Why are there neutral sections?

[Intercity110]

Something i always notice on Electric multiple units is that they sometimes just stop making a motor noise and stop accelerating, and this is always on ones where the pantograph is up. Why is that? (I assume they’re neutral sections due to the fact they do it at seemingly random points)

 

[Lucy1501]

They’re basically a piece of the overhead that aren’t connected to a power supply. It allows parts of the overhead wires to be fed from different power supplies. It also allows specific areas to be isolated for engineering works & stop faults from spreading between sections.

There are signs which mark the start and end of a neutral section, and provide prior warning. Some trains need to drop a pantograph during the section, for example, or else they’ll emergency brake.

 

[skyhigh]

Something i always notice on Electric multiple units is that they sometimes just stop making a motor noise and stop accelerating, and this is always on ones where the pantograph is up. Why is that?

It might also just be that the driver doesn't need to accelerate any further so is just coasting.

 

[GC class B1]

They’re basically a piece of the overhead that aren’t connected to a power supply. It allows parts of the overhead wires to be fed from different power supplies. It also allows specific areas to be isolated for engineering works & stop faults from spreading between sections.

There are signs which mark the start and end of a neutral section, and provide prior warning. Some trains need to drop a pantograph during the section, for example, or else they’ll emergency brake.

I don’t think it is necessary to lower the pantograph at neutral sections. The Vacuum Circuit Breaker opens before the neutral section and closes after it to isolate pantograph from the train electrical system when passing through the neutral section.

 

[hexagon789]

Something i always notice on Electric multiple units is that they sometimes just stop making a motor noise and stop accelerating, and this is always on ones where the pantograph is up. Why is that? (I assume they’re neutral sections due to the fact they do it at seemingly random points)

Over the length of a long electrified line, it may be supplied with electricity from multiple sources, a neutral section breaks these into individually supplied sections.

I don’t think it is necessary to lower the pantograph at neutral sections. The Vacuum Circuit Breaker opens before the neutral section and closes after it to isolate pantograph from the train electrical system when passing through the neutral section.

It isn't in the UK, other countries may differ.

 

[Efini92]

They’re basically a piece of the overhead that aren’t connected to a power supply. It allows parts of the overhead wires to be fed from different power supplies. It also allows specific areas to be isolated for engineering works & stop faults from spreading between sections.

There are signs which mark the start and end of a neutral section, and provide prior warning. Some trains need to drop a pantograph during the section, for example, or else they’ll emergency brake.

What train needs to drop its pantograph through a neutral section?

 

[jfollows]

See https://www.railforums.co.uk/threads/neutral-sections.134509/ for an earlier discussion on the subject.

 

[pdeaves]

and see http://www.railwaycodes.org.uk/electrification/neutral.shtm for the 'random' locations!

 

[John Webb]

Something i always notice on Electric multiple units is that they sometimes just stop making a motor noise and stop accelerating, and this is always on ones where the pantograph is up. Why is that? (I assume they’re neutral sections due to the fact they do it at seemingly random points)

Another reason for the driver going into 'coasting' mode is because there is another train in front of them, and the driver is adjusting the speed of their train to match the speed of the train in front by observing the signal aspects changing as they approach the signals. (There are many more signals than 'Neutral Sections' so this 'coasting' can occur almost anywhere.)

 

[Snow1964]

Some will be because they are between tracks on different circuits, an example is on Royston flyover.

Each end will probably be connected to track that flyover branches from, but obviously don't want a power line linking both circuits, so put short neutral between them.

The neutral section will generally be where trains are coasting, not where they are pulling away from signals, a station, a speed restriction or motoring on steep uphill sections.

 

[43066]

Something i always notice on Electric multiple units is that they sometimes just stop making a motor noise and stop accelerating, and this is always on ones where the pantograph is up. Why is that? (I assume they’re neutral sections due to the fact they do it at seemingly random points)

You might also notice a bang if you’re sat near the pan and the driver forgets to shut off.

There are signs which mark the start and end of a neutral section, and provide prior warning.

There’s a warning board and a commencement board, no termination board provided on any that I sign.

Below is an image of a commencement board (from Wikipedia). The warning board is the same symbol in white on a black background.

 

[CAF397]

You might also notice a bang if you’re sat near the pan and the driver forgets to shut off.

The VCB bangs regardless of whether the driver shuts off or not.

The indication the driver forgot is the instant reduction in power and the sudden jolt

 

[Edvid]

There’s a warning board and a commencement board, no termination board provided on any that I sign.

There are termination and rear clear boards for overlap neutral sections (as seen on the GWML).

On a related subject, it seems inline neutrals for linespeeds above 100mph are back in vogue - two were recently installed on the MML (East Hyde / Sharnbrook) and another is apparently going in at Braybrooke.

