Class 800 pantograph operation

[CrickUK]

As part of my regular commute I travel from Wakefield to Leeds and have noticed that every time traveling North Bound just as passing the electric feeder station approx 200mm south of Copley Hill there is an alarming bang from the roof on Coach A which I take to be the pantograph dropping. Yet on reaching Leeds the front pantograph is always raised. Does anyone know is the pantograph dropped at this location and then raised straight back up again? Or is the noise from something else?

 

[hwl]

As part of my regular commute I travel from Wakefield to Leeds and have noticed that every time traveling North Bound just as passing the electric feeder station approx 200mm south of Copley Hill there is an alarming bang from the roof on Coach A which I take to be the pantograph dropping. Yet on reaching Leeds the front pantograph is always raised. Does anyone know is the pantograph dropped at this location and then raised straight back up again? Or is the noise from something else?

The noise is the vacuum circuit breaker (on the roof) disconnecting just before it goes through a neutral section (ditto all other electric stock)

 

[pdeaves]

The noise is the vacuum circuit breaker (on the roof) disconnecting just before it goes through a neutral section (ditto all other electric stock)

For those who don't know, it is usual to keep the pantograph raised at neutral sections, just stop drawing power for a while. The circuit breaker does that in a controlled manner (and is presumably much quieter when it kicks back in!).

 

[Efini92]

For some reason the VCB on modern stock opens and closes with a loud bang. The 331’s are the same.

 

[Neptune]

For some reason the VCB on modern stock opens and closes with a loud bang. The 331’s are the same.

Nothing will beat the 308’s for loud bangs in neutral sections. It was loud in the saloon but in the brake van it was like a bomb going off.

333’s aren’t that loud. Just a buzz if you sit near the disabled area.

 

[Ianno87]

For some reason the VCB on modern stock opens and closes with a loud bang. The 331’s are the same.

387s are pretty loud.

 

[trebor79]

387s are pretty loud.

Indeed they are. I remember the first time I happened to be near the panto on a 387, I thought there was a serious problem with the train until I twigged what it was. Still made me jump every time, all the way to London.

 

[AM9]

Nothing will beat the 308’s for loud bangs in neutral sections. It was loud in the saloon but in the brake van it was like a bomb going off.

333’s aren’t that loud. Just a buzz if you sit near the disabled area.

My memories of the 308s (and all, the GE MKI EMUs) was that the noise was more of an air blast. I believe that they were air blast breakers where any arc is blown away by a blast of compressed air. Maybe the breakers have been replaced with something else since the '70s.

 

[swt_passenger]

My memories of the 308s (and all, the GE MKI EMUs) was that the noise was more of an air blast. I believe that they were air blast breakers where any arc is blown away by a blast of compressed air. Maybe the breakers have been replaced with something else since the '70s.

Vacuum circuit breakers are the normal fit. I expect someone knows the comparative advantages and disadvantages but they definitely seem to have become standard.

 

[AM9]

Vacuum circuit breakers are the normal fit. I expect someone knows the comparative advantages and disadvantages but they definitely seem to have become standard.

Maybe the sound was that of compressed air actuation. Do you know what VCBs are actuated by?

 

[swt_passenger]

Maybe the sound was that of compressed air actuation. Do you know what VCBs are actuated by?

I don’t I’m afraid. I suspect there’s some powerful springs involved though.

 

[matchmaker]

The air blast breakers in 303s were LOUD - especially in the guards van immediately undeneath them!

 

[coppercapped]

I don’t I’m afraid. I suspect there’s some powerful springs involved though.

Yup! What follows is based on what I learnt in the mid-1960s at English Electric when I worked in the same factory that made the vacuum circuit breakers, albeit using a glass vacuum chamber instead of the ceramic ones used today. They were a thing of beauty...

The opening springs are tensioned and only need to be tripped to open the contacts inside the vacuum chamber. Springs are used so ensure that once triggered the opening sequence cannot be stopped as it is necessary to get the contacts open as quickly as possible to their full separation to break the current. This action is the cause of the bang.

It depends on the design but generally the opening springs are pre-tensioned before the contacts are closed again in case it's necessary to break the circuit again immediately after closure.

