How is OHLE so robust?

[Joseph_Locke]

Dewirements in wind are the typically the result of high mid-span offsets being exacerbated by over-long spans (for the wind speed). And guess what the first thing is that the bean-counters say? "Can we use a longer span to reduce the number of structures and thus the cost ..." We've had to fight long and hard to stop the contractor convincing their client that using maximum span lengths is fine. Of course, headspans are even worse because there is a small degree of interaction between all the wire runs which can lead to stagger change as the headspan sways along track.

 

[Joseph_Locke]

You can occasionally get flashovers.

We tried to get a 13t digger on a flatrol under that very bridge (the lowest wires on the WCML are there) with the same result ...

 

[edwin_m]

British Railways standardised its clearances for 25kV by parking a steam loco under the wire and moving the wire down until flashover occurred, so the effects of steam are taken into account. These are the clearances which have now had to be increased because somebody in the UK standards bodies dropped a clanger when adopting a European standard. However I have a memory from somewhere that steam was banned under Mk3 OLE for some time after it reappeared on the main line in the 70s - does anyone know any more about this? Mk3 equipment did not exist at the end of steam in 1968.

Any deposits of soot etc on the wire would be wiped off by the next passing pantograph.

 

[a_c_skinner]

Was there not an era of steam locos having a yellow diagonal on the cab side showing they were not cleared under the WCML OHLE?

Incidentally do those flashovers trip the supply and leave everything except the culprit at a standstill?

 

[AndrewE]

I wouldn't think so. The exhaust of a steam engine isn't going to be *that* hot, nor is the heat going to be particularly efficiently transferred into the fairly small contact wire. Don't forget that copper is a very good thermal conductor, so any heat added to the wire will quickly be conducted into the rest of the wire. You may get a small amount of expansion, but that is why you have automatically tensioned wires instead of fixed tension. You certainly shouldn't get wire snapping.

I would be more worried about localised corrosion. When it starts it often becomes a runaway process called pitting corrosion (look at the holes in the bottom of an aluminium saucepan with a hand lens to see some!)

 

[furnessvale]

Was there not an era of steam locos having a yellow diagonal on the cab side showing they were not cleared under the WCML OHLE?

Incidentally do those flashovers trip the supply and leave everything except the culprit at a standstill?

I stand to be corrected but wasn't the diagonal more to do with the ability of the fireman to go up above footplate height and put himself in danger on certain loco classes?

AIUI, the breakers go back in after the first flashover, otherwise every roosting pigeon would be stopping the job.

 

[AM9]

Dewirements in wind are the typically the result of high mid-span offsets being exacerbated by over-long spans (for the wind speed). And guess what the first thing is that the bean-counters say? "Can we use a longer span to reduce the number of structures and thus the cost ..." We've had to fight long and hard to stop the contractor convincing their client that using maximum span lengths is fine. Of course, headspans are even worse because there is a small degree of interaction between all the wire runs which can lead to stagger change as the headspan sways along track.

... often followed by a dewirement that not only pulls the offending train's wire down but also pulls down the next headspan that the pantograph encounters meaning that multiple tracks are now involved. I've seen that happen with insulators that aren't fixed to anything solid at either end, flying all over the place.

 

[deltic08]

It is factual posts like this on #13 that makes railforums worth staying with. An island of cerebral pleasure in a sea of froth. Thank you Bald Rick.

 

[deltic08]

Fascinating thread. I will a fuller reply on Monday. Some great material science goes into the design of OHLE systems. The contact wire is not pure copper - too soft and prone to creep under load so used to be alloyed with Cadmium but now silver is the alloying element.

I thought it was alloyed with aluminium. No wonder electrification has become so expensive.

 

[GRALISTAIR]

I would be more worried about localised corrosion. When it starts it often becomes a runaway process called pitting corrosion (look at the holes in the bottom of an aluminium saucepan with a hand lens to see some!)

