Which is better and why - over head or third rail electrification?

[Sapphire]

I’ve no real knowledge on this so interested to know!

 

[GB]

Both systems have their advantages and disadvantages.

OHL has much higher operating speeds, can cope with flood water and is not as susceptible to ice. However it doesn't like really hot weather, windy weather and if the lines come down they often do more damage and cause more disruption.

3rd rail doesn't sag in hot weather like wires and if a tree were to fall onto the line the damage is usually pretty minimal. However 3rd rail has a lower operating speed, needs more substations generally doesn't like flood water or ice. There is also the safety aspect of wires being out the way in the air (usually) but 3rd rail being on the ground for anyone or anything to walk near.

When all said and done overhead line is the way to go.

 

[Tio Terry]

The DfT and ORR have decided the way forward is OHEL and, as they control the purse strings, any other arguments are superfluous!

The current Technical Specification for Interoperability effectively bans any large scale installation of 3rd rail and confines it to minor development only. This is, of course, a matter of interpretation but it seems in reality that there will be no extension of the existing 3rd rail network and no new 3rd rail lines. Instead, there is greater emphasis on dual, or multi, powered traction units so that they can operate on existing 3rd rail then change to an alternative energy source when 3rd rail ceases to be available.

 

[NSEFAN]

From a power efficiency perspective, OLE systems tend to be more efficient means of transmission as they allow higher voltages to be delivered to the trains (25kV for overhead, only 750V for 3rd rail). This means the same amount of power draw pulls less current.

It is also easier to detect short circuits when the voltage is higher, as the normal current load is relatively small. With lower voltages, it becomes tricky to tell the difference between a train and a short circuit!

 

[AM9]

Both systems have their advantages and disadvantages.

OHL has much higher operating speeds, can cope with flood water and is not as susceptible to ice. However it doesn't like really hot weather, windy weather and if the lines come down they often do more damage and cause more disruption. ...

OLE conforming to modern (current) standards is generally more resilient than you say in all but exceptionally windy conditions, and the practice of headspan wiring support is now not applied except in low speed areas like terminus staions and sidings, so the wholsale downing of OLE is far less likely than of yore.

... 3rd rail doesn't sag in hot weather like wires and if a tree were to fall onto the line the damage is usually pretty minimal. However 3rd rail has a lower operating speed, needs more substations generally doesn't like flood water or ice. There is also the safety aspect of wires being out the way in the air (usually) but 3rd rail being on the ground for anyone or anything to walk near.

When all said and done overhead line is the way to go.

For the same reasons mentioned above, current practice for OLE is to apply the correct tension independently of ambient temperatures so sagging in the heat is not an issue as it was with the fixed tension installations e.g. as fitted on the GEML in the 1940s and 50s. 3rd rail (and the high currents that low voltage needs to supply make running in icy conditions difficult or even impossible. In addition, even with expensive feeds every couple of miles, the power losses at the high currents are immense (for each 1000HP of traction, a current of 1000 amps is needed) because of conductor rail and track resistance.

 

[Dr Hoo]

Having had to deal with a couple of electrocutions on third rail equipped lines there is only one answer.

Live conductors at a lethal voltage close to where staff regularly have to walk and work, let alone trespassers, passengers falling from platforms, evacuating from trains, etc., have no place on 'the safest railway in Europe'. (Certainly not an expanded future role.)
Words

 

[al78]

A couple of questions:

1. Why does third rail have lower speed limits?
2. If the 50 year return level for peak wind speed across the UK is known, could OHL be designed and built to withstand the peak winds that correspond to that 50 year return period?

 

[swt_passenger]

A couple of questions:

1. Why does third rail have lower speed limits?

1. Trials years ago apparently found that pickup shoe contact was unreliable above about 100 mph. I suspect that’s become set in stone mainly because the DC network is too busy to allow anything faster, 90 is the typical max speed, 100 mph isn’t that widespread. So there’s probably never been much research into any higher speed pickup.

