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  • Two-hand safety relays

    It would be helpful if the documentation specified that these relays only work with dry contact pushbuttons, and not with solid-state output optical buttons like those sold by Banner. I ended up having the Banner optical buttons each operate a single-pole double throw relay to trigger the safety relay.
    Why worry? If you've done the best you can, worrying won't make it any better

    - Walt Disney


  • #2
    Please provide model numbers and links to devices.

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    • #3
      Originally posted by kewakl View Post
      Please provide model numbers and links to devices.
      https://www.bannerengineering.com/us...art.35094.html

      Why worry? If you've done the best you can, worrying won't make it any better

      - Walt Disney

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      • #4
        I had the N.O. output of the Banners trigger THIS relay as I had them in our inventory.

        Edit: Now, what I don't know is whether Banner has a relay output type, as my customer requested the Banner model.
        Last edited by Todd Dice; 09-11-2018, 01:01 PM.
        Why worry? If you've done the best you can, worrying won't make it any better

        - Walt Disney

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        • #5
          https://www.bannerengineering.com/us...art.35363.html

          Looks like they do.

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          • #6
            Originally posted by techme View Post
            Well darn!!
            Why worry? If you've done the best you can, worrying won't make it any better

            - Walt Disney

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            • #7
              Thanks for pointing this out, I forwarded your posts to the Product Manager who will look into it further.
              If you have an urgent issue, please contact AutomationDirect's Technical Support team at 1(800) 633-0405 or (770) 844-4200. Thank you

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              • #8
                Two- hand safety is implemented where the ophuman operator could be caught by the motion of machinery. Like on a chainsaw, where both hands on determine the best control of the saw. Therefore, two-hands insure that the operator is giving unambiguous command to the machinery while being located at a safe position.
                dry contacts are specific to mechanical switches, including doors, motion limits or reed switches.
                optical contacts are implemented where a complete isolation is needed between the drive of the contact and the signal to the plc.

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                • #9
                  Originally posted by Alexandru View Post
                  Two- hand safety is implemented where the ophuman operator could be caught by the motion of machinery. Like on a chainsaw, where both hands on determine the best control of the saw. Therefore, two-hands insure that the operator is giving unambiguous command to the machinery while being located at a safe position.
                  dry contacts are specific to mechanical switches, including doors, motion limits or reed switches.
                  optical contacts are implemented where a complete isolation is needed between the drive of the contact and the signal to the plc.
                  This is an odd post, almost like a bot posted it. This doesn't answer any of the questions posted here. He's talking about trying to use Opto-touch units w/ solid-state outputs to interface to a 2-hand safety relay that requires C-form dry-contact inputs only.

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                  • #10
                    Originally posted by Todd Dice View Post

                    Well darn!!
                    I use the C-form relay output Opto-touch PB models all the time w/ 2H safety relays and they work fine. Not crazy about having two more devices with mechanical contacts but it's a fairly simple set-up.

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                    • #11
                      usually I don't get in at doing someone's work at choosing parts or programming. however, in this thread I considered useful to clear the water around what a switch is, or what a dry contact or an isolated contact is.
                      two-hand safety is one thing and how the signal is conditioned and transmitted to plc is another. I hope that evens it out.

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                      • #12
                        the banner switch you try to trigger the relay with; you need to check that the power the relay draws can be supplied by banner. pnp is not meant to drive relays though, is just a small signal meant to go straight to plc.


                        the reference to banner points to a switch that outputs a transistor-based signal.
                        a npn switch will provide a path to negative rail on a dc circuit, meaning the plc input needs to close the loop to positive rail. such a switch is called a sinking switch and requires sourcing from plc.
                        a pnp switch will provide connection to positive rail on a dc circuit, which is called sourcing the load (the plc input). it means that the plc output is already tied to negative rail, or sinks the switch.
                        a relay output is just a switch which could close an electrical loop. where the load (the plc input) is located only depends on the nature of plc input.
                        a dry contact is the same with the relay contact.
                        if you need to pass a banner zero pressure complementary (active signal, either pnp or npn) switch through a two-hand safety relay, each relay contact (or dry contact) of the safety relay needs to be closed and then the zero-pressure switch could work as if there was a closed path.
                        I personally don't see the purpose of passing a transistor signal through a dry contact switch on the safety relay. there is still confusion on what you want to achieve.
                        Last edited by Alexandru; 09-18-2018, 01:09 PM.

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                        • #13
                          Originally posted by Alexandru View Post
                          the banner switch you try to trigger the relay with; you need to check that the power the relay draws can be supplied by banner. pnp is not meant to drive relays though, is just a small signal meant to go straight to plc.


                          if you need to pass a banner zero pressure complementary (active signal, either pnp or npn) switch through a two-hand safety relay, each relay contact (or dry contact) of the safety relay needs to be closed and then the zero-pressure switch could work as if there was a closed path.
                          I personally don't see the purpose of passing a transistor signal through a dry contact switch on the safety relay. there is still confusion on what you want to achieve.
                          I'm not sure you understand what the 2H safety relay needs. They REQUIRE each pushbutton to have a C-form relay output. They rarely accept PNP outputs unless you're using a 2H function in a programmable safety controller.

                          He originally solved the problem by powering two small C-form relays from his existing Banner PNP output Opto-touch units not knowing that Banner makes a C-form output Opto-touch model.

                          He's not passing the PNP outputs THROUGH the relays, he's powering the relays to act as an intermediary device to give the safety relay the outputs it requires.

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                          • #14
                            "He's not passing the PNP outputs THROUGH the relays, he's powering the relays to act as an intermediary device to give the safety relay the outputs it requires"

                            I have never encountered such an electrical setup. a safety sensor is not supposed to energize a relay. however,

                            on page 4 of referenced documentation says 150mA max draw per load. since some 24vdc relays draw 30-40mA, that explains why this setup works.

                            't is true I did not understand this setup. it defeats the purpose to use a safety relay in such a setup, I think someone didn't read the specification before working this solution.


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                            • #15
                              Originally posted by Alexandru View Post
                              "He's not passing the PNP outputs THROUGH the relays, he's powering the relays to act as an intermediary device to give the safety relay the outputs it requires"

                              I have never encountered such an electrical setup. a safety sensor is not supposed to energize a relay. however,

                              on page 4 of referenced documentation says 150mA max draw per load. since some 24vdc relays draw 30-40mA, that explains why this setup works.

                              't is true I did not understand this setup. it defeats the purpose to use a safety relay in such a setup, I think someone didn't read the specification before working this solution.

                              While it's not common, using external c-form relays will work fine. The safety relay will pick up any contact weld occurring in the input relays and act accordingly (not allow another cycle until the failure is corrected). It's extremely unlikely that the two relays will fuse contacts at the same moment.

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