(not) touching the third rail



  • Calling all EE's and transport-geeks, foreign and domestic.

    ***Disclaimer:*** *This may be obvious, but my advisor wouldn't let me take Circuits, back in the day, so when electricity scales past 5vdc, I know I'd better ask the experts*.

    So, Washington subway recently had an underground fire, and the news reports today that firefighters were held up 13 minutes while awaiting confirmation that it was safe to enter the tunnel - what with the third-rail and all. (The report, more or less verbatim).

    Got me thinking...(and IRL may have already beat me to it, but sometimes not...)

    Question1: wouldn't it be possible to wire in lights (mounted, say, in or next to the trackbed) that would be lit when the third-rail is live? (And you'd space them pretty frequently)

    Question2: as a supplement to Q1, is there such a thing as a "portable fuse" (made up the word) that rescuers could "toss on the tracks" - if it fries the rail is live.

    Frankly, if I'm the firefighters, I'd like something like this anyway, since I'm not sure I'd completely trust the folks on the other end of the radio/phone when they said "oh yes we've de-energized that segment of track [or the whole system]"

    Thoughts? An embarrassing education for me?


  • SockDev

    for question #1, yep. could be done, but then the lights would draw power off of the third rail increasing the energy draw of the system.

    question #2 : not yet but the idea has potential. N.B. as the design of the third rail is not universal there woulds have to be a few variations on it..... could make good money though if you marketed that thing right. talk to @Polygeekery to get ideas on how to run with that.



  • @ijij said:

    Question2: as a supplement to Q1, is there such a thing as a "portable fuse" (made up the word) that rescuers could "toss on the tracks" - if it fries the rail is live.

    Sounds like a good opportunity for any canaries that lost their jobs in mine closures.



  • @accalia said:

    for question #1, yep. could be done, but then the lights would draw power off of the third rail increasing the energy draw of the system.

    And then someone would have to ensure that all of the lights continued working. If they're not maintained properly, you won't know if they're off because the rail's dead, or because someone forgot to change the bulb.


  • SockDev

    @Keith said:

    And then someone would have to ensure that all of the lights continued working. If they're not maintained properly, you won't know if they're off because the rail's dead, or because someone forgot to change the bulb.

    this too.


  • Discourse touched me in a no-no place

    It's actually pretty hard to fry yourself on a third rail system like the DC Metro. They have safety covers on all sides, and the contact rides in a slot on the side. It's less "touch it" and more "stick shit into it". Makes the fuse concept fairly dangerous to use.

    I suspect Metro dispatch was fucking off and not up to speed on their jobs during an emergency.



  • #2: While I wouldn't suggest literally throwing something on the line (I wouldn't want the risk of more sparks if I was a fire fighter), there should be other ways to detect if a high voltage is on the third rail. For example, there are pen-sized AC voltage sensors that you can find on Amazon...


  • Discourse touched me in a no-no place

    That will go off like a bomb if exposed to this many volts.

    In fact, thinking about it, 'Rail power is off' sounds more like a 'Don't run us over with a fucking train' checklist item than a 'Don't fry us'.



  • @NedFodder said:

    2: While I wouldn't suggest literally throwing something on the line (I wouldn't want the risk of more sparks if I was a fire fighter), there should be other ways to detect if a high voltage is on the third rail. For example, there are pen-sized AC voltage sensors that you can find on Amazon...

    They would need to confirm that it was staying off though.


  • SockDev

    @Weng said:

    That will go off like a bomb if exposed to this many volts.

    well the contactless ones would if you got them too close, but they'd probably be able to tell you if the thing is off from 10 feet away or more!



  • You would have them fail-safe, the only illuminate if the rail's power is OFF, then you space them so that at least 3 can be seen. still not flawless, but better. Would also have to have cable break detection to ensure that they didnt report failed power if the connection between them and the rail broke.

    Do magnetic field detectors exist yet? wouldn't it make sense for firefighters to just have a device that can detect the magnetic field from the third rail if its live?


  • SockDev

    @algorythmics said:

    Do magnetic field detectors exist yet

    yes. it's what those contactless AC tester pens use, the ones that just tell you whether there is current nearby anyways. the more advanced ones are... well fancier, but still probably the same underlying technology.



  • third rail has big power running through it, thats why it's dangerous. big power, big field, seems like the best way to be safe.


  • SockDev

    @algorythmics said:

    big power, big field,

    yep. and since it's AC it's a changing field. you detect AC by making a loop of wire and seeing if there's current induced in it.

    the more current induced the more power is flowing nearby.

    that doesn't work for DC (no changing magnetic field) you need a different detector for DC



  • @Keith's lights could work the same way, and would be pretty resistant to cable breaks as well, although an internal oscilloscope type setup could be used to detect cable breaks if necessary(or the same technology they use to make a LAN out of your power circuits at home)

    I think a portable bit of kit is definitely going to be the most cost effective though. I would be amazed if each unit cost more than $100 too. Take it to the Fire Department, tell them we have their problem solved



  • @algorythmics said:

    third rail has big power running through it, thats why it's dangerous. big power, big field, seems like the best way to be safe.

    We still need to keep in mind the excellent point made by @Weng above - the power may be off right now, but do the folks in charge of the switch intend to keep it off
    (... and not run anybody down with a train.)


    My idea seemed like a little too easy of a fix.


