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Discussion in 'Infrastructure & Stations' started by Edders23, 1 Dec 2019.
If the trains drag the voltage down then the current through any electric shock would reduce by the same proportion. But it's only about 30% maximum so unlikely to make much difference to the outcome.
The actual flashover distance is only a few centimetres but understandably standards and procedures require a much greater separation. People standing on platforms may be closer than the latest standards require for new installations, re-design due to meet these standards being one of the reasons for delay and extra cost to recent electrification. However there's never been any case of a 25kV flashover to someone on the ground or a platform as far as I'm aware, except for one where the wires had partly fallen but had not touched the ground which would have cut the supply: http://www.railwaysarchive.co.uk/documents/RAIB_SuttonWeaver2014.pdf
OK, if you are being pedantic, the third rail officially carries 'low voltage'. In power distribution terms, low voltage is in the range 120VDC -1500VDC whereas 25kV is 'medium voltage'. If you touch it the name is irrelevant though - it's dangerous, and the chances of people touching it are infinitely higher than wires at least 4165mm from the rail/crossing road surface. Irrespective of the voltage on the wire being 33 times higher than the 3rd rail. Even if you take the extreme case of the peak voltage at the maximum permissable ac line voltage (29kV), it will be just over 41kV which will still only initiate an arc over a distenace of less than 50mm in air, so at 2.5 metres with a possible 1 metre additional arm reach, the tallest men in the world, (if they were stupid enough) couldn't even deliberately touch the conductor wire.
As a rule of thumb, electricity will flashover at a rate of about 1mm per kV in air. Much of the additional clearance is because OLE is not rigid so there are dynamic clearances to maintain.
I don't think anyone is denying that third rail is more dangerous than OHLE, it's whether the additional risk means that we can't have any more
After all when I walk down the street there's a risk that a vehicles might leave the road and mount the pavement, but you wouldn't ban cars because of it.
Having seen all the over-reaction to 3rd rail, here and elsewhere, in recent times, it does seem to me that there have been a number of recent incidents with 25Kv, without an equivalent number of 3rd rail ones. One could almost say that the ORR have got their ban the wrong way round.
Not a first for the ORR, of course.
The inherent dangers of having lethal voltages at ankle level where even trained staff suffer life-changing or even life-extinguishing injuries may for a while enjoy grandfather rights inherited from a less enlightned age, but to exacerbate the risk by installing more would be unacceptable to the majority of the population.
First, the incidents where a person has ignored or defeated the clear protection measures provided, should not be categorised in the same way as accidental events. I don't have actual figures, but I would wager that the true incidents of unintentional contact with high voltage (both 3rd rail and OLE) show OLE to be far safer than 3rd rail. Typically, although a mass evacuation might be classed as trespassing, some causes may ultimately be regarded as likely to provoke one. In those cases, if the automatic removal of 3rd rail power worked, the result would be far more widespread disruption that with OLE where the power is rarely removed. AFAICT, the only case of truly accidental electrocution by OLE has been where a driver has not seen sagging wires that by chance hadn't shorted to ground causing power removal.
Only the official record of investigated events should be used to may judgements about the relative safety of the two systems. Wiring shocks/burns seem to make better headlines so news media reports are notoriously poor sources of facts.
I don't think that third rail is unacceptable to the majority of Londoners, who actually enjoy two live rails(+420V, -210V) rather than one and are having extensions built and proposed.
All that's really wanted in the SR are the Wokingham- Ash and Shalford Jn - Reigate gaps filled. Uckfield is probably a battery job and Ashford could be 25kV (as could Pirbright Jn - Weymouth!).
All movement poses risk - you can only avoid it by avoiding life.
I’m sorry if someone thinks it isn’t appropriate to have this discussion in this topic. My opinion is that if there’s something dangerous it’s better to talk about it, wherever the conversation may lead. It’s interesting and believe it’s just trying to take something good out of this horrible incident.
Well, the user below yours seems to be denying that!
I think that in this case you have a tested and verified system which could replace a (slightly?) more dangerous one. Why not?*
It’s not like having an OHLE will make UK the land of innovation and mysterious tech. These “wires and poles” have been in use in most EU countries since the beginning of last century, and since we’re still going for it, 100 years later, I believe we can safely say that it works and it’s not particularly dangerous.