 

[Sapphire]

I’m very aware that this probably sounds very dumb and I do live in third rail land so don’t really stand on many platforms with overhead lines, but anyway ...! after watching the old educational videos posted in another recent thread one of the dangers was the current arcing and it not just being the actual wire that is dangerous. The gantries (?) for all the wiring seem really close together and run along the platforms. What risks are there of an accidental electrocution?

 

[jopsuk]

A couple of questions:

1. Why does third rail have lower speed limits?

Multiple reasons, but one is that each section of 3rd rail has to have ramps at the ends. This creates gaps, which are fine when moving at speed because the momentum of the train carries them over. The higher the line speed, the shallower the angle the ramp needs to be, to avoid taking the (sprung) shoes off. But shallow ramps mean longer gaps, which means that if trains do have to stop, there's an increased risk of not being able to start again. So steep ramps in station areas, with no need for higher speeds, shallow ramps on plain line out of town, but sill limiting to ~100mph.

OHLE meanwhile incorporates its "gaps" into the wire with insulated sections, and doesn't need gaps at points.

 

[edwin_m]

I’m very aware that this probably sounds very dumb and I do live in third rail land so don’t really stand on many platforms with overhead lines, but anyway ...! after watching the old educational videos posted in another recent thread one of the dangers was the current arcing and it not just being the actual wire that is dangerous. The gantries (?) for all the wiring seem really close together and run along the platforms. What risks are there of an accidental electrocution?

The gantries themselves are not live. You will see "insulators" which look rather link a stack of small saucers, and only the metalwork between the insulators and the wire itself is live. The rules forbid any live equipment directly above a platform and on the latest electrification schemes the standards have been tightened so any live equipment will be further from someone on the plaform. The high voltage in overhead lines can "jump" a significant distance so even being within a few metres of the live parts is dangerous.

As mentioned, a higher voltage is only possible in overhead line but has the advantage of less energy being lost and allowing the places that feed the power into the system to be tens of miles apart instead of maybe two or three miles on a 750V third rail route. On a typical railway these advantages outweigh the need for each train to carry a large and heavy transformer that turns the high voltage into a lower voltage that the motors can use. However many bridges etc will need re-building to provide clearance for the overhead line, and it is more unsightly than a third rail. Despite this pretty much every railway that has started electrifying since the 25kV system was invented has chosen that voltage, and even some with older systems at lower voltages have chosen to do new electrification at 25kV instead.

However on a metro line where trains are frequent and stations close together, a low voltage may be a better choice as the losses are less important and the trains can be lighter and cheaper without a transformer. Not having overhead line may also allow the tunnels of an underground metro to be smaller. Hence most metros use a third rail, but for new metros it will be a bottom contact system where the top and sides of the rail are insulated, as on Docklands Light Railway. This is safer for anyone who gets onto the track, although usually a metro won't allow any workers on the line unless the trains are stopped and the power shut off.

Older metros, including later extensions, may have a top contact third rail or even third and fourth rails like London Underground. Third rail on a main line railway is unusual with the lines south of London being probably the largest example in the world. The system started off on the suburban lines and was chosen partly because it allows through running with the Underground under certain circumstances. However it was then gradually extended onto longer distance routes where if they had been starting from scratch a 25kV system would certainly have been used instead. The third rail on the Southern is pretty much at the limits of the technical capability of such a system.

Tramways also have small light vehicles and frequent stops and also run in the street so any voltage greater than 750V is considered unsafe. However for obvious reasons they use overhead line rather than third rail - except for a few which use a system where the third rail is divided into short sections and each one is energised only when underneat the tram.

 

[twpsaesneg]

I’m very aware that this probably sounds very dumb and I do live in third rail land so don’t really stand on many platforms with overhead lines, but anyway ...! after watching the old educational videos posted in another recent thread one of the dangers was the current arcing and it not just being the actual wire that is dangerous. The gantries (?) for all the wiring seem really close together and run along the platforms. What risks are there of an accidental electrocution?

