Overhead Neutral Sections (OHNS)

[John1976]

What purpose do overhead neutral sections (OHNS) serve on the AC wires? Is it some sort of "border" between differing overhead sections for ease of isolating without switching off the entire network? Also do trains have to lower their pantographs when they coast through them? Thanks to your responses in advice

 

[DaveNewcastle]

Each section is independently fed and independently switched, frequently on different phases of the supply, and due to the constantly changing demand, they will frequently be at differernt voltages. Where they meet, it is necessary to separate them electrically, but as a simple insulator is likely to break down at these voltages (greater than 100k instantaneous peak to peak voltage difference between phases), then a greater distance between them is requred to prevent leakage or flashover. That 'distance' is formed of a 'neutral section', a non-conductive section which is mechanically aligned with the contact wires at either end of it, so that pantograph pick-ups can pass under them unimpeded.

The supply is disconnected automatically before a neutral section by means of a circuit breaker (usually a large and clunky device) on-board such that there is no need to lower the pantograph.

 

[RogerB]

.........
The supply is disconnected automatically before a neutral section by means of a circuit breaker (usually a large and clunky device) on-board such that there is no need to lower the pantograph.

Why do they use a circuit breaker? Isn't the supply is disconnected ipso facto when the train hits the neutral section?

 

[John Webb]

Why do they use a circuit breaker? Isn't the supply is disconnected ipso facto when the train hits the neutral section?

The uncontrolled arcing that would take place if the train was drawing current when it came off the energised section onto the neutral section would damage the pantograph and possibly the overhead wire. By cutting off the current with a circuit breaker before reaching the neutral section, no current is being drawn through the pantograph and so the arcing is minimised, if not eliminated completely.

 

[RogerB]

Thanks John, obvious when you think about it!

 

[scotraildriver]

You must also be very careful as a driver not to become stuck in one. They are sometimes located at awkward places with low speed limits and if a passenger activates the passcom or you get an unsolicited brake application you can become stuck in the dead section and require assistance.

 

[The Planner]

They have a habit of being right after station stops as well. Not ideal when you are trying to pull away.

 

[6Gman]

You must also be very careful as a driver not to become stuck in one. They are sometimes located at awkward places with low speed limits and if a passenger activates the passcom or you get an unsolicited brake application you can become stuck in the dead section and require assistance.

There is a well-known story of an Up train grinding to a halt on the NS north of Stafford. The driver of a northbound train was asked to check what was going on, drew up alongside ... and made it two stranded trains!

 

[455driver]

There is a well-known story of an Up train grinding to a halt on the NS north of Stafford. The driver of a northbound train was asked to check what was going on, drew up alongside ... and made it two stranded trains!

Yes I have heard that one, first train comes to a stop so the Northbound driver pulls alongside and asks "whats up" and the first driver replies "same as you", Northbound driver looks confused and is then asked "check your line light" and realises the problem!

Words might be different but thats the gist of it!

 

[Crossover]

There is a well-known story of an Up train grinding to a halt on the NS north of Stafford. The driver of a northbound train was asked to check what was going on, drew up alongside ... and made it two stranded trains!

That is rather amusing, I must say, even if it wasn't for the people involved!

As an aside, with the Pendolinos, are they long enough that a panto would be out of the neutral section (having 2 of them) or are they also liable to get stuck too?

 

[O L Leigh]

It is actually quite hard to become "gapped" in a neutral section. That's not to say that it's not impossible, as I know of at least one driver that has done it. However, it is just the one driver and not many more as you might imagine.

Although the magnets that trigger the automatic power control relay (APCR) that opens the train's vacuum circuit breaker (VCB) are further apart, the actual length of the dead wire is just 15 feet. If you do find yourself stopped with the pan within the neutral section it's always worth trying to reset the VCB from the cab just in case it's still on a live section. It might just be enough to get yourself out of a tight spot.

O L Leigh

 

[rdeez]

There is a well-known story of an Up train grinding to a halt on the NS north of Stafford. The driver of a northbound train was asked to check what was going on, drew up alongside ... and made it two stranded trains!

Hahaha! Red faces all round.

 

[DownSouth]

As an aside, with the Pendolinos, are they long enough that a panto would be out of the neutral section (having 2 of them) or are they also liable to get stuck too?

A neutral section is short enough that it wouldn't be a problem for a Pendolino, if it is stopped with the active pantograph in the neutral section the driver would just raise the other one - unless a few other things go wrong and there is not enough compressed air left to raise the other pantograph.

 

[tsr]

There is a well-known story of an Up train grinding to a halt on the NS north of Stafford. The driver of a northbound train was asked to check what was going on, drew up alongside ... and made it two stranded trains!

This is probably a silly question, but why can't the neutral sections be staggered by a short distance, so that adjacent lines are not all neutral at the same geographical location? Wouldn't this prevent such problems, if done at sufficient intervals, unless a train examining an adjacent line was particularly heavy and so couldn't draw enough juice to accelerate away again after safety checks?

 

[Lockwood]

Someone I know is ex-S&T, and they were telling me a while back about some potential (no pun intended) problems with OHLE

One of the things mentioned was that a live cable could induce a parallel isolated cable.
I would guess that having 2 charged wires at different potential and possibly different phases trying to induct each other could end badly?

 

[swt_passenger]

I'm thinking keeping the neutral sections alongside each other is sensible so that when the sort of damage arises that brings down the wires, they cannot parallel two separate feeds accidentally.

 

[John Webb]

Someone I know is ex-S&T, and they were telling me a while back about some potential (no pun intended) problems with OHLE

One of the things mentioned was that a live cable could induce a parallel isolated cable.
I would guess that having 2 charged wires at different potential and possibly different phases trying to induct each other could end badly?

The possibility of induced currents/voltages between adjacent OHL is the reason that any OHL being worked on must be comprehensively earthed at intervals along the line to prevent the build-up of such induced voltages to the level where those working on the line could be hurt.

Induction can also cause problems for signalling equipment - hence the need for signalling systems to be 'AC Immunised' when a line is being electrified.

Induction can also affect non-railway equipment. For example at the preserved St Albans South box we get induced voltages on the wiring of our simulator particularly the inputs to the computer. To avoid problems our wires are either held to the +volts by a resistor or switched to the 0v of the supply - this has prevented false signals upsetting the simulator.

 

[plastictaffy]

Reading a few posts back about Neutrals being near stations. Berkswell Neutral is a bit naughty on the up - it's just out of the station, and on an incline as well. While I was route-learning NMP-BHM, one of the drivers went through it and didn't shut off early enough as he was battling for enough speed to coast through it. There was an enormous jolt!! On arrival at Canley (the next station) our hero came back to have a look at the pan, as he was quite concerned. He also apologised profusely down the tannoy to me and the punters for the jolt!!! I like walking through the train when you hear the breaker come out for a Neutral - the looks on the punters faces is priceless when they hear the bang!!!!