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Author Topic: Tim Harris speaks about Severn Tunnel  (Read 5125 times)
infoman
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« on: June 28, 2019, 07:44:36 am »

Tim Harris speaking about concerns for the electrification as regards to "the straps"

through  Severn Tunnel on Radio Bristol.

Listen again approx 07:35am in, on Friday 28 June 2019.
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johnneyw
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« Reply #1 on: June 28, 2019, 10:15:04 am »

Just had a listen. Looks serious. The straps were expected to last 25 years and are now failing less than 4 years after installation. Salty, humid conditions (you don't say!) are blamed. The report said this could have a severe impact on electrification to Wales.

Edit:  IIRC the straps referred to were "earthing straps".
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Oxonhutch
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« Reply #2 on: June 28, 2019, 01:09:37 pm »

Could you please post a link?  Thanks.
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johnneyw
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« Reply #3 on: June 28, 2019, 01:40:17 pm »

It's quickly available on the news at the top of the hour on BBC Radio Bristol through the iPlayer. It's been on all the bulletins this morning. Not sure about a precise link.
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grahame
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« Reply #4 on: June 28, 2019, 01:46:44 pm »

Could you please post a link?  Thanks.

https://www.bbc.co.uk/sounds/play/p07c22hk

1 hour 35 minutes and a few seconds in.

Earthing straps intended for 25 years but decaying after 4.  Expert suggests that a compromise solution is available in running the trains on diesel for 6 or 7 km.  "You would not notice" switch from electric to diesel, and you'll still get electric trains in Cardiff.   However, it's said that Network Rail are also looking for alternatives.
« Last Edit: June 28, 2019, 01:51:46 pm by grahame » Logged

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Rob on the hill
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« Reply #5 on: June 28, 2019, 02:08:54 pm »

https://www.bbc.co.uk/news/uk-wales-48621944
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stuving
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« Reply #6 on: June 28, 2019, 06:38:49 pm »

Some of this is a bit garbled, isn't it? For example, from Railnews:
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However. it has emerged that recently-installed earthing straps connected to the overhead conductor rails inside the tunnel are already breaking down. Network Rail had estimated that they would be all right for 25 years.

Well, if the overhead conductor rail really is earthed, there won't be any electric running anyway!

I guess the issue is earthing straps (or bonding) of all the steel supports bolted to the roof. All OLE has to have its steel structures securely earthed, using its own piles and wires alongside the track joining all the bits together, plus bonding to the rails as well. However, these roof supports are extra bits, and there are a lot of them - at a maximum of 12 m apart x 12 km that's a thousand or more, so replacing that many would be a big job. Obviously the same goes for replacing the supports, which as they look like galvanised mild steel may also degrade quite fast.

The BBC words refer to two quite different issues:
Quote
Earth straps - a safety feature on the overhead connectors holding up the power line - were corroding in the salt water environment within months when they are supposed to have a 25-year lifespan, meaning the line through the tunnel could not be electrified safely.
Media captionInside the four-mile-long underwater Severn Tunnel

It means the electrification system within the brickwork tunnel - the longest underwater tunnel in the world for more than 100 years after it opened in 1886 - struggles to maintain voltage and regularly trips.

Network Rail contracted experts from Swiss company Furrer and Frey, specialists in developing power cables inside tunnels, to design a solution but none have so far worked in the salty climate.

If the material of the straps corrodes, the first thing I'd expect to see is they fail a visual inspection. That would make it unsafe to energise, but wouldn't make it trip. Now, I'd guess the damp makes the insulators disinclined to insulate, and too high a leakage might cause the power to trip. But for slightly dodgy earthing to make that worse needs something extra to be involved.

I did wonder, when this was first mentioned some time last year, if Furrer + Frey might know a lot about tunnels through Alps, but still be short of experience of undersea ones (due to the lack of sea in Switzerland).