Vanishingly small unless you approach the live equipment to a far smaller distance than is possible from a platform without climbing something or waving something metallic at the OLE.
The UK practice for many years has been to ban completely placing live equipment over a platform surface, although it is permissible in certain circumstances given enough clearance and has been done in very very limited circumstances.
The much maligned GL/RT1210 standard has pushed the requirements for safety clearance up to even greater values.
I am not aware of any incident involving electric shock in the UK to a member of the public from OLE which hasn't involved some kind of misadventure on the part of the member of the public.

It's an awful lot easier for someone to trespass and fall over onto a live rail than climb something and get zapped.

 

[apk55]

The power available on 3rd rail is quite limited. The best 3rd rail can do is about 4MW and even this is difficult to achieve as it requires substation every 1.5 miles or less. And many sections are limited to 2MW because of power supply issues. And it is not very efficient at doing this as the voltage drop mid section of a train on full power is over 100v implying a power loss of 15% just from the substation to train. One of the problems is return voltage drop, if this exceeds a few volts it implies safety issues and corrosion problems. Even on Manchester Metrolink they had supply problems (ok overhead but still 750V) and had to put in extra substations, for example at Brooklands Road as the voltage drop in that area was too high. (The two substations either side were at Timperley and Dane Road about 3 miles apart) And this is with units of about 1MW (double tram).
In contrast high voltage AC can easily cope with train loads of 8MW for example Pendelno or double headed freight train. And this is with substations more than 10 miles apart

 

[Railcar]

Should the judgement be based only on technical factors?

OLE looks a mess. All those support girders, support wires, conductor wires, insulators, hangers, all the bits and bobs that spoil the look of the railway and its surroundings. With third rail, well, there's just an inconspicuous third rail.

 

[AM9]

Should the judgement be based only on technical factors?

OLE looks a mess. All those support girders, support wires, conductor wires, insulators, hangers, all the bits and bobs that spoil the look of the railway and its surroundings. With third rail, well, there's just an inconspicuous third rail.

It should be judged on suitability for purpose, i.e. providing a transport service, that is safe for both users and non-users, at an acceptable through-life cost, meeting environmental legislation (which doesn't just mean not spoiling the view for a few people).

 

[jopsuk]

Third rail is at an advantage when creating a largely tunnel-ed system, as it allows you to use a smaller tunnel for the same size of train

 

[edwin_m]

Should the judgement be based only on technical factors?

OLE looks a mess. All those support girders, support wires, conductor wires, insulators, hangers, all the bits and bobs that spoil the look of the railway and its surroundings. With third rail, well, there's just an inconspicuous third rail.

The GW scheme has been particularly unfortunate in this respect. The visual impact has been considerably less for previous overhead line schemes on the classic network, others that are being done at the same time and in particular the sort of equipment used on high speed lines which must meet a higher specification than GW. Agreed any of these equipments are obtrusive when looking along the line of the railway, but most "normals" are more interested in the view from a distance and from an angle, where a good design has very little impact.

 

[Groningen]

With a third rail 100 miles/160 kilometers is still posible. However with a third rail there might be more suicide if the railway is not more isolated from the surroundings.

 

[AM9]

Third rail is at an advantage when creating a largely tunnel-ed system, as it allows you to use a smaller tunnel for the same size of train

Which is fine for the tube or a metro system, but on lines with 100mph speed limits tight tunnels add yet more energy inefficiency to that caused by distribution of power at low voltages. The majority of tunnels built/rebuilt in recent years are generally of sufficient bore to accommodate well designed OLE. Similarly, a great many of the original tunnels built on the classic mainlines are nearing the end of their working lives, so the impact of their rebuilding costs are not necessarily unique to the OLE case. A recent example was some of the tunneling on the SWML between Basingstoke and Southampton.

