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Thread: push button contact

  1. #11
    Join Date
    Apr 2017
    Location
    Ulsan, Korea
    Posts
    3

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    Quote Originally Posted by petersonra View Post
    How often does this happen?

    It could be that you are using poor quality contact pushbuttons. If so, changing to a better quality of pushbutton may solve your problem.

    Maybe there is some environmental factor that is causing the failure. Could be corrosive fumes, even vibration. Maybe sealed contact blocks would help with corrosion. Vibration is a little tougher but you might be able to find mounting locations that are less susceptible to vibration.

    It could also be that you are using them outside of their ratings, although that seems unlikely.

    Without more information it is hard to give you any good advice.

    I don't know how going to some kind of voting scheme is going to solve the failures. Better to fix the problem than to mask it.

    I dont think it happen much, maybe once a 5 years even though we have huge amount of push button swich in our site.
    I know voting scheme is not a best solution and cannot apply all of push button considering efficiency but it can be reduce failure rate.
    As you mention above, we are trying to mitigate environment factor to push button, for example, exmergency stop switch is hard to do maintenace during normal operation.
    More, if it' faiure could happen, we are going to lose alot of money on refinery plant characteristic.
    If you have any experience with that kind of matter, I would love to share the best practice or let us know if you know where I can refe to code/standard.
    I think if we apply voting scheme for pushbutton, we must set criteria up considering engineering effiency.


    Please let us discuss and I want to be always with you as engineer.

    from Korea

  2. #12
    Join Date
    Dec 2007
    Location
    NE Nebraska
    Posts
    32,049
    Quote Originally Posted by webfst View Post
    I dont think it happen much, maybe once a 5 years even though we have huge amount of push button swich in our site.
    I know voting scheme is not a best solution and cannot apply all of push button considering efficiency but it can be reduce failure rate.
    As you mention above, we are trying to mitigate environment factor to push button, for example, exmergency stop switch is hard to do maintenace during normal operation.
    More, if it' faiure could happen, we are going to lose alot of money on refinery plant characteristic.
    If you have any experience with that kind of matter, I would love to share the best practice or let us know if you know where I can refe to code/standard.
    I think if we apply voting scheme for pushbutton, we must set criteria up considering engineering effiency.


    Please let us discuss and I want to be always with you as engineer.

    from Korea
    Switch built to higher specifications sounds like a start, presuming you aren't already using what you have beyond it's design specifications. Most of the 30 mm switch designs seem to take more abuse then 22 mm switch designs when it comes to general purpose pushbuttons and their associated contact blocks.

  3. #13
    Join Date
    Jul 2006
    Location
    San Francisco, CA, USA
    Posts
    6,534
    Quote Originally Posted by GoldDigger View Post
    One switch with three contacts or three switches ganged together with one contact each.
    Switches feed into logic that determines the state of the switch by either all three or any two out of three being in a particular state.
    Majority rules.
    Most safety relays (ala "Pilz" mentioned above) use a dual contact channel, not 3, but yes they "vote" in that both sets must agree.
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    It should be noted however, a Safety Relay monitoring system will not PREVENT a shutdown if an E-Stop contact fails, it will prevent OPERATION if one of the contacts fails to CLOSE. The problem being described here is almost exactly the OPPOSITE of what a Safety Relay is intended for, in that you are undoubtedly getting nuisance Emergency Stops. That to my mind would preclude the use of a "voting" system because for EMERGENCY stop you do NOT want to interfere with initiating the command by having something 'decide" whether or not the circuit should be open.

    The suggestion that you use better quality contact blocks is probably your better option here. There are a few mfrs out there who's contact blocks are prone to open on high vibration; ABB is notorious for that. I'm not sure of what's readily available in Korea, but if you are using ABB, that's something I would suggest changing out.
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