Forth Bridge: how would you electrify it?

[K.o.R]

Umm why 3kV? Is there any particular reason why that system should be used to electrify the Forth Bridge?

I ask this because back in 1955 the British Railways Modernisation Plan actually called for all OHL systems to use the 25 kV AC standard BUT on the Great Eastern for example, parts of the route was instead electrified at 6.25 kV AC due to low clearances under bridges until they found out that it wasn't a issue after all!

So why not use the 6.25 kV AC standard again to electrify routes where 25 kV AC is going to be rather difficult, note I said difficult not impossible.

OR even go one further and look at our Germanic friends and use the 15 kV AC to electrify these routes which as above will be difficult to electrify at 25 kV AC but again not impossible.

Wasn't 6.25kV chosen because some locomotives had 4 sets of transformer coils, and could use 25kV by switching them into series, or 6.25kV by switching them into parallel?

 

[Aictos]

No specific reason why 3kV other than that it's in use on the Continent so it should be easier to find ready-to-go equipment. Any voltage reduction reduces the clearance required, and 3kV seems like a happy medium before conductor thickness starts becoming and issue.

Yes but by that argument you could also use 15 kV as that is also in use on the Continent so like your example ought to make it easy to find if I can quote you "ready-to-go equipment".

 

[NotATrainspott]

15kV on the continent is at 16.7Hz, not 50Hz. It's a technology which was useful in the early days of AC OHLE for reasons unrelated to electrical clearance but is now technically superseded by 25kV.

If the issue is clearance between the pantographs and the diagonal cross-members it might not matter how low the supply voltage is.

 

[snowball]

If the issue is clearance between the pantographs and the diagonal cross-members it might not matter how low the supply voltage is.

I think it's electrical clearance, not physical clearance, and the lower the voltage, the less the gap that can be jumped.

But adding sheets of plastic to the girders in question still seems to me a simpler solution.

 

[najaB]

Yes but by that argument you could also use 15 kV as that is also in use on the Continent so like your example ought to make it easy to find if I can quote you "ready-to-go equipment".

Indeed, it is and could be. But if the name of the game is reducing electrical clearance requirements, the lower the voltage the better.

But adding sheets of plastic to the girders in question still seems to me a simpler solution.

It is and should be the first option investigated. However, given that it's an exposed location subject to salt spray it may be that 'they' don't find that to be an acceptable solution.

 

[edwin_m]

The clearance reduction at lower voltages is in fact very modest, and earlier posts suggest the problem is more about the ends of the pan fouling the diagonal girders which would be an issue at any voltage.

It's quite possible that within the next decade or so, new electric trains will have as standard a battery or other auxiliary power source capable of moving them a few miles or powering the auxiliaries for several hours. The "all-electric" versions of the IET fleet already have this in the form of a diesel engine. If so then the Forth Bridge problem would simply be solved by omitting the wiring completely and ensuring that no older electric units use it.

 

[deltic08]

However, given that it's an exposed location subject to salt spray it may be that 'they' don't find that to be an acceptable solution.

No more exposed to salt than 25kv up the coast of North Northumberland and over the Border north of Berwick.

The only showstopper I can see is ends of the pan fouling the bracing girders and nothing else. We are still going round in circles on this one speculating. Someone somewhere surely knows if this is possible or not as it must have been measured by now.

 

[deltic08]

It's quite possible that within the next decade or so, new electric trains will have as standard a battery or other auxiliary power source capable of moving them a few miles or powering the auxiliaries for several hours. The "all-electric" versions of the IET fleet already have this in the form of a diesel engine. If so then the Forth Bridge problem would simply be solved by omitting the wiring completely and ensuring that no older electric units use it.

I think SR have ordered enough 385s to last them more than 10 years so no electrification into Fife before then unless there is massive growth needing a follow on order of modified EMUs.

 

[najaB]

No more exposed to salt than 25kv up the coast of North Northumberland and over the Border north of Berwick.

True. But they don't have reduced clearance to factor into the equation as well. Plastic with a film of conductive brine on it is less of an insulator than clean(ish) plastic.

 

[apk55]

Maybe a solution is to have trains with a narrow pantograph head. If units were fitted with two photographs they could swap to the narrow one for crossing the bridge.
I have seen on continental railway stock some twin phonograph units that fit in the space of one so should not be a problem to convert existing stock.

 

[najaB]

Maybe a solution is to have trains with a narrow pantograph head. If units were fitted with two photographs they could swap to the narrow one for crossing the bridge.
I have seen on continental railway stock some twin phonograph units that fit in the space of one so should not be a problem to convert existing stock.

This suggestion has come up a couple of times already on the thread. It's definitely an option, but if you read the earlier posts it does come with some drawbacks.

 

[deltic08]

Maybe a solution is to have trains with a narrow pantograph head. If units were fitted with two photographs they could swap to the narrow one for crossing the bridge.
I have seen on continental railway stock some twin phonograph units that fit in the space of one so should not be a problem to convert existing stock.

Each unit would have two photographs? That is a very cheap conversion.
Twin phonographs is a different kettle of fish.

 

[AndrewE]

Each unit would have two photographs? That is a very cheap conversion.
Twin phonographs is a different kettle of fish.

One in each first-class coach, of course!