit ones came along later.

 

[The exile]

Yes. I can't quote any specific incident but know it has happened.

There was a completely wireless section of an electrified mainline in East Germany (can’t remember whether wires would have got in the way of a runway approach or electrical interference have ruined some surveillance equipment). Whatever the reason, it occasionally caught loco-hauled trains by surprise! (No pantograph anywhere else on the train to act as a get out of jail free card!)

 

[jfollows]

Single pantograph AC electric locomotives sometimes got stuck in neutral sections. The one that comes to mind is the one at Kidsgrove on the up line from Crewe, initially located about a train’s length from the platform signal. With DVTs, the locomotive was at the rear of the train and if the train stopped at the signal it could get stuck. I believe the neutral section was relocated. The modern type can’t be energised.

https://www.avantiwestcoast.co.uk/t...r-trains/railfans/electricity-and-pantographs says

The neutral section itself can be anything from 100 metres to 400 metres in length.

 

[Falcon1200]

Has anything ever got stuck in a neutral section

Oh yes! It doesn't help when a neutral section is located next to a station, which is the case on the Up Argyle Line at Rutherglen, and not assisted further when the line there is on a sharp curve! If a train cannot gravitate onto live overheads it would require assistance.

 

[scrapy]

It would be a shame not to mention the driver who, the story goes, was asked by the signaller to stop adjacent to a train on the opposite line that appeared to have stopped mid-section, to find out what was wrong. He duly did so, and leant out to ask the other driver whether he had a problem. "Yes", came the reply as he pointed up to the neutral section above them both, "the same problem as you..."

Yes, there was also a certain TOCs control that had the idea of doing a train to train evacuation from a train stuck in a neutral section. Thankfully they were told by the driver why it wouldnt be a good idea.

 

[SansHache]

Opening and closing of the VCB (Vacuum Circuit Breaker) on the pantograph vehicle is triggered by detection of the APC (Automatic Power Control) magnets positioned on the approach to and exit from the neutral section. The distance from the magnet to the neutral may vary with the speed limit at the specific location. The standard positioning for high speed lines (>100mph) is 30m before and after the neutral, 60m between the two magnets.
Modern neutral sections have an insulating rod connected in the contact wire. I think this is typically 2m to 3m long.
When the train passes the first APC magnet the VCB is opened, ensuring there is no current flowing when the pantograph runs off the contact wire onto the insulator. When the train passes the second APC magnet the VCB is re-closed allowing the train to start drawing power again.
On Class 390 the VCB is always held open for a minimum of 5 seconds regardless of the reason for opening. This will be the delay between opening and closure of the main VCB at most neutral sections. If the train is travelling slowly over the neutral section the VCB will remain open until the train crosses the second APC magnet.
The APC Receiver is mounted on the bogie closest to the pantograph to ensure the APC control system can function correctly when running in either direction.
The loud noise from the VCB is caused when opening. Reclosure is normally quieter, but still audible in the saloon.
A 9-car Class 390 has two VCBs and an 11-car has three VCBs. All VCBs open simultaneously at the neutral section but the reclosure is staggered with a 4-second delay between VCBs after the main VCB has reclosed.

 

[6Gman]

It would be a shame not to mention the driver who, the story goes, was asked by the signaller to stop adjacent to a train on the opposite line that appeared to have stopped mid-section, to find out what was wrong. He duly did so, and leant out to ask the other driver whether he had a problem. "Yes", came the reply as he pointed up to the neutral section above them both, "the same problem as you..."

I have seen that reported as happening near Stafford, from a credible source.

 

[SansHache]

One additional point to add. If a train comes to a stop with the VCB open between the two APC magnets the driver can re-close the VCB by operating the Pan Up / Reset button on the driver's desk. This will allow the train to take power unless the train has stopped with the pantograph on the insulated section.

 

[Elecman]

• Has anything ever got stuck in a neutral section

Yes a few times when DVTs were first used on the WCMLas the loco was then at the rear and at certain signals if at red the loco pan was on the NS

 

[edwin_m]

One additional point to add. If a train comes to a stop with the VCB open between the two APC magnets the driver can re-close the VCB by operating the Pan Up / Reset button on the driver's desk. This will allow the train to take power unless the train has stopped with the pantograph on the insulated section.

Is there then a risk of striking an arc if they power through the actual neutral part? Or isn't that a problem at low speed or do they have to drive away from it?

 

[SansHache]

Is there then a risk of striking an arc if they power through the actual neutral part? Or isn't that a problem at low speed or do they have to drive away from it?

If the VCB (Vacuum Circuit Breaker) fails to open before reaching the neutral section then yes, an arc will be drawn from the pantograph. Worst case the arc can be drawn across the complete insulated section and short the two power supplies together, resulting in the feeders tripping. Such events are normally a consequence of a defective APC Receiver or a displaced APC magnet. There have been examples of trains with defective APC equipment tripping multiple feeders at successive neutral sections, but once this is apparent the defective train is soon brought to a stand by the control room.
If a train comes to a stand with the pantograph between the first APC magnet and the insulated section then the driver can manually re-close the VCB. When the train sets off it will be taking relatively low current. An arc will be drawn as running onto the insulator, but this is unlikely to be sustained and cause a supply trip. When the train reaches the exit APC magnet the VCB will open again as the control is out of sequence. This requires another manual reset by the driver before continuing.
Of course, on a Class 390 there is always the option to switch to the other pantograph which will be well clear of the neutral section.