The springs are quite strong! You wouldn't want to get your finger caught in one!

The only moving part inside the vacuum chamber is the moving contact which is sealed from the outside world by a thin flexible stainless steel bellows. All the operating stuff is outside the vacuum.

And the reason for the vacuum is that there is no air or oil to be ionised by the arc from the opening contacts. In fact be shaping the contacts the arc can be made to self-extinguish by the way it then moves in the magnetic field it generates itself.

All very cunning...

 

[swt_passenger]

Yup! What follows is based on what I learnt in the mid-1960s at English Electric when I worked in the same factory that made the vacuum circuit breakers, albeit using a glass vacuum chamber instead of the ceramic ones used today. They were a thing of beauty...

The opening springs are tensioned and only need to be tripped to open the contacts inside the vacuum chamber. Springs are used so ensure that once triggered the opening sequence cannot be stopped as it is necessary to get the contacts open as quickly as possible to their full separation to break the current. This action is the cause of the bang.

It depends on the design but generally the opening springs are pre-tensioned before the contacts are closed again in case it's necessary to break the circuit again immediately after closure.

The springs are quite strong! You wouldn't want to get your finger caught in one!

The only moving part inside the vacuum chamber is the moving contact which is sealed from the outside world by a thin flexible stainless steel bellows. All the operating stuff is outside the vacuum.

And the reason for the vacuum is that there is no air or oil to be ionised by the arc from the opening contacts. In fact be shaping the contacts the arc can be made to self-extinguish by the way it then moves in the magnetic field it generates itself.

All very cunning...

Thanks for that. I did some googling and found a few diagrams, but that’s the best explanation I’ve read...

 

[eMeS]

The noise is the vacuum circuit breaker (on the roof)...

Is this done automatically, or under the control of the driver?

 

[CW2]

Is this done automatically, or under the control of the driver?

It is automatic, triggered by the train passing over trackside magnets (APC magnets).

 

[edwin_m]

Doesn't the VCB also open (first) if the driver hits the button to drop the pan?

 

[Elecman]

Doesn't the VCB also open (first) if the driver hits the button to drop the pan?

Yes

 

[O L Leigh]

Vacuum circuit breakers are the normal fit. I expect someone knows the comparative advantages and disadvantages but they definitely seem to have become standard.

Air blast breakers are mechanically similar to vacuum breakers, but rather than using the vacuum to extinguish an arc it uses, as the name suggests, a blast of compressed air is used instead. I suppose the advantage of the vacuum type is that it doesn't require any outside power source or air supply when falling into it's failsafe (open) state making it more reliable.

 

[coppercapped]

Air blast breakers are mechanically similar to vacuum breakers, but rather than using the vacuum to extinguish an arc it uses, as the name suggests, a blast of compressed air is used instead. I suppose the advantage of the vacuum type is that it doesn't require any outside power source or air supply when falling into it's failsafe (open) state making it more reliable.

As I tried to demonstrate in my post above the point of the vacuum is that there is no[1] air between the contacts to ionise. Any 'arc' which is drawn as the contacts open under load consists of metal vapour from the contact surfaces which condenses rapidly on nearby cold surfaces so no 'arc' is supported.

Pedant mode! After I wrote my first post I remembered that, strictly, the vacuum chamber and its contacts is called the 'vacuum interrupter'; the whole kaboodle of springs and operating gubbins is strictly the 'circuit breaker'.

At the low pressures present in the vacuum interupter the 'mean free path', that is the average distance a gas molecule or metal atom will travel before hitting another gas molecule is in the order of 10mm to 100mm at normal temperatures. This means that a particle emitted from one of the contact surfaces is very likely to hit the contact surface opposite or the walls of the vessel and be adsorbed there before it hits and ionises any of the residual air molecules in the chamber.

The advantage of the vacuum interrupter is that it is smaller than the switch needed for breaker opening in the air, which at these power levels also needs a blast of air to blow the arc out and is often accompanied by arc shutes to limit the arc's spread. It is also smaller than all the gubbins needed to surround the opening switch with sulphur hexafluoride which has also been used. The contact surfaces also last a lot longer as they are not eroded by hot arcs or exposed to the elements.