 

[A0]

Third rail is at an advantage when creating a largely tunnel-ed system, as it allows you to use a smaller tunnel for the same size of train

Well more importantly it allows 'tube' size tunnels and trains - which would be impractical for OHLE.

Some of the benefits being put out in this thread about OHLE are actually more to do with using AC current over DC - specifically the longer distances between substations. You can have DC OHLE but that is more expensive because of the additional substations needed.

Conversely - in theory you could have an AC 3rd rail..... in practice it probably introduces far more issues than it solves.

3rd rail does work well where there is limited space - hence its use on many Metro or Underground systems. Out in the open the picture changes a bit - yes OHLE is more unsightly, but it is, in many respects a much, much safer system. Yes, people shouldn't trespass on the railways, but they always have and always will and for that reason leaving an exposed, high voltage power source at ground level isn't a great idea. As others who actually work on the railways have pointed out, it also affects wildlife, particularly foxes or badgers that get fried by the conductor rail which causes fires - that said I have seen a pigeon manage to short itself out on the OHLE at a station - went with a bang and flash of light.

For all the 'de-wirements' on OHLE, there are similar conductor rail issues with 3rd rail installations - on other threads posters such as Bald Rick have given examples.

On balance, the approach in use now is probably the right one - in the longer term some of the Southern network should probably be re-electrified using OHLE - the LSW Mainlines to Portsmouth and Bournemouth would make some sense. So, potentially would the Brighton mainline - which would mean coming full circle as the original electrification on part of that line by the LBSCR was OHLE at 6600v AC - which was changed to 3rd rail following Grouping.

 

[broadgage]

I’m very aware that this probably sounds very dumb and I do live in third rail land so don’t really stand on many platforms with overhead lines, but anyway ...! after watching the old educational videos posted in another recent thread one of the dangers was the current arcing and it not just being the actual wire that is dangerous. The gantries (?) for all the wiring seem really close together and run along the platforms. What risks are there of an accidental electrocution?

Negligible, as has already been said, only the contact wire and metallic parts connected thereto are dangerous. The gantries and supports are safe.
Under favourable conditions, 25KV will only jump an inch or two. Much greater clearances are required to give a generous margin for adverse conditions such as rain, fog, smoke, steam, birds and clouds of flying insects.
The main risks are to trespassers, and to passengers carrying long items on platforms. IIRC, there was a fatal accident on a platform caused by someone carrying a carbon fibre fishing rod.
Long items should be carried with care to ensure that no part of the article is above head height.
In the early days of electrification, steam locomotives were still in general use, and there was at least one fireman killed by climbing atop the coal in the tender and touching or very closely approaching the overhead. In daylight, only a fool would do this, but in the dark it would be an easy mistake to make.

 

[coppercapped]

It's all to do with Ohm's Law...

...which states that the drop in voltage (V) along a body conducting electricity is equal to the current (I in amps) passing through it multiplied by the resistance (R in ohms) of said body. That is

V = I x R.

Now, power dissipated in said body equals Volts (V) times current (I), that is VxI.

So, multiplying both sides of the first equation by I gives Power (in watts) equals Current times Current times Resistance

V x I = I x I x R, and the right hand side is spoken as 'I squared R'.

So, any given level of power can be obtained by a low voltage and a high current or a high voltage and a low current. The 'I squared R' term describes the power (heat) dissipated in the body carrying the electrical current, so the level of current is important.

Suppose you have in one case, call it A, a voltage which is a hundred times greater than in case B, then for the same power the current in case A will be a hundred times less than case B. As an example take these as 1 amp in A and 100 amps in B. But looking at the right hand side the heat dissipated in case A will be 1 times 1 times R and 100 times 100 times R in case B. So case B dissipates ten thousand times more power than case A.

This effect is real, Network Rail published a study some years ago (sorry, can't find the reference off-hand!) which demonstrated that only some 75% of the energy fed into the low voltage third rails on the southern networks actually moved trains - the rest simply heated the cables, third rails and rectifiers. On the other hand the heating losses in 25kV distribution was only a few per cent.

