Electrification- overhead vs 3rd rail?

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pete12345
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Electrification- overhead vs 3rd rail?

Postby pete12345 » Tue Apr 22, 2014 10:28 pm

Hi all.

Just a matter of interest regarding electrified railways. I've noticed that in the southeast of the UK, electrified track is pretty much all 3rd rail, whereas just about everywhere else, overhead lines dominate. I guess there's an element of keeping what's already in place (e.g the southern railway used 3rd rail so it makes sense to keep it) but are there any overriding advantages to either for various situations? I'd assume that 3rd rail is cheaper to install, at the cost of not being so good for long distances due to the lower voltage.

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Re: Electrification- overhead vs 3rd rail?

Postby bike2steam » Tue Apr 22, 2014 11:05 pm

The LB&SCR was the first railway to 'electrify' in the south of England, and was overhead. When the Southern Railway came up with the decision to standardise on one system it chose the 3rd- rail only because in the intervening years the LSWR had electrified more mileage using 3rd rail, than the LB&SCR had with overhead.

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Bufferstop
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Re: Electrification- overhead vs 3rd rail?

Postby Bufferstop » Tue Apr 22, 2014 11:25 pm

Third rail with relatively low voltage is easier to install on an already built railway, as there are no problems with clearance from other structures. Because of the higher currents involved substations have to be fairly close together to ensure an adequate current supply. Masts and catenary for overhead supply aren't vastly different in cost, but the cost of raising bridges, moving signals, trimming back platform canopies etc. soon bumps up the cost of using OH. supply. In the crowded south east third rail makes sense because of the frequency of bridges, where as on the long stretches through open countryside with occasional bridges, the saving in substations makes high voltage overhead lines a better choice. When the WCML through the West Midlands was electrified in the sixties it created havoc in the built up areas. Most of the bridges had to be raised which in turn meant moving, relaying or simply working around all of the services that were buried in the roads across them. Not only that but a 25KV cable within a couple of feet of the trunk phone and TV cables with what amounts to a travelling earth fault rushing by every few minutes has the potential to disrupt the signals being carried so those cables have to be moved into steel pipes where they cross the bridges and any changes to the length of the cable can require a whole lot of work to ensure it still performs in the same way. That's what costs when you use 25KV Overhead.
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Re: Electrification- overhead vs 3rd rail?

Postby knottinbotswana » Thu Apr 24, 2014 7:25 pm

Bufferstop wrote:... a travelling earth fault rushing by every few minutes ...


This needs to be worked into a Wikipedia article on electrified railways.

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Re: Electrification- overhead vs 3rd rail?

Postby b308 » Thu Apr 24, 2014 7:31 pm

There is also speed, you can go much faster using OHLE than 3rd rail, and use far larger voltages which give more power...

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Re: Electrification- overhead vs 3rd rail?

Postby Bufferstop » Thu Apr 24, 2014 7:59 pm

Bufferstop wrote:... a travelling earth fault rushing by every few minutes ...


Anyone who doubts this description (by the BR guy responsible for electrical safety) should look at the base of an overhead mast. Just above ground you'll see a big thick cable bolted to the steel mast. You'll also see the same cable fastened to the rails. It's there to make sure that the return current flows through the return cables not the ground, on its way back to the substation. Whilst people with a ground spread of less than 1m and usually wearing shoes would probably not notice if the return current flowed through the ground beneath their feet, it could prove fatal to farm animals.
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Re: Electrification- overhead vs 3rd rail?

Postby b308 » Thu Apr 24, 2014 9:38 pm

Not as big or as thick as those used on a 3rd rail set-up though!

We're told to avoid at all costs if that cable is not connected at both ends btw... Though with either form of electrification I give everything a wide berth!

I remember when i first started as a guard and myself and a colleague did a trip to Southampton in an HST on a wet day... We were put in the centre road just outside SOU station to reverse and the driver (an old hand) had to swap ends... We saw him jump down between the two live rails and walk down to the other end when he could have got down on the other side of the train and there's have been no live rails at all... No nerves, those old drivers!! :shock:

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Re: Electrification- overhead vs 3rd rail?

Postby Bufferstop » Thu Apr 24, 2014 11:32 pm

Went to Liverpool a few weeks ago, they've got one of the old OH Railway cars in the museum. On the bulkhead in the drivers cabin there's an isolating switch, one of those old open knifeblade jobs that you only see in films and telephone exchanges. Full traction voltage, open contacts and terminal block, not even a cage to prevent accidental contact. It was still running like that in the '50s.
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6C
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Re: Electrification- overhead vs 3rd rail?

Postby 6C » Fri Apr 25, 2014 11:07 am

Depends if you want to griddle or fry your railway trespassers !

Locally we have that choice - 650V 3rd rail and 25kv OHL :lol:
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Re: Electrification- overhead vs 3rd rail?

Postby Bushey Troughs » Fri Apr 25, 2014 12:02 pm

bike2steam wrote:The LB&SCR was the first railway to 'electrify' in the south of England, and was overhead.

The Volks Railway 1883, or City and South London Railway 1890.
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Re: Electrification- overhead vs 3rd rail?

Postby b308 » Fri Apr 25, 2014 1:11 pm

Insert the word "Mainline" before railway... ;)

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Re: Electrification- overhead vs 3rd rail?

Postby Bushey Troughs » Fri Apr 25, 2014 3:00 pm

b308 wrote:Insert the word "Mainline" before railway... ;)

No mention of "Mainline" in the post. :wink:
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Re: Electrification- overhead vs 3rd rail?

Postby b308 » Fri Apr 25, 2014 10:20 pm

bike2steam wrote:The LB&SCR was the first "mainline" railway to 'electrify' in the south of England


Colin, see above, the two you mention were not mainline railways...

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Re: Electrification- overhead vs 3rd rail?

Postby Bushey Troughs » Fri Apr 25, 2014 10:27 pm

pete12345 wrote:Hi all.

Just a matter of interest regarding electrified railways. I've noticed that in the southeast of the UK, electrified track is pretty much all 3rd rail, whereas just about everywhere else, overhead lines dominate. I guess there's an element of keeping what's already in place (e.g the southern railway used 3rd rail so it makes sense to keep it) but are there any overriding advantages to either for various situations? I'd assume that 3rd rail is cheaper to install, at the cost of not being so good for long distances due to the lower voltage.

Pete

Just show me where this post says "Mainline". :roll:
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b308
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Re: Electrification- overhead vs 3rd rail?

Postby b308 » Fri Apr 25, 2014 10:32 pm

I wasn't quoting that post, though, was I! You quoted (correctly) two other lines in the south that predated the LBSCR, I just pointed out that theirs was the first mainline one... Calm down, kiddo! :)


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