[1] The pressures in a vacuum interrupter are typically 10^-4 to 10^-5 torr (in old money), 0.00013 millibar to 0.000013 millibar in new(ish) money. 1 torr (named after Torricelli) is the pressure exerted by a column of mercury 1mm high; 1 bar is atmospheric pressure.

 

[Philip Phlopp]

I

Air blast breakers are mechanically similar to vacuum breakers, but rather than using the vacuum to extinguish an arc it uses, as the name suggests, a blast of compressed air is used instead. I suppose the advantage of the vacuum type is that it doesn't require any outside power source or air supply when falling into it's failsafe (open) state making it more reliable.

The VCB designs now in use also support the upgraded OLE fault current of 12kA, ABBs were only designed/certified to 10kA. 12kA compliant VCB now needed to use NR's 25kV AC OLE.

 

[gsnedders]

As I tried to demonstrate in my post above the point of the vacuum is that there is no[1] air between the contacts to ionise. Any 'arc' which is drawn as the contacts open under load consists of metal vapour from the contact surfaces which condenses rapidly on nearby cold surfaces so no 'arc' is supported.

[...]

[1] The pressures in a vacuum interrupter are typically 10^-4 to 10^-5 torr (in old money), 0.00013 millibar to 0.000013 millibar in new(ish) money. 1 torr (named after Torricelli) is the pressure exerted by a column of mercury 1mm high; 1 bar is atmospheric pressure.

Also to be further pedantic: electricity can still arc in a vacuum, especially when the contacts exist within a magnetic field (oh, hey, the earth has one of them!), hence the need to separate the contacts quickly, to make the arc as the contacts separate as short-lived as possible.

 

[supervc-10]

I feel like this video from the wonderful Alec at Technology Connections is appropriate to this discussion

 

[trebor79]

Also to be further pedantic: electricity can still arc in a vacuum, especially when the contacts exist within a magnetic field (oh, hey, the earth has one of them!), hence the need to separate the contacts quickly, to make the arc as the contacts separate as short-lived as possible.

Yep. Spectacular and very dangerous failures are not unknown. I used to work in factories that had hundreds of 415v breakers, some 11kV and some 25kV. Fortunately never witnessed why if the higher voltage stuff fail, but did witness a 415v fail when someone threw the isolate whilst a large current was being drawn. It all worked as designed with most of the blast escaping to the rear, but the cubicle door did bulge a bit.
A few years previous someone hadn't been so lucky, the cubicle door hadn't been screwed shut properly, flew open and broke his jaw, but fortunately in the process shielded his face from most of the flames and molten copper.

 

[gsnedders]

Yep. Spectacular and very dangerous failures are not unknown. I used to work in factories that had hundreds of 415v breakers, some 11kV and some 25kV. Fortunately never witnessed why if the higher voltage stuff fail, but did witness a 415v fail when someone threw the isolate whilst a large current was being drawn. It all worked as designed with most of the blast escaping to the rear, but the cubicle door did bulge a bit.
A few years previous someone hadn't been so lucky, the cubicle door hadn't been screwed shut properly, flew open and broke his jaw, but fortunately in the process shielded his face from most of the flames and molten copper.

They're something with relatively few unspectacular failure modes! (Getting stuck open is about the only good failure mode they have!)

 

[bspahh]

Yep. Spectacular and very dangerous failures are not unknown. I used to work in factories that had hundreds of 415v breakers, some 11kV and some 25kV. Fortunately never witnessed why if the higher voltage stuff fail, but did witness a 415v fail when someone threw the isolate whilst a large current was being drawn. It all worked as designed with most of the blast escaping to the rear, but the cubicle door did bulge a bit.
A few years previous someone hadn't been so lucky, the cubicle door hadn't been screwed shut properly, flew open and broke his jaw, but fortunately in the process shielded his face from most of the flames and molten copper.

This tweet https://twitter.com/i/status/1292535103839772674 has a 400kV, 600MVA air blast circuit breaker tripping in a planned test.

