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Continuity Relay

  1. Sep 30, 2006 #1
    Just curious about something. I work on elevators and there are countless safety circuits to prevent accidents. One example is an elevator hatch door. There are contacts that have to "make" in order for the elevator to run, to prevent the elevator from taking off with the door open. These contacts usually are 120AC and are easily the cause of most shutdowns on any elevator. After so many times of opening and closing the door, carbon builds up on the contacts and the circuit eventually fails. Why is it necessary to send voltage through these contacts? Cant you have a relay make up the safety circuit based on continuity instead? I wondered if any such type of relay exists but then i thought they must. How else would your meter beep when you test continuity.
    Without arcing the contacts would not build up carbon and be much more reliable. On the other hand i guess it is job security for guys like me.:biggrin:
     
  2. jcsd
  3. Sep 30, 2006 #2

    Danger

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    Maybe it's just because I'm not that great at electricity, but I don't quite understand the question. What do you mean by 'a relay based on continuity'? Relays create continuity in response to a signal. :confused:
     
  4. Sep 30, 2006 #3

    dlgoff

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    Inorder to check for contunity with your meter, you're sending a small current through the circuit you're checking.

    Your 120vac safety interlocks probably energize a large relay/contactor that runs the elevators motor (480vac perhaps).

    Yes, job security. Switches are cheap.
     
  5. Sep 30, 2006 #4

    Integral

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    This is a safety feature. The in circiut with the switchs is either a brake, that requires power to open or a line essential to the operation of the motor. IF the switch is open, the elevator cannot move either due to a brake or a disabled motor. Sure you can do the same thing with signal voltage but the safety people may not get warm fuzzy feelings about this solution when human lifes are at stake. Sensors fail in the worst possible way at the worst possible time.
     
  6. Sep 30, 2006 #5

    Danger

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    I think that I see what you mean now. It's the same as the difference between 'fail safe' and 'fail secure' with an alarm system. 'Fail safe' allows exit in the case of a power failure, such as during a fire. 'Fail secure' locks everything down if the power goes out.
     
  7. Sep 30, 2006 #6

    chroot

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    If you used a lower voltage to test the closing of the contacts, you'd have to use some kind of additional circuitry to switch on and off the brakes, or whatever safety devices it controls. Sure, you've made the switches fail less frequently, but you're now stuck with another couple of circuit elements which are more likely to fail -- and in a worse way -- than was the original switch.

    Small aircraft are full of these kinds of unfortunate "low-tech-is-the-only-way" sorts of safety systems. It'd be really nice if small airplanes could use water-cooled engines with computer-controlled ignition, just like cars. They'd use much less gas, produce much less pollution, be quieter and run more smoothly. Unfortunately, they'd also kill more pilots -- so we stick with the low-tech, bullet-proof air-cooled naturally aspirated magneto engines, and just wear thicker, less-comfortable headsets.

    - Warren
     
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