when lightning strikes

[L&Y Robert]

Thunderstorms forecast for tomorrow (15/09/16) and the day after. Slow moving, could be multiple lightning strikes, says the nice weather girl on the TV. Well, what happens if lightning strikes the overhead? Or the track, perhaps. I never heard of any special measures, but there must be some. Anybody know?

 

[Carntyne]

It certainly has fun with axle counters and trackside equipment!

 

[ainsworth74]

See what blows up and then fix it I believe is the standard procedure

 

[John Webb]

Lightning hitting the OLE will jump across insulators to the earthed portal frame or headspan wires, probably at a number of nearby locations. Insulators and other equipment supporting the OLE may be damaged but not always.

A train body, being metal, acts as a 'Faraday Cage' since it is well earthed to ground through the track so it's particularly safe to be in - but the surges around the train may affect its electronic equipment.

 

[SpacePhoenix]

What if the 3rd rail gets struck in a 3rd rail area?

 

[edwin_m]

What if the 3rd rail gets struck in a 3rd rail area?

Similar I guess, but it's so close to the ground that it isn't very likely.

 

[Greybeard33]

Several Manchester Metrolink substations sustained damage yesterday evening from lightning strikes on the 750V DC OLE. Services on the Bury line are still restricted pending completion of repairs.

 

[Bald Rick]

Most modern signalling has lightning protection. It localises the problem - so there will still be a signal failure, but it doesn't spread to everything else connected. There has been some detailed academic work on how the electrical energy dissipates from the point of the lightning strike, and how that will affect signalling (as the most vulnerable kit). Unfortunately if an equipment room or location case gets a direct hit, there's not much you can do. In the same way that the consequences for your house and all the electrical kit in it are rather dire in the event of a direct hit.

In my days managing signallers, I had an incident where lighting struck the track some distance away a the same time as a siggy was pulling a lever, and he got thrown across the box. He had a sore head the next day, but was otherwise unharmed. I rather hope he picked some lottery numbers that weekend.

 

[Llanigraham]

A friend was manning Onibury some years ago when one of the barriers was struck when it was open. It melted into a lump of ali and plastic and fried all the electrics in the Box.

 

[Deepgreen]

Most modern signalling has lightning protection. It localises the problem - so there will still be a signal failure, but it doesn't spread to everything else connected. There has been some detailed academic work on how the electrical energy dissipates from the point of the lightning strike, and how that will affect signalling (as the most vulnerable kit). Unfortunately if an equipment room or location case gets a direct hit, there's not much you can do. In the same way that the consequences for your house and all the electrical kit in it are rather dire in the event of a direct hit.

In my days managing signallers, I had an incident where lighting struck the track some distance away a the same time as a siggy was pulling a lever, and he got thrown across the box. He had a sore head the next day, but was otherwise unharmed. I rather hope he picked some lottery numbers that weekend.

Given how unlucky he had been, he may have picked one number away from the jackpot!

 

[Tio Terry]

The majority of lightning strikes are intracloud - a strike between clouds - which is not normally of great interest. Its cloud to ground or ground to cloud lightning that causes the damage. Basically, the electrical charge needs to be dissipated so the shortest, lowest resistance/inductance path will always be chosen.

Without going in to great detail the best protection is to provide that low resistance/inductance path to help the discharge. There is a danger from Rise of Earth Potential (ROEP), from the point of strike the charge dissipates with distance relative to ground resistance which can cause problems if there are multiple earths that are not connected together, that's a common cause of equipment damage and failure.

In the UK our designs for things like signalling equipment rooms are nowhere as good as, say, France. They design their equipment rooms to be Faraday Cages - which is the case for HS1 - whereas the UK uses REB's which don't give the same kind of protection. The rule for lightening protection conductors is as straight and as short as possible - no curly pigs tail like connectors, they form inductors - with an earth resistance - rod or mat - of not greater than 4 ohms, the lower the better. Also, do make sure all metallic items along a route - including fences, return conductors, location cases, structures, et al - are all connected together and earthed at regular intervals.

The probability of a direct lightning strike on either the 25kv catenary or 3rd rail is extremely low, they are not easy paths to earth, more likely to be a structure - signal gantry, OHEL support, lineside structure than a non-earthed item.

 

[D6975]

I was on the 1805 Man Picc - Plymouth as far as Bristol on Tuesday. As we left Picc a quite magnificent light show was put on by a huge thunderstorm. We sat at Stockport for nearly half an hour because the Stoke line was stationary due to signalling problems caused by the storm. We eventually got diverted via Crewe, stopping for permission at every signal until we reached the Crewe panel area.

 

[jopsuk]

How often are individual trains hit? Airliners, flying up where most of the lightning is, are stuck on average (globally) once a year.

 

[AM9]

The probability of a direct lightning strike on either the 25kv catenary or 3rd rail is extremely low, they are not easy paths to earth, more likely to be a structure - signal gantry, OHEL support, lineside structure than a non-earthed item.

The problem with 3rd rail is slightly different as traction return, by definition cannot be grounded continuously as it creates DC ground currents that would cause damaging electrolytic corrosion. Because of this, a lightning discharge to the running rails can cause havoc with track circuits and because of parallel conductor paths, also upset the S&T.

 

[Tio Terry]

The problem with 3rd rail is slightly different as traction return, by definition cannot be grounded continuously as it creates DC ground currents that would cause damaging electrolytic corrosion. Because of this, a lightning discharge to the running rails can cause havoc with track circuits and because of parallel conductor paths, also upset the S&T.

Agreed. But the answer is to provide a low resistance/impedance path to earth where ever you can. This will minimise lightning strikes to live or running rails. Structures, signal gantries, metallic footbridges, just about anything you can earth you should earth. Whilst this would not eliminate strike damage it would limit it. Lightning will always seek the path of least resistance.

 

[edwin_m]

How about putting a galvanised earthed metal mast alongside all third rail lines, reaching well above train height, every 50 metres or so on both sides?

[/stirring]

 

[snowball]

In the UK our designs for things like signalling equipment rooms are nowhere as good as, say, France. They design their equipment rooms to be Faraday Cages - which is the case for HS1 - whereas the UK uses REB's which don't give the same kind of protection.

What's an REB?

How often are individual trains hit? Airliners, flying up where most of the lightning is, are stuck on average (globally) once a year.

Just to clarify what you probably meant, I think it's once a year per aircraft, isn't it, rather than one strike a year to some aircraft somewhere in the world?

 

[najaB]

What's an REB?

I *think* it stands for a Remote Equipment Building - basically the cabinets you see by the side of the railway.

 

[snowball]

That looks pretty Faraday-cage-ish to me - a closed all-metal box?

 

[najaB]

That looks pretty Faraday-cage-ish to me - a closed all-metal box?

One of our PW forum members could confirm, but I think a lot of them are metal frames covered with fiberglass (or similar) panels.

 

[SpacePhoenix]

I *think* it stands for a Remote Equipment Building - basically the cabinets you see by the side of the railway.

Will that contain what would otherwise be split between multiple normal signalling cabinets?

 

[Tio Terry]

I *think* it stands for a Remote Equipment Building - basically the cabinets you see by the side of the railway.

Re-locatable equipment building.

You need to look at the construction of the building to determine if it really is a Faraday Cage. You have to prevent circulating currents and the design of the earthing and cable tray systems within the building is also very important. There's a lot more to it that just the outside.