*There sure are reasons why not but I don’t think these are safety related.
It isn't just a safety issue. 750VDC is much less energy efficient than 25kV (however the train collects its power). The infrastructure to maintain a usable supply to modern trains is both extensive and inefficient, - and that is with trains deliberately crippled to prevent them drawing full power needed to get the full performance. Thus the route is saddled with lower performing trains and limits that frequently restrict the number of trains working in a locality, (not anything to do with signalling limitations). High current DC (33 times that drawn for an equivalent power on a 25kV ac line) causes many anodic corrosion issues, and requires complex grounding installations at stations where domestic ac supplies are interfaced with remotely supplied traction power.
Much of the infrastructure on 3rd rail routes requires replacement, and will be replaced with OLE compatible structures, so even the often used arguments about OLE not fitting into 100 year old tunnels and bridges doesn't really hold, so despite the shelving of the Electric Spine project, OLE will graduallly reduce the 3rd rail area until it becomes a series of islands whereupon it will eradicated.
We all know why not; well, all except those who live in a world of make believe. OLE is massively expensive, we can't afford it and Network Rail have demonstrated that they can't be trusted to deliver an OLE project on schedule at a sensible price. That was the reason MML electrification was cut short.
Third rail, and on London Underground third and fourth rail, is used by literally millions of people every day. As a Londoner I assure you we don't use it with fearful trepidation. We don't cower and tremble while saying our final prayers. We use it without a moment's thought. The ORR and a handful of rail enthusiasts might froth and splutter about, say, the North Downs Line being giving its long overdue third rail electrification, but people in the Home Counties would be calm and happy.
Apologies if this has already been said but whether the source is DC or AC, it only takes milliamps to stop a human heart. Given that most household appliances are fused at 13a and I only need 50mA to potentially cause me death, I'm not likely to believe 750 V DC or 25kV AC are any different in terms of risk to life.
When I left school many years ago, I worked in a Telephone Exchange where equipment used 50v however a watch strap or finger ring would have quickly melted if caught on the bus-bars and within the instance of blowing the fuse of significant size. So, the message was, Volts jolt, mills Kill!
The ORR relies on the Electricity at Work regulations 1989 as a legal justification to prevent more 3rd rail. Yet the Jubilee Line was built 10 years later on the surface over quite some distance from North Greenwich to Stratford, seemingly without issue.
I think the vast majority of urban metro systems round the world select 3rd rail running, including new lines and new systems.
The DLR likewise has extended their 3rd rail system recently without issue. Although under-running it is of course still exposed, and the collector shoes now pointing upwards, not down, on the opposite side of the train are exposed, all live.
A county council employee was killed after misguidedly running a metal tape measure over the side of an overbridge on an electrified line and a young lad died of injuries caused by getting a 25kV jolt due to carrying his carbon fishing pole over his shoulder as he crossed the Lea Valley route at (I believe) Slipe Lane crossing. I also seem to recall that someone firing a steam special got an arc travel down the shovel he was wielding. I'm sure there are many more.
Its been interesting to see the amount of "one upmanship" in post relating to 3rd rail vs OHLE, and the electrical theory that goes with it. Can we just simply agree that the railway is a dangerous place to be, and that irrespective of it being 3rd rail or OHLE, coming into contact with electricity used in the railway environment will seriously damage your health.
To try and get this back on topic, has there been any update on the poor person in question, or how he came to be in contact with the OHLE.
The point of that post is that I was clarifying the distance that an arc might typically occur. The reason for the safe distance being in metres is to do wioth dynamic changes in the gap (e.g. wind, oscillation from pantograph pressures, etc.), and more relevant to this thread, intrusions into the safety zone. Clearly climbing onto a train roof qualifies for that but there are more subtle risks such as passengers ignoring rules about carrying long objects high above platforms or dangling things over bridges. In those events, the actual arcing zone can be penetrated causing an arc or even direct contact. 750V would need to be as good as touched (i.e. less than 1mm) but the nature of 3rd rail electrification is that such an event is much more likely.
