> On 25 Feb 2021, at 9:41 am, Matthew Geier matthew@...> wrote:

>>

> The UK used to have 6.5kv under bridges. I understand it's all been removed as the trains needed voltage change equipment (tap changing transformers) and they found other ways of dealing with the clearances for 25kv AC.

>

> The article is referring to the western line electrification around Cardiff. It's possible where the clearances are really tight, they have a dead zone and just have a contact bar so the pantograph doesn't have to drop. All the trains are bi-mode so they don't have to worry about getting 'gapped'.

>

> Their talk about compact surge arrestors seems to be to indicate the voltage isn't lowered under the bridge or has a dead section, but they are making dammed sure it can't spike any higher.

>

> There are places on the GWR electrification already where they have dead sections and one where the wire changes height so suddenly to go under a low bridge that is rather close to a level crossing (where maximum height is desired) that there are speed restrictions on electric traction that are not applicable to diesel traction.

>

> I've seen a driver comment that you can get though that section faster by dropping the pantograph and coasting at diesel line speed!.


That’s interesting that the voltage drop isn’t done for tight clearances any more, and they are confident the surge protection is sufficient to ensure safety. Back in the 50s one of the tests for clearance with 25kV overhead that BR did was to get a really filthy, sooty fire going in a 9F, with solid black smoke, and lowered the hot 25kV wire till it flashed over through the smoke to the stack. The wire got under 3 inches (~70mm) before it arced over, and the smoke being emitted was much dirtier than any competent fireman would allow. They also must be pretty confident the paint is applied competently to ensure it works adequately too.

Coasting trains through gaps in the wire isn’t unusual. On the US Northeast Corridor the former New Haven 11kV 25hz electrification had long gaps at opening bridges and trains would coast through at track speed with the pantographs fully extended. I thought this had been eliminated when the electrification was modernised to 12.5/25kV 60hz with the extension to Boston, but I saw a video un Youtube recently that showed the gaps still exist at the bridges. There was a time when electric trains here would drop pans and coast through worksites where the overhead was being worked on, but it’s not done any more because that work is done on weekend closedowns now, under full track possession.

Tony