  • SockDev

    @algorythmics said:

    Take it to the Fire Department, tell them we have their problem solved

    that's why i suggested @ijij do that.

    i'll not even want any credit or royal;ties for my suggestion. ;-)



  • @accalia said:

    that's why i suggested @ijij do that.

    i'll not even want any credit or royal;ties for my suggestion. ;-)

    There are many others better suited...

    But I'll take a t-shirt or a mug, or other swag if the whole thing works out for somebody.
    (None of that Finnish licorice stuff, tho)



  • @accalia said:

    that doesn't work for DC (no changing magnetic field) you need a different detector for DC

    This. I'm not sure about modern ones, but historically transit systems haven't always used AC power. ... From a quick glance at http://en.wikipedia.org/wiki/List_of_current_systems_for_electric_rail_traction, it appears AC is used in some catenary (overhead wire) systems, but all third-rail systems use DC.



  • @accalia said:

    i'll not even want any credit or royal;ties for my suggestion.

    I doubt anyone would make you a royal, but somebody might give you a tie, maybe even a royal one:



  • @algorythmics said:

    Do magnetic field detectors exist yet? wouldn't it make sense for firefighters to just have a device that can detect the magnetic field from the third rail if its live?

    A magnetic field only exists if there is actual current flowing through. If there are no trains on that part of the track, it's possible the third rail is charged but with no current flow.


  • SockDev

    @HardwareGeek said:

    but all third-rail systems use DC.

    ISTRT the boston subway system uses AC...

    certainly not every system uses DC current: http://en.wikipedia.org/wiki/List_of_current_systems_for_electric_rail_traction



  • @ijij said:

    (... and not run anybody down with a train.)

    For dutch railways, a safety device is used which creates a short between the two rails (current supply is overhead). This makes the traffic detection systems think there is a train at that location, turning all relevant signs to red.


  • SockDev

    @HardwareGeek said:

    I doubt anyone would make you a royal, but somebody might give you a tie, maybe even a royal one:

    heh. nice typo. and nice tie.

    still you try to put that around my neck and.... well teeth marks will be involved. :stuck_out_tongue:

    it's not nice to make a lady wear a tie.



  • That's the same article I linked to. I didn't read beyond the contents box, but they list only DC systems as "conductor rail."


  • SockDev

    hmm... that is a point. AC only shows up on the list for the overhead systems... I wonder why that is?



  • It might say in the article, which I'm not going to read at this time because I should be leaving for work in about 15 minutes, and I have more than 15 minutes worth of showering, dressing, dog feeding, lunch making, etc. to do before I leave.



  • Missed that very valid point.

    I think the cheapest solution all round has to be a fireman's switch in a locked cabinet with keys at the fire department and at the station offices, allowing firemen to shut down the power on site without outside assistance. the addition of other supporting detection equipment (lights or otherwise) would allow an added layer of confidence that the switch has worked, but that's the only legitimate solution that would work with all the requirements presented I think.


  • Discourse touched me in a no-no place

    Yeah, on the DC Metro system that would be tampered with and used to shut down everything once a week.

    The DC Metro doesn't have bathrooms. This is intentional. Station staff is typically one or fewer.



  • Surely a locked steel box, potentially even one with a fully internal lock only openable wit NFC/RFID/Whatever would prevent tampering? I recognize it needs securing, but that doesn't seem too difficult.


  • Discourse touched me in a no-no place

    Never underestimate the ability of a bum who wants those fucking trains to stop waking him up every 15 minutes. After all, the reason the escalators never work is because they are damaged by being used as makeshift toilets.



  • @Weng said:

    escalators... used as makeshift toilets.

    :wtf: How does that work? You'd have to keep breaking your squat to avoid falling off the end of the escalator. I legitimately cannot think of a worse place to perform a defecation...



  • Not everyone squats to pee.


  • Discourse touched me in a no-no place

    My understanding is that it isn't the actual escalator part, but the machinery hatches that you poop in.



  • @tar said:

    defecation

    @mott555 said:
    pee.

    :wtf:



  • @Weng said:

    machinery hatches

    That makes a little more sense, they'll probably want to start locking those...



  • @tar said:

    :wtf:

    @Weng said:

    After all, the reason the escalators never work is because they are damaged by being used as makeshift toilets.

    says nothing about the type of bodily function other than it being something you'd typically do at a toilet.


  • BINNED

    @mott555 said:

    something you'd typically do at a toilet

    tampon art?



  • @mott555 said:

    something you'd typically do at a toilet

    So you were actually responding to @Weng?



  • No.


  • Discourse touched me in a no-no place

    Locks? See above comments about bums and their abilities.


  • mod

    @accalia said:

    i'll not even want any credit or royal;ties for my suggestion.

    yes, but what about royalties?



  • I am confused as to why we're talking about peeing when my post clearly mentioned #2s...



  • Your post mentioned #2, but it was in response to Weng's post which did not mention that. I was attempting pedantic dickweedery and it fell flat on its face.



  • @mott555 said:

    it fell flat on its face

    Was it squatting on a moving escalator?



  • @HardwareGeek said:

    Was it squatting on a moving escalator?

    Any advise on how to get the mental image of @mott555 squatting on an escalator out of my head?



  • Try imagining me standing on the sliding handhold and peeing off the side of the escalator as I ride up out of the subway station.



  • I'm not sure that helps.



  • @mott555 said:

    Try imagining me standing on the sliding handhold and peeing off the side of the escalator as I ride up out of the subway station.

    Indeed ... Now it's even worse



  • Also: lol-ing in the company restroom seems socially unacceptable


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