 

[Kneedown]

155ft is the length of a neutral section (in MML land at least) between the two APC magnets (that open, and close the VCB.
Broken down, there is 100ft of live "run in" wire, 5ft of insulated wire, 5ft of earthed wire, then another 5ft of insulated wire, before 40ft of electrically live "run off" wire. Therefore, only 15ft of the neutral is electrically dead. Normal procedure is to reset the VCB first in the hope that the pan is on the 140ft of live wire. If that doesn't work, you attempt to roll out if on a downhill gradient, otherwise assistance is required. A single 360, or pair with only one pan up, cannot roll due to the auto hold brake. An 810 could raise the other pan, or switch to diesel.
Yes, it has happened.

 

[plugwash]

155ft is the length of a neutral section (in MML land at least) between the two APC magnets (that open, and close the VCB.
Broken down, there is 100ft of live "run in" wire, 5ft of insulated wire, 5ft of earthed wire, then another 5ft of insulated wire, before 40ft of electrically live "run off" wire. Therefore, only 15ft of the neutral is electrically dead. Normal procedure is to reset the VCB first in the hope that the pan is on the 140ft of live wire.

Having succesfully reset the VCB does the driver need to determine somehow whether they are on the "run-in" or the run-out wire?

 

[AM9]

Way back in the '70s, I was commuting on the GEML, and there was a neutral section somewhere near Forest Gate station, (that part of the line was 6.25Kv in those days and the breakers were compressed air types rather than vacuum). On more than one occasion, the breakers didn't open and each time there was a loud buzz and crackle with an arc glow visible against the cutting walls.
I remember in one of the rail magazines in the late '60s an article about OLE isolator sections and describing a new type comprising a glass fibre section of contact wire sheathed in ceramic beads. These were rapidly installed across the network as being less disruptive to operation and suitable for speeds up to 100mph. The types they replaced were the 'snowshoe' try prs which were limited to speeds under 60mph, and most commonly used at junctions and crossovers. For high speed sections there was the parallel wire changeover (effectively two succesive wire-end overlaps with the centre stretch of conductor grounded).

 

[Kneedown]

Having succesfully reset the VCB does the driver need to determine somehow whether they are on the "run-in" or the run-out wire?

No way of telling visually. If on the run off then happy days. If on the run in, then hope you can build enough momentum to carry you through, otherwise rules for "stuck in the dead section" will apply.

 

[swt_passenger]

I’m surprised the distance between magnet and neutral section is typically so short, at a high line speed the magnet/VCB reaction time must need to be a fraction of a second.

 

[edwin_m]

I’m surprised the distance between magnet and neutral section is typically so short, at a high line speed the magnet/VCB reaction time must need to be a fraction of a second.

The VCB is designed to operate very quickly, as it must do in the event of a fault on the train.

 

[swt_passenger]

The VCB is designed to operate very quickly, as it must do in the event of a fault on the train.

Ah yes, that makes sense. I had wrongly assumed that the driver might have time to react to a failure of the system. Was I probably thinking more of auto power changeover locations for bimodes?

 

[8ace]

Does third rail traction have anything similar? Or is it not required because of the lower voltage and/or more durable con rail/shoegear?

 

[swt_passenger]

Does third rail traction have anything similar? Or is it not required because of the lower voltage and/or more durable con rail/shoegear?

Not required because there are no neutral sections, with a few exceptions the Southern DC network is operated with everything connected in parallel, as used in the naming of lineside ‘track paralleling huts’.

That’s not to say the system can’t be sectioned for maintenance or reaction to train or infrastructure faults, but in the normal day to day it’s all a big system.

 

[brad465]

Does third rail traction have anything similar? Or is it not required because of the lower voltage and/or more durable con rail/shoegear?

I thought they don't need such sections because it's DC, which is less of an issue to mix between feeders?

Getting stuck in Neutral Sections, while rare, is going to become an even rarer occurrence as batteries and/or bi/tri-modes prevail, as the unit can simply use an alternative power source to get out of the NS.

 

[jfollows]

Does third rail traction have anything similar? Or is it not required because of the lower voltage and/or more durable con rail/shoegear?

The issue with AC is the phase differences between different feeds. DC is just voltage, it doesn’t matter. Simplistically put anyway.

 

[MarlowDonkey]

Does third rail traction have anything similar? Or is it not required because of the lower voltage and/or more durable con rail/shoegear?

There are gaps in the third rail at junctions, level crossings or where it changes side. Mulltiple units may have several pick ups so may not be affected. It was a potential problem for locomotives, hence the development of the electro-diesel and before that locos with flywheels.

 

[Taunton]

I believe there have been significant difficulties in positioning neutral sections on the freight etc connecting lines in north-west London between different electrified routes, especially where the connection is short, uphill, and low speed. The current project on the "Poplars" from GWML at Acton up to the WCML is one, and I believe the reason for it not being installed at all on the link from the MML to the Goblin at Junction Road is because it's not possible. Others likely know more.