Copper is particularly bad for pitting corrosion. Copper is usually alloyed with tin and on certain series silver. Aerospace - airplanes use aluminium alloyed with about 2.7 % copper

 

[6Gman]

Was there not an era of steam locos having a yellow diagonal on the cab side showing they were not cleared under the WCML OHLE?

Incidentally do those flashovers trip the supply and leave everything except the culprit at a standstill?

I believe that standards were changed around the time that wiring reached Rugby (bear in mind that the WCML was effectively wired from Liverpool/Manchester to London) and that clearances were reduced south of Rugby (and possibly in the West Midlands). However some steam loco classes had higher boiler fittings and therefore posed a higher flashover risk - these received the yellow cab flash which basically meant "not South of Crewe".

Quite separately the top bracket for headcode lamps was repositioned lower on some locos (not sure how widely) for staff safety. Mind you, I always thought firemen placed themselves at risk trimming coal on the tender and perhaps when watering from columns - all those chains and (wet) piping.

 

[eMeS]

...

AIUI, the breakers go back in after the first flashover, otherwise every roosting pigeon would be stopping the job.

Are you serious? What's the capacity of a pigeon?

 

[Bald Rick]

Are you serious? What's the capacity of a pigeon?

Surely the better question is "What's the capacitance of a pigeon?"

To which the answer is "not enough"

 

[eMeS]

Surely the better question is "What's the capacitance of a pigeon?"

To which the answer is "not enough"

Thank you! I'm getting rusty in my old age.

 

[Bald Rick]

Has does our failure rate compare with other European countries? I imagine many of our lines get more intensive use, plus the UK is a very windy country which doesn't help

Not sure there is comparative data available for failure rates. However the OLE does come down in other countries; indeed the wires were down on the LGV-Nord not long ago. Naturally we in the UK won't hear about overhead line issues in Europe.

It's not just limited to Europe - my historic travels on the US NE corridor have a 100% rate of being affected by a major OLE failure.

 

[Cowley]

I stand to be corrected but wasn't the diagonal more to do with the ability of the fireman to go up above footplate height and put himself in danger on certain loco classes?

AIUI, the breakers go back in after the first flashover, otherwise every roosting pigeon would be stopping the job.

I’m not sure if it was just that. I think it was also to do with reminding firemen not to be swinging the fire irons (that were stored in the tender lockers) above the height of the cabs?
Some of the equipment they used to break the clinker down on the grate was metallic and many feet long...

 

[deltic08]

The series 1 equipment on the GWML uses CuAg, although it is worth pointing out that it is typically only around 0.1% Silver, leaving it at ~ 99.9% Copper

That is still a lot of tea pots and candelabra. How many tons does one wire run weigh?

 

[Taunton]

Given that solid bar overhead is installed in "difficult" places like the Severn Tunnel, I wonder why it is not used throughout, overcoming some of the above issues.

15-20% caused by extreme weather outside the design capability of the system (usually very, very hot temps, or hurricane force winds)

Would have expected icing in freezing weather to lead here.

 

[Shaw S Hunter]

Copper is particularly bad for pitting corrosion. Copper is usually alloyed with tin and on certain series silver. Aerospace - airplanes use aluminium alloyed with about 2.7 % copper

And in increasingly smaller quantities as composite materials take over. In theory these will have lower whole-life costs thanks to being lighter but the proof of that will only come when such aircraft come to be scrapped and recycled...

 

[Cowley]

The whole OLE system wears over time.

1) the contact wire wears over time. I have in my possession a short length of original contact wire off the GEML (taken out in 2011), and it has a cross section slightly less than a semi circle (it was circular when installed in 1948/9). Contact wire does break on it's own although that is relatively rare. Incidentally the new contact wire going in on the GEML is about 75% of the cross section of what is coming out. The original was installed for 1500v DC and was necessarily thicker to deal with increased current. Incidentally (2) I'm reasonably sure that the contact wire on all running lines of the WCML was changed south of Crewe, not just the fasts.

