London Underground 4th Rail

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phillyrube
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London Underground 4th Rail

Post by phillyrube » Thu Jan 17, 2019 2:03 pm

Started watching some video on youtube, about the Underground. Noticed a 4th rail, used as a ground, for the power. Any ideas why, since we widely use the running rails as the ground?

David Benton
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Re: London Underground 4th Rail

Post by David Benton » Thu Jan 17, 2019 7:58 pm

I have been asked this before , and found the answer, but can't remember what it was.
Found this answer , but I'm not sure it is correct.
https://www.quora.com/Why-does-the-Lond ... four-rails" onclick="window.open(this.href);return false;
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David Benton
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Re: London Underground 4th Rail

Post by David Benton » Thu Jan 17, 2019 8:01 pm

http://www.trainweb.org/tubeprune/tractioncurr.htm" onclick="window.open(this.href);return false;
Seems it is correct..
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Benny
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Re: London Underground 4th Rail

Post by Benny » Thu Jan 17, 2019 8:52 pm

Also Milan metro line 1 has four rails.
The official reason was to avoid that eddy currents damage the main rails but unofficially the choice was made to ease an eventual future conversion to rubber tyres use (in the 60s there were strong pressures by some industries to put on use a system called "strada guidata" (driven road)

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phillyrube
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Re: London Underground 4th Rail

Post by phillyrube » Fri Jan 18, 2019 8:08 pm

Found anothr youtube that explained it in some depth. I'm guessing that since the "neutral" is insulated, there would be potential on that as well, similar to USN ships.

David Benton
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Re: London Underground 4th Rail

Post by David Benton » Sat Jan 19, 2019 12:32 am

That's what I am thinking too. If the middle rail is -260 volts, and the running rails are earthed ,then the running rails are 260 volts positive to the centre rail. Certainly enough to prove fatal.I always thought the refuge hole in the centre of the tracks was a safe place to fall, but not with 260 volts D.C in the middle.
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The only train trips I regret are the ones I didn't take.

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