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Now you can no longer use MCR for safety?

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  • Now you can no longer use MCR for safety?

    Back in the early 2000's when I did controls, I would use a multi contact mechanical relay as my safety device where I would wire one of the NO contacts in parallel to the start switch and have this contact wired to the relay's coil. Then in series all the safety switches and e-stop would be wired to the coil. If any safety switch tripped or E-stop pressed or the stop button pressed, then it would not come back on until you pressed the start button again.

    Now getting back into controls in 2019, I can no longer do this design because some recent safety law passed that states I must use a safety relay? What directive is this that requires that I do it like this way now? Also, apparently I now have to use coded switches for safety instead of just regular ones? Man, things have changed so fast in just a few years.


  • #2
    The standard are toward requiring systems where two or more methods of cutting the power must fail at the same time to create an unsafe situation. In safety light curtains there must be cross checking between circuits of different types to prevent common failures. The use of forced guided relays with monitoring contacts make it so that welded relay contacts prevent a start. The coded switches etc are to guard against stupid or malicious operators from causing a problem for people. The emphasis is preventing harm to people as paramount. Seek out a safety design course.
    thePLCguy

    Bernie

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    • #3
      Don't have light curtains in this design. So I'll also have to use a safety relay for guard doors? Im designing the controls for a 25 HP shredder machine where a side panel to the hopper is to have a safety switch in place. I'll update it to use a safety relay and coded switch instead of a sealing master control relay and regular switch. Thanks.

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      • #4
        It is best if you use a two-channel safety relay on your application. I use THIS relay. One normally open circuit of the relay powers THIS force guided relay. I use a reset pushbutton to enable the safety relay, and the reset pushbutton is wired in series with the NC circuit of the force guided relay in the enable circuit. I came by this wiring method from a safety wiring manual published by Allen-Bradley. However, I found THIS Idec document on the web, and the third page illustrates what I've described.
        Last edited by Todd Dice; 09-08-2019, 10:14 AM.
        Why worry? If you've done the best you can, worrying won't make it any better

        - Walt Disney

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        • #5
          There are other things to consider as well if that shredder is still rotating when your door is open. That's not generally allowed either. Are you using a drive or just direct wiring to AC for the motor?

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          • #6
            Originally posted by Adisharr View Post
            There are other things to consider as well if that shredder is still rotating when your door is open. That's not generally allowed either. Are you using a drive or just direct wiring to AC for the motor?
            +1. Good catch!

            If an operator can get into the area where you're shredding, and blades are still spinning after an E-stop is implemented, you should use solenoid locking switches like THIS, so an operator cannot open a door until after all motion is stopped.
            Why worry? If you've done the best you can, worrying won't make it any better

            - Walt Disney

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            • #7
              Originally posted by Todd Dice View Post

              +1. Good catch!

              If an operator can get into the area where you're shredding, and blades are still spinning after an E-stop is implemented, you should use solenoid locking switches like THIS, so an operator cannot open a door until after all motion is stopped.

              This discussion and your link to the interlock switches brings up a situation that we also want to resolve. We have chop saws at work that we use to cut up scrap plastic into smaller parts so they can be sent into a grinder and re-used. The motor for the saw is a AC wired motor, no drive. The way the manufacturer had implemented a delay for opening the door to access the saw blade was a long bolt that would take several minutes to unscrew, by that time, the saw blade will have stopped. These are now long gone as they wore out and after repeated welding, cannot be restored. They used a stop gap method of installing padlocks on the doors that only a foreman has, so the operator can't just open the door.

              I am thinking I could use these interlock switches, I could add a switch on the panel that the operator would select between "running" and "open doors", when the open doors is selected, it would cut off the coil to the contacter that powers the saw, then using something like the Fuji time delay relay, would start a timer for say, 60 seconds, then once the timer expires , would energize the interlock switches to allow opening the door. It would also not allow the saw to operate if the doors are not closed.

              Do you think this is a viable solution?

              Mike

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              • #8
                Originally posted by mikeexplorer View Post


                This discussion and your link to the interlock switches brings up a situation that we also want to resolve. We have chop saws at work that we use to cut up scrap plastic into smaller parts so they can be sent into a grinder and re-used. The motor for the saw is a AC wired motor, no drive. The way the manufacturer had implemented a delay for opening the door to access the saw blade was a long bolt that would take several minutes to unscrew, by that time, the saw blade will have stopped. These are now long gone as they wore out and after repeated welding, cannot be restored. They used a stop gap method of installing padlocks on the doors that only a foreman has, so the operator can't just open the door.

                I am thinking I could use these interlock switches, I could add a switch on the panel that the operator would select between "running" and "open doors", when the open doors is selected, it would cut off the coil to the contacter that powers the saw, then using something like the Fuji time delay relay, would start a timer for say, 60 seconds, then once the timer expires , would energize the interlock switches to allow opening the door. It would also not allow the saw to operate if the doors are not closed.

                Do you think this is a viable solution?

                Mike
                It sure sounds like it is.
                Why worry? If you've done the best you can, worrying won't make it any better

                - Walt Disney

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                • #9
                  On a machine we updated the controls for that had a spinning component with a lot of inertia, we used a zero speed sensor in conjunction with locking door switches.

                  https://www.automationdirect.com/adc..._safety_relays
                  -Lee

                  “Felix, qui potuit rerum cognoscere causas” - Happiness is understanding how things work.

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                  • #10
                    And to add to the Discussion..

                    We have a Fairly Large Glass Enclosure around some filling Equipment.. it has Time Delay Locks on the Doors Lock Safety Latches.. So humans cant get in till three seconds after the last machine stops moving ( tach readings from the Servo Drives ).. however the 'Glass Room' is large enough to fit a few people in it.. the problem is, they used 'Power On' door Unlocks.. so when the Power Drops, you are locked in the Glass Room!..

                    We have had to add 'Pull Releases' on the inside of the Room to be able to escape when the machine is powered down for Maintenance.. Cause if you are doing work in the machine, and the Door Closes, your Trapped..

                    Not a great design.. but it's a Safety Circuit, and we cant change the Logic to 'Power Lock'.. but we can add the Safety Release..

                    Cap

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                    • #11
                      Originally posted by Cap View Post
                      And to add to the Discussion..

                      We have a Fairly Large Glass Enclosure around some filling Equipment.. it has Time Delay Locks on the Doors Lock Safety Latches.. So humans cant get in till three seconds after the last machine stops moving ( tach readings from the Servo Drives ).. however the 'Glass Room' is large enough to fit a few people in it.. the problem is, they used 'Power On' door Unlocks.. so when the Power Drops, you are locked in the Glass Room!..

                      We have had to add 'Pull Releases' on the inside of the Room to be able to escape when the machine is powered down for Maintenance.. Cause if you are doing work in the machine, and the Door Closes, your Trapped..

                      Not a great design.. but it's a Safety Circuit, and we cant change the Logic to 'Power Lock'.. but we can add the Safety Release..

                      Cap
                      A lot of the newer door lock designs have an inside door opening mechanism now to prevent that but they're fairly pricey.

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