It is only dangerous if abused
I like to think I've been more knowledgeable than most about railways all my life, but have never encountered any such rule stated to passengers anywhere.
Should I believe that a 3rd rail full installation is cheaper? Are there any numbers around?
I've been on a station where there were some children with helium balloons and an announcement was made asking them to keep them under control due to the live wires.
It's just about acceptable to discuss safety here, but can we keep cost issues for one of the many other threads that have discussed it already?
I’m not sure about that. Most metro systems I see around the world have OLE - often ‘low voltage’ D.C., but the conductors are in the air. Almost every tram system for a start.
For some reasons the incidents that happen on D.C. don’t seem to feature so prominently in mainstream news. I’m aware of at least two fatalities of trespassers on the third rail network in south London in the last few months. Neither have made it to these pages to my knowledge. I don’t know why that is; I have two theories:
1) sadly, the death of young people in traumatic circumstances is pretty much a daily occurrence in London, and ‘another one’ caused by someone walking on tracks is perhaps not as newsworthy as a stabbing etc
2) when someone is killed through contact with the OLE they are usually (although not invariably) doing something very obviously and visibly stupid, ie on top of a train. When someone is in contact with the third rail, it is usually by something as simple as crossing the tracks or jumping off a platform / over a fence to retrieve a lost item. Now whilst most people on this forum will recognise that walking on the tracks is dangerous, I have a view that a significant minority of the population don’t see it as a problem, or recognise the danger. Put another way, in my near three decades of involvement in the railway, I have never seen any member of the public on top of a train, but I have regularly seen trespassers on or about the line.
“Dangerous” is a relative term.
I seem to recall there are statistics out there to show that third rail is (slightly) riskier, but the additional risk is largely towards people who are doing something they shouldn’t - ie trespassers.
Third rail is incredibly safe in absolute terms. Millions of passenger journeys take place over it every year, on some of the busiest parts of the U.K. rail network.
The disadvantages come from D.C. being an inefficient method of electricity supply, necessitating a substation every two to three miles, as opposed to every twenty or thirty miles for AC.
I couldn’t agree more!
Assuming "tra" is "tram" then they generally have to use an overhead supply for safety in the street. Limited exceptions are the Alstom APS system which energises sections of the third rail only when the tram is above them, and a few routes that have battery operation over short distances.
However many new metros do use third rail, such as those in Saudi Arabia. These are fully segregated either on viaduct or in tunnel, with platform screen doors, so it's extremely difficult to get onto the track bed by accident.
Plenty of both historic and modern systems used a conduit system embedded beneath the road surface, such as the streetcars in San Francisco. Such a system would be impractical for heavy-rail use and expensive to install on tramways today, but it does solve the problem of exposed electrical supply.
There are signs on the platforms at Newport station (and, for all I know, elsewhere on the newly electrified GWML) telling passengers not to make contact with the OHE. I can't remember the exact wording, but it includes things like selfie sticks, balloons, etc.
As there has been no large third rail project for decades, it's probable there are no contemporary figures. However in the 1960s while the OLE of the southern half the WCML was costing around £200 million, Woking to Bournemouth was done for about £12 million.
'Third rail' covers top-, side- and bottom-contact systems. The third rail on the London Underground and NR networks is top-contact, which means it's easy for things to accidentally come into contact with it. Side- and bottom-contact is safer still because it's easier to provide shielding. The DLR and most new third-rail metro systems are bottom-contact, so you'd have to go to some real effort to come into contact with the live voltage.
The Jubilee and Northern line extensions are able to use the existing system because that's necessary for the extensions to work at all. If LUL wanted to build a whole new line, they'd have to use bottom-contact like the DLR or overhead like the Elizabeth line. Even then, the third/fourth rail extensions are still designed to more modern standards which would further reduce the risk of accidental contact. The new tracks have maintenance/evacuation walkways to the side which mean it is possible to stay further away from the live voltage for longer.
Deleted - Used google.
It would have to be a ridiculously long selfie stick for it to come anywhere near the OHLE. I think they typically don't extend beyond about 1 metre, so even if you held it above your head fully extended I can't imagine it being even slightly dangerous.