This device is used to bond sensitive brushes & other infrastructure, where a direct connection to traction earth is not permitted.

In normal conditions the device is an insulator, and isolates the bridge from the railway. But if the voltage in the bridge rises above a certain point, electricity jumps the gap & the fault is detected.

They were widely used historically, but their weakness is that they have a ltd number of uses before becoming ineffective, and there's no way way to check them.

These are also commonly known as "Soulays", after the French company that makes them. A Soulay is a good example of a Voltage Limiting Device (VLD), a family of devices that also includes Surge Diverters (more on them down Thread) and Non Linear Resistors.

Of course, once I'd found one, they kept popping up everywhere e.g. right out the window at South Hylton... I was being trolled by Spark Gaps.

A more sophisticated device that does a similar job is the surge arrester aka the surge diverter.

In 2019 a certain Scottish railway engineer cheekily posted this video and asked what was unusual about it...

It can't have escaped the notice of railway-adjacent people in UK that a lot of work is currently going into reducing the cost of electrification here. There are lots of different facets to this. One v large cost is when you need to reconstruct a bridge because it is too low.

Think about the air gap between the roof of the train & the bridge. Into that gap we must fit 25kV live parts. We must have sufficient air gap above AND below live parts to prevent flashover, either to bridge or to train

If the air gap isn't big enough, you will get a flashover. For reference, a "good" air gap would be 270mm. BUT: there is a big misconception that this air gap there for 25kV withstand: it isn't. The air gap needed to withstand 25kV is much smaller.

So what's going on?

The air gap is there for LIGHTNING. Lightning loves OLE, it provides a lovely low impedance path to earth and there's loads of it laid out across the countryside, sticking up into the air screaming "COME AT ME BRO"

Once lightning hits and breaks down the air gap (or flashes across an insulator) you have an electrical arc; traction current will then flow across this path. So even once the lightning dissipates, you still have a traction fault to deal with.