Some of this is a bit garbled, isn't it? For example,
from Railnews:
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:
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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