Changeover from DC to AC at Speed

[76020]

Does anybody know what is the speed limit for changing over from 3rd Rail to 25KV at speed, obviously Eurostar trains used to do this until 2007.
So it is 2019 and you are on a train from London Waterloo to Weymouth and it is approaching Basingstoke, will the train have to slow down to change to 25KV, or will all trains except the Salibury/Exeter service will have to stop there?

 

[A-driver]

There is no official speed limit for this-when it comes to dropping pans and raising them on the move the rule book states 80mph unless authorised by the specific train operator. Both FCC and east coast allow you to do it at line speed (so mark4 sets can do it at 125mph). The restriction tends to be more the traction-for example you can't raise a pan on a 365 above 3mph.

As for changing over I would Imagen it will be defined by the length of the overrun - you need enough time to fully drop the pan and stop if you are having any problems before you run out of wire/3rd rail or else you may be stranded!

 

[jopsuk]

The over-run is probably what will limit it- as previously explained many times on this topic, for several reasons you want to limit quite how long your dual-voltage section is

 

[Holly]

In a context then of so-called discontinuous (overhead) electrification, could there be mileage (pun not intended) of having third rail DC coverage of the gaps in overhead electrification (assuming they are implemented as overhead neutral sections)?
That is to have a pantograph on neutral wire long before (or while) it starts a lowering manoeuvre at full speed.

 

[A-driver]

Yes, or at least if I have understood what you mean.

On the NCL the third rail starts at the start of Drayton park platforms. The overhead son the down line start at the start of the platform but on the up line the wire continues to the end if highbury station but is a dummy (neutral). This is to stop the top if the pan being ripped off by the tunnel mouth should anyone forget to drop it. You could have a dummy wire above a 3rd rail for as far as you wanted it.

 

[W230]

Does anybody know what is the speed limit for changing over from 3rd Rail to 25KV at speed, obviously Eurostar trains used to do this until 2007.

I'd never really thought about this until you'd mentioned it here. On TL when dropping/raising the pan at Faringdon this is obviously done while stationary.

In the past when drivers have forgotten to drop the pan it easily breaks when the wires end if it's still up, causing just a few problems when you need to switch back to AC later! I'm guessing that must be a risk too when swapping between AC/DC. As you could end up stranded on a short intermediate DC section.

 

[swt_passenger]

So it is 2019 and you are on a train from London Waterloo to Weymouth and it is approaching Basingstoke, will the train have to slow down to change to 25KV, or will all trains except the Salibury/Exeter service will have to stop there?

It will be based on comparison of costs. They'll compare the business case advantages for having a subset of services that don't stop over the cost of providing the specific additional arrangements just for them.

In other words there's no guarantee that alternate fast trains will continue to run through.

 

[Holly]

Yes, or at least if I have understood what you mean.
On the NCL the third rail starts at the start of Drayton park platforms. The ... This is to stop the top if the pan being ripped off by the tunnel mouth should anyone forget to drop it. You could have a dummy wire above a 3rd rail for as far as you wanted it.

So basically you could switch many times at speed between AC overhead and third rail on a line (I am thinking of Crewe-Chester). Provided you kept an unenergised overhead wire above the actual third rail sections and an energised overhead where where the third rail is absent.

The point being that (presumably) the clearance required for an unenergised overhead wire is less than that for an energised wire thus avoiding a need to lower the trackbed and having a goodly number of short third rail sections (preferable to dead sections with risk of stranding).

 

[A-driver]

I don't think there is much difference, if any, for clearance with energised or unenergised wire. There are some bridges where the wires are practically you touching the roof and they are energised. I may be wrong about this though as I know by little about this kind of thing. But I see what you mean in theory-as long as there is some wire there, live or not, that will prevent the pan from breaking its back if not dropped.

 

[LNW-GW Joint]

So basically you could switch many times at speed between AC overhead and third rail on a line (I am thinking of Crewe-Chester). Provided you kept an unenergised overhead wire above the third rail.

The point being that (presumably) the clearance required for an unenergised overhead wire is less than that for an energised wire thus avoiding a need to lower the trackbed and having a goodly number of short third rail sections (preferable to dead sections with risk of stranding).

There's an article on this in the December Modern Railways by Ian Walmsley.
It quoted 76 pan drops between Chester and Crewe (that's the number of low bridges) and a journey time longer than with steam.
The gist of it was that discontinuous electrification is useless in the real world.

 

[Eagle]

What's the speed that 377s do at the changeover on the WLL?

 

[causton]

What's the speed that 377s do at the changeover on the WLL?

Lower than linespeed! Feels somewhere around 20mph, but I am unsure...

Edit: Wait, apologies. That's for 378s. 377s stop and do it while stationary.

 

[Holly]

There's an article on this in the December Modern Railways by Ian Walmsley.
It quoted 76 pan drops between Chester and Crewe (that's the number of low bridges) and a journey time longer than with steam.
The gist of it was that discontinuous electrification is useless in the real world.

It would appear then that third rail would be cheaper than OHLE (or a mixture) for Crewe-Chester?

 

[transportphoto]

So it is 2019 and you are on a train from London Waterloo to Weymouth and it is approaching Basingstoke, will the train have to slow down to change to 25KV, or will all trains except the Salibury/Exeter service will have to stop there?

I wasn't aware any South West Trains units were fitted with Pantographs? Aren't they all 3rd rail only?

TP

 

[Eagle]

I wasn't aware any South West Trains units were fitted with Pantographs? Aren't they all 3rd rail only?

TP

No, but they will have to get some in the next few years when Basingstoke to Southampton (and/or beyond, depending on what you read) has its third rail ripped out...

 

[LNW-GW Joint]

It would appear then that third rail would be cheaper than OHLE (or a mixture) for Crewe-Chester?

You couldn't retrofit Pendolinos with 3rd rail.
So while the infrastructure might work, the trains won't (except maybe for Merseyrail EMUs).
Most of the bridges are for farms and would be demolished for AC I suspect.
I can't believe anybody is thinking of 3rd rail beyond Chester, except possibly to Wrexham.

 

[ainsworth74]

I wasn't aware any South West Trains units were fitted with Pantographs? Aren't they all 3rd rail only?

They are fitted for but not with. They would need a Pantograph and transformer to enable them to operate under OHLE and they've been designed with this in mind.

 

[Peter Mugridge]

Lower than linespeed! Feels somewhere around 20mph, but I am unsure...

Edit: Wait, apologies. That's for 378s. 377s stop and do it while stationary.

I've been on at least one ( late running ) 377 that did the changeover northbound at around 30mph a couple of years ago...

 

[pendolino]

I've been on at least one ( late running ) 377 that did the changeover northbound at around 30mph a couple of years ago...

Ssshh, you'll get someone into trouble.