Sunday, May 15, 2011

DANGER Third Rail!

Third rail on the Red Line
I have always been interested by electricity. My father is an electrician by trade and has always tried to teach me everything he knows. I have become very interested in the electrified third rail of the subway. Most subway systems in the world use an electrified third rail to power their trains. I wondered how it actually powered the train sets and if it was really as dangerous as people have made it out to be.

The MBTA does not use a protective covering
 like most Transit Systems 
First, I found out how it actually powered the cars. The third rail in the MBTA system is a steel conductor rail that provides 600v D.C. power to the subway cars on the Orange, Red, and Blue lines. The electricity that is constantly flowing through this steel rail is then picked up by the trains as they drive past. Each train in the system is equipped with two steel shoes that jut out from each set of wheels. The shoes glide along the top of the rail and collect electricity as the train rolls forward. Other transit authorities in the U.S have converted their third rail systems to allow for pick up by having the shoe glide underneath which would allow for covering of the top of the rail which could prevent electrocution.

Now onto the actual danger of the third rail! The third rail is advertised to passengers as an extremely dangerous thing. They basically say if you touch it you will be electrocuted. This is not entirely true though.  In order for electricity to flow through your body you must first be grounded. This means that one of your limbs needs to be touching the electrical conductor while the other is touching a neutral object or the ground in order for the electricity to flow.

Let's take a hypothetical situation here and say a person has just fell onto the tracks at Park Street. They are wearing rubber soled shoes, which act as an insulator, and are about to use the third rail to boost them self up to the platform. They proceed to jump and make it so both feet are on the third rail at the same time. They proceed to the platform unharmed. Now, if the person were to put one foot on the third rail and one on the ground below they would have been electrocuted. This is also why pigeons, rats, and other small animals can run freely along these tracks without harm.

So contrary to what you may have believed, touching the third rail does not result in an automatic death sentence. With that being said, the third rail is something NO ONE should mess around with. High voltage electricity of any kind can be very lethal.

Third rail demonstration image courtesy of HowStuffWorks.

6 comments:

  1. Personally I wouldn't trust my rubber soled shoes...it's much easier to just jump up on the center platform side where there's no third rail! Stations with only one platform have the third rail on the opposite side.

    Also, while jumping you'd have to make sure you don't touch the platform with your hands. But the rubber soles are why MBTA inspectors can reach under the third rail to retrieve cell phones and such.
    http://iridethet.blogspot.com/2010/11/cell-phones-on-t.html

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    Replies
    1. The rubber soled shoes are irrelevant If you touch nothing but the electrified third rail i.e. by jumping onto an unshielded top contact electrified rail. You aren't grounded so no current can flow. It's the same principal as a bird sitting on a bare electrified wire. As long as it touches nothing else the current has nowhere to flow and it's unharmed. That said in practice there is always the danger of arcing if conditions are right but the voltage on third rail systems isn't high enough to arc in typical dry conditions. I would not trust any rubber soled shoe to insulate me from ground in this type of senario. More likely than not the current will go right through the shoe. It is not a good enough insulator.

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  2. In your hypothetical scenerio, I hope you're not confusing electrical ground with the actual ground-ground because as far as the electrons are concerned, the platform is also the "ground" and someone who successfully jumped onto the third rail had better not touch anything or anyone lest the circuit be completed.

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  3. I'd also caution not to confuse the MBTA's DC-current, 600v 3rd rail with the Amtrak 25k-vac catenary overhead power for their trains. The 25kv lines won't allow you to touch them - they will reach out and zap you if you get close enough, sort of like a spark plug cable on a car. In this case "close" is enough, like in horse shoes.

    Let's also remember if you find yourself in the pit, there is a ladder at either end of the platform

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  4. I got shocked to know that there is a railway track which is passing very near from active volcano in Japan & it is connecting Takamori to Tateno Station in Minamiaso.There are few world's most terrifying and gusty railway tracks , which gives you goose bumps.

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  5. Very interesting blog. Alot of blogs I see these days don't really provide anything that I'm interested in, but I'm most definately interested in this one. Just thought that I would post and let you know. Phone and Data Cabling

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