If all the right information is given to the right people (metallurgists, for example) you'd think that should produce an answer to the problem. Put that way, there is an implication that what was installed wasn't chosen by such a process - perhaps by picking the standard item most likely to cope.
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Timmer
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« Reply #7 on: June 29, 2019, 12:58:23 pm »

The wrong kind of humidity.
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Electric train
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« Reply #8 on: June 29, 2019, 07:33:00 pm »

The OLE structures in the tunnel will be galvanised steel, the hard drawn copper alloy contact wire is in an aluminium "conductor beam" this is supported from the structures by polymeric insulators.  All of the OLE structures will have a multi strand aluminium between them as a bonding conductor (in layman's terms and earth wire), the aluminium bonding conductor will be fastened to the structures by a stainless steel clamp.

It is quite likely that there has been a higher level of electrolytic action between the dissimilar metals, zinc, stainless steel, aluminium than expected causing the damage.  Its possible not helped by the fact that the contact system has been bonded out for the last 4 years allowing circulating currents caused by the electrolysis to flow.

I have visited many tunnels in my railway career including Seven Tunnel a number of times, whilst all Victorian tunnels are damp and dank places Seven Tunnel in in a league of its own in regards dampness
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« Reply #9 on: June 30, 2019, 02:49:47 pm »

It is quite likely that there has been a higher level of electrolytic action between the dissimilar metals, zinc, stainless steel, aluminium than expected causing the damage.  Its possible not helped by the fact that the contact system has been bonded out for the last 4 years allowing circulating currents caused by the electrolysis to flow.

It's been along time since I did A level physics but could one have replaceable sacrificial  anodes similar to those on my cousins steel hulled narrow-boat. He's got through about 3 sets of 4 in 20 years.
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broadgage
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« Reply #10 on: June 30, 2019, 04:43:46 pm »

Presumably the damp and corrosive atmosphere in the tunnel was not known about in advance, and could not have been found by prior inspection ?
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It has space for cycles, surfboards,luggage etc.
A 5 car DMU is not a proper inter-city train. The 5+5 and 9 car DMUs are almost as bad.
stuving
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« Reply #11 on: June 30, 2019, 06:26:26 pm »

Presumably the damp and corrosive atmosphere in the tunnel was not known about in advance, and could not have been found by prior inspection ?

If you think through what "prior inspection" would involve, I suspect that's right. As ET has pointed out (including when this topic came up before) that tunnel is know to be as bad as it gets for corroding metals. And among tunnels with OLE inside, let's say it was known beforehand to be worse than anywhere done before. What next?

Assume Plan A is to choose, among kit that's available to buy, whatever has the best performance in this respect. (I know that in practice it may have been F+F's system or nothing, but put that to one side for now). Is there a better plan B? You'd need to quantify how much worse this tunnel is than the standard bad tunnel the design was based on ... if it was. What do you sample? Where? How do you test it?

Ultimately, I suspect the answers would be of limited use, and you'd end up needing to do some full-scale tests - installing lengths of OLE to see what happens, ideally of several possible designs. Then, of course you wait and see - for at least ten years to show that Plan A would have been OK ten years ago. Of course you'd find out quicker if you need a new system. So ask the boss, "do we have ten years?"

I'm beginning to see the attractions of plan A.
 
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Oxonhutch
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« Reply #12 on: July 01, 2019, 07:05:20 am »

It's possible not helped by the fact that the contact system has been bonded out for the last 4 years allowing circulating currents caused by the electrolysis to flow.

I think ET put his finger on the main driver. That was not part of the original design which envisaged electric trains into South Wales by now.
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Adrian
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« Reply #13 on: July 03, 2019, 07:56:26 pm »

Earthing straps intended for 25 years but decaying after 4.  Expert suggests that a compromise solution is available in running the trains on diesel for 6 or 7 km.  "You would not notice" switch from electric to diesel, and you'll still get electric trains in Cardiff.   However, it's said that Network Rail are also looking for alternatives.

Running IETs on diesel through the tunnel might be possible, but it would rule out any possibility of using 387s as crowd-busters on event days in Cardiff.
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