 

[matchmaker]

Yep. Spectacular and very dangerous failures are not unknown. I used to work in factories that had hundreds of 415v breakers, some 11kV and some 25kV. Fortunately never witnessed why if the higher voltage stuff fail, but did witness a 415v fail when someone threw the isolate whilst a large current was being drawn. It all worked as designed with most of the blast escaping to the rear, but the cubicle door did bulge a bit.
A few years previous someone hadn't been so lucky, the cubicle door hadn't been screwed shut properly, flew open and broke his jaw, but fortunately in the process shielded his face from most of the flames and molten copper.

Many years ago I did training in a large coal fired power station. Adjacent to it was a transformer farm and numerous switchgear to connect to the National Grid.

One very windy morning I was passing when - somewhere - an earth fault occurred on a 275kv line and the breakers opened. It was the biggest bang and flash I have ever heard. I very nearly sh!t myself!

 

[coppercapped]

Also to be further pedantic: electricity can still arc in a vacuum, especially when the contacts exist within a magnetic field (oh, hey, the earth has one of them!), hence the need to separate the contacts quickly, to make the arc as the contacts separate as short-lived as possible.

Yup! I know the contacts open quickly - earlier I wrote that you wouldn't want to get your fingers caught in the operating springs.

Metal and gas atoms and molecules on the surfaces of the contacts are exposed to the influence of the high electric field strengths (volts per metre) between the separating surfaces. These high fields draw material off the surface and this material can ionise and conduct electricity from one surface to the other. This is the reason why the surfaces are designed to separate as quickly as possible to a distance at which the local electric field strength is below that which will draw significant quantity of material off the surfaces to permit a conducting path to be maintained. I would also point out that the magnetic field strength generated by the current through the contacts, and therefore the self-induced magnetic field as the contacts open, is higher than the earth's local field.

However, because of the very low pressure within the vacuum chamber many of the 'particles' drawn from the surfaces of the contacts will hit a nearby cold surface and stop before they hit and ionise any of the residual atmospheric molecules in the chamber or other particles drawn off the surfaces. As I wrote earlier the 'mean free path' (MFP - the average distance between collisions) at the vacua typically found in a vacuum isolator is 10mm to 100mm - at atmospheric pressure the MFP is in the order of 60 nanometres (60*10^-9 metres) roughly a hundred million times smaller. This means that a hot energetic particle emerging from a contact surface in atmosphere will certainly hit and probably ionise a molecule of the surrounding air thus easily creating an arc.

This last process cannot occur in a vacuum chamber as there is not sufficient air to ionise to form a conductive path. An arc consisting of ionised air cannot occur within a vacuum interruptor. This is what I was trying to explain.

 

[trebor79]

Many years ago I did training in a large coal fired power station. Adjacent to it was a transformer farm and numerous switchgear to connect to the National Grid.

One very windy morning I was passing when - somewhere - an earth fault occurred on a 275kv line and the breakers opened. It was the biggest bang and flash I have ever heard. I very nearly sh!t myself!

I had a moment like that at the same factory I talked about in the post you quoted. In the boiler house at 2am, stood neat the turbine discussing something or other with the boiler house operator. Suddenly there was a bang from one of the 25kV breakers, the lights dipped briefly and the alternator made a very loud "electrical buzzing noise" for a second. Grid fault somewhere had tripped us into island mode, we both jumped.
The other time I needed a change of underwear was when I'd crawled underneath an evaporator to open the main steam valve during a start up. I'd just cracked the valve (6 foot diameter!), when suddenly they was a bang followed by an almighty roaring. It would have been nice if someone had told me they were going to lift the safeties on one of the boilers, the exhaust not very far away from where I was working! 50bar superheated steam makes a heck of a racket, akin to a small explosion. For a moment I though something had failed on the evaporator and I was about to get scalded to death.

 

[eMeS]

Yup! I know the contacts open quickly - earlier I wrote that you wouldn't want to get your fingers caught in the operating springs.
... This is what I was trying to explain.

Many thanks for your explanation.
I read "light current electrical engineering" and missed the exciting stuff, but my supervisions were held next to the prototype of the world's first commercial scanning electron microscope.