2) the catenary (support) wire also wears. Obviously not through pantograph contact, but usually at mechanical contact points ie where it is attached to registration arms, anchors and the like. It is usually a stranded wire (as opposed to a single thick 'rod' for contact wire) and the typical failure is of some of the strands, which causes additional stress on what remains, and eventually they all go.

3) the most common thing to break is a dropper. This is the short lengths of wire that connect the catenary wire to the contact wire. These break routinely, normally at the connectors. I should think that one comes off somewhere on the network every day. Fortunately you can get away with the odd one being broken, so it's not an urgent job. Unless of course it is hanging below the contact wire and then it has to be removed ASAP (10 minute job once someone gets there and has the juice off)

4) other key components that fail are
a) insulators, particularly older designs which are susceptible to water penetration and frost shatter.
b) pulley wheels, usually due to lack of maintenance (lubrication); although these don't cause a failure themselves they can trigger something else, cause the wire to go out of alignment which then causes a pan to fall off
c) registration arms. These can occasionally break / bend badly, but that is pretty rare.
d) insulation failures. Where live components are closer to structures etc than the standard, they must be insulated. This insulating can perish over time (particularly if under a road bridge that has regular salting), which can then cause a flashover which will burn through the relevant component in a trice.
e) all the connections that hold it all together, basically nuts and bolts. The OLE system is designed to be flexible to deal with thermal expansion, wind, snow loading etc. Some of the connections are thus designed to travel, and if they get too tight (through corrosion, wear, being particularly dirty, lack of lubrication) then they can seize, and cause part of the system to lose that flexibility. This is why 'problems with the OLE' are more likely in very cold or very warm weather.

Finally the OLE support structures do wear, albeit often in unseen ways. Some obviously corrode, although that's not usually a problem. The main issue is failure of the structure foundations, particularly in places where they are 'stray' currents around from heavy industry and particularly other (DC) railways. this can cause the bolts between the concrete foundations and masts to corrode rapidly.

All of this is inspected regularly, and some components are changed on an as required basis. Often it is decided to do a 'campaign' replacement of some components on a certain stretch of line, eg all insulators. I've done that myself in a number of places.

In terms of failure of the OLE itself, as opposed to the distribution equipment (transformers, substations etc) my guess for failure causes, based on personal experience, is as follows:

20-25% caused by the train, usually pantograph failure, but sometimes other parts of the train hitting the wires

20-25% caused by 'outside parties' coming into contact with the live part of the system, thence causing failure through mechanical failure or electrical flashover. Often the 'outside party' is of avian origin, landing or flying at just the wrong place / wrong time. Pigeons seem to be particularly conductive.

20-25% caused by the failure of one or more component, broadly split between a failing of maintenance, or because of a hidden flaw in that component.

15-20% caused by extreme weather outside the design capability of the system (usually very, very hot temps, or hurricane force winds)

The rest fall into the category of no-one is really sure.


Hope this helps.

Bald Rick - I saw your post earlier today and thought maybe I need a bit of time to ingest it properly.
I’ve had that time this evening and you’ve considerably improved my understanding of the subject.
Thanks for putting in the time to explain it to us ‘amateurs’. Much appreciated, and very interesting.

 

[goblinuser]

You can occasionally get flashovers.

I've always wondered how steam locos blast steam at high voltage cables from 30cm away and don't electrocute everyone. But it's never occured in over 70 years of steam operating under electrics so it seems to be fine.

 

[Cowley]

Is there a risk of electrocution to the driver or passengers when this happens? I've always wondered how steam locos blast steam at high voltage cables from 30cm away and don't electrocute everyone.

Very interesting footage Harbonite. I hadn’t seen it before.

 

[AndrewE]

Are you serious? What's the capacity of a pigeon?

Surely the better question is "What's the capacitance of a pigeon?"

To which the answer is "not enough"

I would have thought the question is how much current flows up the outside of the bird before it is vapourised, rather than how much charge it can absorb?

