On 24/2/21 11:44 pm, Tony Galloway wrote:
> Hi Dudley

>

> From reading the article I’d  say the paint has sufficient insulating

>  properties to prevent flashover, and maybe to reduce any induction

> effect into the steelwork of the bridge from the high voltage AC. They

> are vague about whether it is a simple insulator or does more than

> that, which might be why it’s “special”.

>

> It also looks like the line voltage is reduced under the bridge to

> allow closer clearance between train roofs and the overhead. 70mm is a

> big reduction in electrical air gap when high voltages are involved.

>

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!.