# The Zen of Third Rail Power **URL:** \ **Category:** Factual Questions\ **Created:** [November 20, 2001, 7:20pm UTC](https://boards.straightdope.com/t/the-zen-of-third-rail-power/94447 "2001-11-20T19:20:55Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [November 20, 2001, 7:20pm UTC](https://boards.straightdope.com/t/the-zen-of-third-rail-power/94447/1 "2001-11-20T19:20:55Z")
So there we were, my brother and me, eatin’ our lunches and discussing the odd thoughts that pop into our heads. Today it was: what is the grounding apparatus like at a sub-station on a third-rail train? We’re both technologically savvy and understand the principles of third-rail power transmission but were clueless as to the arrangement for completing the circuit to the train. I’ve done Google searches on this and not come up with much in the way of particulars. So, any electric train types out there among the Teeming Masses? What’s the spacing like for sub-stations? Is it AC to the sub-station and then DC to the train? Does groundwater effect circuit impedance? And most importantly, what is the grounding equipment like? My brother’s guessing a big metal grid, but I’m thinking a twenty foot metal rod would be enough. Thanks so much in advance… ---
**Author:** ![GaryM](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garym/32/241_2.png) [@GaryM](https://boards.straightdope.com/u/GaryM)\ **Post date:** [November 20, 2001, 8:42pm UTC](https://boards.straightdope.com/t/the-zen-of-third-rail-power/94447/2 "2001-11-20T20:42:36Z")
This site [http://www.nycsubway.org/irt/irtbook/ch5/index.html](http://www.nycsubway.org/irt/irtbook/ch5/index.html) contains information that may answer some of your questions. It concerns the IRT division on the NYC subway. I’m sure some of that equipment is still in use. This site [http://www.nycsubway.org/index.html](http://www.nycsubway.org/index.html) contains links to the URL above, but also has a lot of additional information. ---
**Author:** ![Dewey\_Cheatem\_Undhow](https://avatars.discourse-cdn.com/v4/letter/d/13edae/32.png) [@Dewey\_Cheatem\_Undhow](https://boards.straightdope.com/u/Dewey_Cheatem_Undhow)\ **Post date:** [November 20, 2001, 9:49pm UTC](https://boards.straightdope.com/t/the-zen-of-third-rail-power/94447/3 "2001-11-20T21:49:14Z")
Well, damn, that was disappointing. I was hoping for a question along the lines of “Will peeing on the third rail kill you?” You know, the real burning questions of the ages… ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [November 20, 2001, 10:27pm UTC](https://boards.straightdope.com/t/the-zen-of-third-rail-power/94447/4 "2001-11-20T22:27:49Z")
> [@](#): > > \*Originally posted by Dewey Cheatem Undhow \* > **Well, damn, that was disappointing. I was hoping for a question along the lines of "Will peeing on the third rail kill you?"** My thoughts exactly… I once saw a pidgeon go to land on the third rail and was thinking that flying rodent was toast. As you probably guessed the little bugger landed all safe and sound and was unharmed by the third rail (the bird obviously wasn’t touching anything else). However, the arriving train a minute later did get the pidgeon (no kidding…it tried to get away but was too slow…EWWW’s all around the platform were heard). Moral of the story? Don’t mess with the third rail because even if it doesn’t get you the train probably will. Sorry I have no help for the OP…just had to share. ---
**Author:** ![manhattan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/manhattan/32/7_2.png) [@manhattan](https://boards.straightdope.com/u/manhattan)\ **Post date:** [November 20, 2001, 11:23pm UTC](https://boards.straightdope.com/t/the-zen-of-third-rail-power/94447/5 "2001-11-20T23:23:35Z")
> [@](#): > > \*Originally posted by Whack-a-Mole \* > \*\* > > > [@](#): > > > > \*Originally posted by Dewey Cheatem Undhow \* > > **Well, damn, that was disappointing. I was hoping for a question along the lines of "Will peeing on the third rail kill you?"** > > My thoughts exactly…\*\* Well, we can’t be disappointing our customers, now can we. [Cecil Adams](http://www.straightdope.com/classics/a1_222b.html) on urination and the third rail. ---
**Author:** ![octothorpe](https://avatars.discourse-cdn.com/v4/letter/o/b77776/32.png) [@octothorpe](https://boards.straightdope.com/u/octothorpe)\ **Post date:** [November 21, 2001, 12:05am UTC](https://boards.straightdope.com/t/the-zen-of-third-rail-power/94447/6 "2001-11-21T00:05:46Z")
**MonkeyMensch** wrote: > [@](#): > > We’re both technologically savvy and understand the principles of third-rail power transmission but were clueless as to the arrangement for completing the circuit to the train. I’ve done Google searches on this and not come up with much in the way of particulars. While i can’t be entirely sure (never having worked on such a system), i would suppose that either (or both) of the other rails would be adequate for the ‘return’ portion of the circuit. > [@](#): > > What’s the spacing like for sub-stations? Is it AC to the sub-station and then DC to the train? Does groundwater effect circuit impedance? And most importantly, what is the grounding equipment like? My brother’s guessing a big metal grid, but I’m thinking a twenty foot metal rod would be enough. The spacing for the substations would be dependant on the voltage drop of the third rail. The motors of the locomotive have a specific voltage rating at which they operate. Once voltage drops below this rating current will rise to the point that the insulation of the motor windings will fail causing the motor to burn out. I would expect that it would be AC to the substation (although there may be areas in NYC still served by DC) as AC is much easier to transform. I would also expect the substation grounding to be similar to any other substation. Specifically, a ‘grounding grid’ made up of numerous interconnected bare copper conductors tied to 8’-10’ driven ground rods every 20’ or so. All substation apparatus would be tied to this grid. The effect of ground water would be miniscule. * * * Expect the worst, that way you’re never disappointed. # ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [November 21, 2001, 12:08am UTC](https://boards.straightdope.com/t/the-zen-of-third-rail-power/94447/7 "2001-11-21T00:08:56Z")
To answer the grounding question, I think the (main or first two) rails provide the return path for the current taken from the third rail. For the sake of providing such a return path, it doesn’t matter whether they are grounded (though they may well be for all I know). Another question about the main rails: what is the effect of current flowing between them and the surrounding soil, wooden ties, et cetera? Over time, electrochemical corrosion of large metal structures is a big problem. If you look around the end of a bridge, for instance, you can often find special electodes driven into the ground to create an earth-powered battery that gradually destroys the electrode (called a “sacraficial electrode”) but guarantees the bridge structure will not be eaten by current flowing from the earth. I bet if the train circuit is DC there are special precautions taken to avoid causing this kind of problem. ---
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [November 21, 2001, 12:14am UTC](https://boards.straightdope.com/t/the-zen-of-third-rail-power/94447/8 "2001-11-21T00:14:52Z")
Thanks so much you guys. I knew you all would come through as you always do.