 

[cambsy]

I think the reason when a steam loco causes a flashover it doesn’t electrocute every one on loco is the Farrady cage, which means the electricity is earthed or something like that, someone else may know more in depth about it, have seen it proved on tv programs where a car is put in a lightning rig and zapped with someone sitting in it and it looks spectacular and can mean the cars electrics are fryed , the occupant may need a change of pants, but the occupant gets out fine.

 

[furnessvale]

I would have thought the question is how much current flows up the outside of the bird before it is vapourised, rather than how much charge it can absorb?

OK, Maybe they don't blow the breakers but they certainly go "pop" when they try to headbutt a bridge whilst sitting on the wire!

Incidentally, my original point still remains to be corrected if wrong. I believe the breakers go back in, I think twice more, after an initial blow before giving up and waiting for human attention.

 

[edwin_m]

I've always wondered how steam locos blast steam at high voltage cables from 30cm away and don't electrocute everyone. But it's never occured in over 70 years of steam operating under electrics so it seems to be fine.

The steam loco is made of metal and its wheels provide good electrical contact to the rails, so if a flashover happens it is just between the wire and the chimney. Even if someone is standing on the platform and touching the train there is no significant voltage between the two.

Incidentally, my original point still remains to be corrected if wrong. I believe the breakers go back in, I think twice more, after an initial blow before giving up and waiting for human attention.

I believe this happens at least once. See this RAIB report for what happens (or should happen) when there is a short circuit - this is HS1 but I assume Network Rail is similar.
https://www.gov.uk/raib-reports/overhead-line-failure-at-st-pancras-international

After 6 seconds the feeder station circuit breaker reclosed automatically, as it was designed to do, and then opened owing to the passage of an excessive current to earth through the wire resting on the train.

 

[Bald Rick]

When the system trips (and tripping is a very regular occurrence), what happens next depends on the system that controls it, very recent history, and if anyone is working on the kit nearby.

Some systems reclose the breakers automatically, some need manual intervention from the electrical control office. Typically the breakers will be closed twice in succession (ie original trip, close, repeat trip, close) but not a third time. On the third occurrence the power stays off until the section has been inspected.

If anyone has a permit to work on any electrical sections abutting to, or near the area of tripping (eg on an adjacent line) they will not be closed at all until the permit holders have been contacted and confirmed that all staff and equipment are clear of the section that has tripped. This was an issue at London Bridge during the rebuild, as there was always someone working on parts of the track, so any tripping in the area needed a rapid response to check all was well.

 

[Bald Rick]

That is still a lot of tea pots and candelabra. How many tons does one wire run weigh?

A typical length of contact wire on plain track will weigh a couple of tonnes or so. Add in the catenary wire, droppers, and current carrying droppers you can quadruple that.

But it's not the weight that requires all the support, it's the tension required in the wire to limit the sag over the wire run (or to keep it level for high speed railways), combined with the absolute necessity to keep the wire positioned in the right place laterally to the track.

 

[Bald Rick]

Given that solid bar overhead is installed in "difficult" places like the Severn Tunnel, I wonder why it is not used throughout, overcoming some of the above issues.

Would have expected icing in freezing weather to lead here.

Conductor bar actually has contact wire attached to the underside of it. It suffers from thermal expansion more than just wire, as it can't sag. That's not a problem in tunnels, or short lengths, which are the only applications in the UK that I'm aware of. Finally it is very, very expensive, as being much heavier it needs more support structures to hold it up.

Icing is rarely an issue. It causes trippings, and lots of them, but rarely brings the wires down.

 

[Domh245]

A typical length of contact wire on plain track will weigh a couple of tonnes or so. Add in the catenary wire, droppers, and current carrying droppers you can quadruple that.

In the context of Copper Silver alloys, it is only the Contact wire that has that composition though, is it not? The droppers, catenary wire and other wires are usually other compositions.