 

[800301]

The VCB bangs regardless of whether the driver shuts off or not.

The indication the driver forgot is the instant reduction in power and the sudden jolt

and this would happen because there is track fitted equipment small yellow blocks that talk to train fitted equipment that tells the VCB to open and close when passing through these sections. While the driver should always shut off while passing through a neutral section, as long as the APC (Automatic power control) equipment is working, it should prevent anything bad happening if the driver forgot to shut off, if it’s defective each train will be stopped at a suitable place and instructed to shut off while passing the section just to be sure

 

[59CosG95]

There are termination and rear clear boards for overlap neutral sections (as seen on the GWML).

On a related subject, it seems inline neutrals for linespeeds above 100mph are back in vogue - two were recently installed on the MML (East Hyde / Sharnbrook) and another is apparently going in at Braybrooke.

The one at Sandridge (which East Hyde replaced) is due to be removed too.

 

[GRALISTAIR]

And dont forget this free download from Garry Keenor which explains and illustrates them too. http://www.ocs4rail.com/

 

[Bald Rick]

The one at Sandridge (which East Hyde replaced) is due to be removed too.

Whilst it’s called Sandridge neutral, it is very much in St Albans! (Right next to the tip, as it happens)

Something i always notice on Electric multiple units is that they sometimes just stop making a motor noise and stop accelerating, and this is always on ones where the pantograph is up. Why is that? (I assume they’re neutral sections due to the fact they do it at seemingly random points)

To answer the OP, with a specific point, and trying to simplify….

AC electricity from the grid is generated and transmitted in 3 separate ‘phases’, each at 50hz; essentially the alternating of the current happens at slightly different times 50 x a second for each of the three phases. This frequency is carefully controlled by the grid and is the same across the whole country.

Where the railway electrification is supplied from the grid, it taps into one of these phases for any given section of electrical supply. (At many supply points, it taps into two phases, and uses them separately for two different sections.)

It is very important that these phases do not mix, as it can cause severe overloading of the electrical equipment.

Hence the neutral sections are there to keep the phases separate. Trains must disconnect from the power supply to prevent them bridging the neutral section and causing an overload both to the train and the power supply, hence the circuit breaker opening well in advance of the neutral, and closing well after. The neutral section itself is only a couple of metres long, usually.

 

[GRALISTAIR]

It is very important that these phases do not mix, as it can cause severe overloading of the electrical equipment.

25,000V x sqrt 3

 

[TurboMan]

There are termination and rear clear boards for overlap neutral sections (as seen on the GWML).

There were, but they've been removed as they were non-standard. They were installed for the carrier wire neutral sections which are around 250m long. The warning and commencement boards also had a 'LONG' suffix board but they've gone too.

 

[edwin_m]

25,000V x sqrt 3

You'd also get a dead short of (I think) 25000 x sqrt 2 through the wire connecting the feeds at the different phases, which would hopefully open the breakers before something melted.

 

[Efini92]

and this would happen because there is track fitted equipment small yellow blocks that talk to train fitted equipment that tells the VCB to open and close when passing through these sections. While the driver should always shut off while passing through a neutral section, as long as the APC (Automatic power control) equipment is working, it should prevent anything bad happening if the driver forgot to shut off, if it’s defective each train will be stopped at a suitable place and instructed to shut off while passing the section just to be sure

It’s a bit less technical than that, the yellow blocks are magnets.

 

[Falcon1200]

and this would happen because there is track fitted equipment small yellow blocks that talk to train fitted equipment that tells the VCB to open and close

Unless the APC magnets have been stolen! (which happened in the Motherwell area a few years ago).

There are also neutral sections at locations where it has not been possible to achieve clearance between the OLE and structures; For example, on one line (the Down Inner Circle) between Queens Park (Glasgow) and Pollokshields East, due to a pipe overhead (at an angle, hence only one line affected) and at Tams Brig, Newton-on-Ayr.

 

[jfollows]

I also remember when the neutral section between Levenshulme and Heaton Chapel was moved (quite a few years ago) that the old APC magnets were left behind for a while, which was a bit of a pain.

 

[Sly Old Fox]

There were, but they've been removed as they were non-standard. They were installed for the carrier wire neutral sections which are around 250m long. The warning and commencement boards also had a 'LONG' suffix board but they've gone too.

They’re still there at St Brides.

 

[TurboMan]

They’re still there at St Brides.

I just had a look at the route video...so they are! After all the fuss about the signage at Maidenhead being non-standard and having to be recovered...

== Doublepost prevention - post automatically merged: 26 May 2023 ==

I also remember when the neutral section between Levenshulme and Heaton Chapel was moved (quite a few years ago) that the old APC magnets were left behind for a while, which was a bit of a pain.

I suppose at least if they were all left in place, the VCB would open and then close again. Whereas if they'd left one set behind...