So, high voltages reduce distribution losses - but need to kept well out of the way of people so are restricted to overhead systems. Low voltage systems tend to be used in suburban and metro type operations, as other posters have pointed out, but as legacy systems they were installed before safety standards were so strict. Extensions to top contact third rail networks will not happen.

 

[Esker-pades]

In snow, the most resiliant form of electrification is the DLR method (3rd rail but pick-up is the bottom of the rail). When everything else in London is canned because of snow, the DLR keeps plodding along.

 

[swt_passenger]

In snow, the most resiliant form of electrification is the DLR method (3rd rail but pick-up is the bottom of the rail). When everything else in London is canned because of snow, the DLR keeps plodding along.

The DLR set up has a pretty low maximum speed though. The figure slips my mind but it’ll be in a previous discussion. It wouldn’t be of practical use on the wider DC network.

 

[Esker-pades]

The DLR set up has a pretty low maximum speed though. The figure slips my mind but it’ll be in a previous discussion. It wouldn’t be of practical use on the wider DC network.

Oh quite. I wouldn't suggest rolling it out for a London to Weymouth service at any point.

 

[AM9]

In snow, the most resiliant form of electrification is the DLR method (3rd rail but pick-up is the bottom of the rail). When everything else in London is canned because of snow, the DLR keeps plodding along.

It's rare for snow to prevent OLE operation, and even ice on conductor wires is only a problem in very cold countries like Russia and some parts of eastern Europe. As I've mentioned upthread, I'm talking about modern OLE installations, not the original fixed tension ex 1500VDC lines or the cheapo MKIIIb cats cradle headspan knitting.

 

[Sapphire]

Negligible, as has already been said, only the contact wire and metallic parts connected thereto are dangerous. The gantries and supports are safe.
Under favourable conditions, 25KV will only jump an inch or two. Much greater clearances are required to give a generous margin for adverse conditions such as rain, fog, smoke, steam, birds and clouds of flying insects.
The main risks are to trespassers, and to passengers carrying long items on platforms. IIRC, there was a fatal accident on a platform caused by someone carrying a carbon fibre fishing rod.
Long items should be carried with care to ensure that no part of the article is above head height.
In the early days of electrification, steam locomotives were still in general use, and there was at least one fireman killed by climbing atop the coal in the tender and touching or very closely approaching the overhead. In daylight, only a fool would do this, but in the dark it would be an easy mistake to make.

See, this bothers me, carry long items with care and not above head height. What about umbrellas held high? Numerous umbrellas in a rush hour platform? I honestly find the overhead equipment pretty scary (which I guess is a good thing, better than being blasé). Agree it’s very unsightly (was travelling from Bristol back to London yesterday for the first time since the electrification completed, which was what got me thinking about this)

 

[apk55]

See, this bothers me, carry long items with care and not above head height. What about umbrellas held high? Numerous umbrellas in a rush hour platform? I honestly find the overhead equipment pretty scary (which I guess is a good thing, better than being blasé). Agree it’s very unsightly (was travelling from Bristol back to London yesterday for the first time since the electrification completed, which was what got me thinking about this)

The latest recommendations are for live parts to be at lest 3.75M from a public access area presumably cope with a tall person holding an umbrella.

Another good 3rd rail system (now abandoned) was the original Manchester Bury line which used a side contact system, and that was also good in icy conditions.

The Swiss achieve their reliable train service using high voltage overhead

 

[61653 HTAFC]

OHLE isn't ugly or unsightly, it's just there. From a distance it's barely noticeable. It's certainly less unsightly than the black sooty residue left behind from diesel exhaust, not to mention the noise.

 

[Harbornite]

It should be judged on suitability for purpose, i.e. providing a transport service, that is safe for both users and non-users, at an acceptable through-life cost, meeting environmental legislation (which doesn't just mean not spoiling the view for a few people).

Well said.