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Thread: sticking contacts on a safety relay

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  1. #1
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    sticking contacts on a safety relay

    I have a robot surrounded with a cage, the doors in the cage have magnetic safety sensors and pallet openings have light curtains. My question is I have a company rep from the palletizer wrapping company (which included the robot) and a questionable co-worker that keeps stating the safety relay in the enclosure is stuck (he claims stuck on). My position is in the PLC (Allen Bradley RS Logix 500) something is made, telling the safety circuit to activate. Theres a 11 flash code (Allen-Bradley Guardmaster) on the safety relay. Robot guys are saying based on what this co-worker is telling them that the safety relays sometimes "stick" (whatever that means). and needs replacing. I have never heard of a safety relay "sticking". Anybody ever hear of something like this? These co-workers, (not electricians) keep powering the panel down and powering it back up to clear the safety relay fault. My contention is that if something were stuck, burned, defective, whatever, it would repeat constantly. This happened once in a while, like once every day or every other day out of a 12 hour operation.

    Your thoughts?

  2. #2
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    If the safety relay clears on a power cycle it is not "stuck".
    However it may be latching into the ON position because of a short transient pulse and not releasing thereafter until power cycled. Possibly a race condition in the programming of the PLC?
    I am not familiar with the general design of a safety relay.

  3. #3
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    Quote Originally Posted by GoldDigger View Post
    If the safety relay clears on a power cycle it is not "stuck".
    \
    This is the point I was making to them, can't be stuck, no such thing as "stuck"

  4. #4
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    Quote Originally Posted by Tbrzezicki View Post
    This is the point I was making to them, can't be stuck, no such thing as "stuck"
    I would be a lot more willing to believe a faulty sensor or bad programming of the PLC rather than a relay problem.

  5. #5
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    The situation they are describing is exactly what a "safety relay" is designed to NOT allow to take place. Safety relays have what are called "force guided contacts" and monitoring contacts of the contacts. If it is really "stuck" that would make it serious fault and that fault would be, per safety rules, non-resettable because the relay would deem itself defective and be required to fail safe. Cycling power would NOT be able to reset it.

    The relay is doing exactly what it is being told to do. Something is taking place out of the proper sequence and it is staying off because it is SUPPOSED to do that. Now the tricky part is slogging through the logic and sequence of operations, then comparing that to what the operators are doing (and whether or not that's what they were trained to do) to find the problem.
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  6. #6
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    I had found one. Robot with a caged area would trip the safety circuit and not reset. It would take a lot of opening and closing of the cage doors, tapping on the relays to jostle bad contacts, guys had tried jumping things out. I believe the error would come up on the robot pendant as an E Stop but it was in the door circuit.They had the problem before and changed the safety board inside the Fanuc robot.

    Eventually I found an Omron safety relay, socket mounted on the safety board, inside the Fanuc robot control box, had burned contacts. Changed that off their spare board and it worked perfectly. I had suspected there was an improper implementation, switching transients were not suppressed or too much, burning the contacts. They would close but must have looked open in the circuit.
    Lasciate ogne speranza, voi ch'intrate

  7. #7
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    Now the co-worker that said the safety relay had sticking contacts has a code 11 on the safety relay which he looked up as an internal fault. Since I originally posted this last year it has happened maybe 4 times. Now they want to change out the safety relay. I said "fine go ahead,and when that one fails then what" So now the "Co-worker" went out and purchased two of them and the company wants it replaced. Guess they just don't listen. Quest Robots says it's the safety rely so this guy is listening to them, Allen Bradley has never had this problem being specifically caused by the safety relay alone. I'm pretty much giving up as this co-worker is the person they are listening to and not me.

  8. #8
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    It seems to me if the relay is diagnosing itself as having some kind of internal fault that the only option is to replace it. I don't know why you are opposed to replacing a piece of equipment that is telling you it has fail and should be replaced
    Bob

  9. #9
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    Quote Originally Posted by petersonra View Post
    It seems to me if the relay is diagnosing itself as having some kind of internal fault that the only option is to replace it. I don't know why you are opposed to replacing a piece of equipment that is telling you it has fail and should be replaced

    Because the guy that does this "diagnosing" knows little if nothing about PLC's, Safety relays or electric circuits. But he thinks he does. Like for instance his remedy so far was to buy 3 new safety relays. He usually buys a qty of 2 new of everything but in this case he bought 3. Doesn't know how to log into a SLC 500 to really see what tripping it so he keeps replacing the relay. We are on number 2 now and $1,700 dollars worth of relays, still not fixed.

    BTW theres nothing wrong with the relay I was told to work on something else but this robot still faults out and he just keeps power cycling it.

  10. #10
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    If the relay is diagnosing itself and saying that it has an internal fault that is not him making the diagnosis that is the relay. They're supposed to do that. In any case there is no reason why you should have to log into a PLC to diagnose a safety relay issue. The safety relay functions are supposed to be completely separate and generally PLC control functions and actual safety functions should not be mixed together.

    I don't have any way to judge his level of Competency from here but I don't have a major issue with buying spare parts. It's Not Unusual to find that whatever broke the thing in the first place breaks the replacement especially if it's some kind of wiring problem where the wrong voltage somehow got put into the safety relay circuit.

    These kind of things can be maddening to debug because often the Diagnostics available are pretty much go no-go and it's not that unusual for them to be misapplied or apply in ways that make it very difficult to debug. Maybe you're lucky you're not involved. Let somebody else pull his hair out and beat his head against the wall trying to fix it.

    Incidentally I don't believe that the relay is stuck either. If it was stuck it would stay stuck. My guess is that something is maladjusted or just wasn't done correctly in the first place. A lot of safety circuits are horribly done by people who don't really understand what they are supposed to accomplish.
    Bob

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