connecting a safety relay to PLC
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    Default connecting a safety relay to PLC

    Hi guys

    I am currently in the planning/design stage of upgrading a machine which has a lot of redundant relays and other such controls. The main panel itself is a mess of wires and connectors everywhere and not a single drawing exists. I will be installing a PLC and removing a lot of the mess. The thing is this machine has 16 doors on it all with their own safety switch (obviously) which are controlled and monitored through 2 safety relays. My question is how do i incorporate the relay into the PLC i know the relay has an output that connects back to the PLC for monitoring but im not sure how to write this piece of software code into the program. Is it just a matter of a NC contact and if it opens the PLC will not run? Also, if i put the doors and emergency circuit into the relay do i just include the normal stop buttons into the PLC software?

    Thanks in advance guys

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    In my experience, (and I'll defer to Motion if he chimes in). Unless you are using a safety rated PLC ($$$$), you want all of your e-stop / safety circuit hard wired and self-contained. The PLC is only monitoring it and shutting things down if it's activated (How much monitoring is up to you).
    Note that the new rules (several years old now) require a full dual-channel, monitored safety circuit. Thus you basically now need a "safety controller", whether that be a safety rated PLC or a stand alone controller. (Banner, Siemens, Allen Bradley, Balluff, etc all have these available.). You are monitoring for contact close time between both circuits.
    Also, now most door switches are available with RFID keys. Thus the key must match the switch via RFID code or the switch doesn't show closed. This is to prevent people from sticking any key in the switch to bypass it.

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    realistically you may need to conduct a HIRA with the machine operators and the safety committee who oversees this machine (assuming their is a safety committee).

    You mention that the machine has two safety relays at present. See if they comply with whatever safety standard that you will be held to in the event of an incident. I would think that you are to design to ISO13849 and/or IEC62061 . . . This is something you should figure out before starting.

    If the machine is designed well with a properly implemented safety circuit design, it should be fairly well isolated from the control aspects of the machine apart from the Safety Relay outputs which will be dual channel contacts that interrupt power and/or safety inputs on inverters or pneumatic dump valves or hydraulics, etc.

    This is probably not the kind of project that should be your first PLC implementation without someone with experience mentoring you along the way.

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    What type of robot control (brand)? Most robots require e-stop and some type of fence circuit for their safety functions. They are typically (depending on the vintage) dual channel dry contacts that you must interface with your safety relay. You will run door interlocks, light curtains, safety mats etc into a safety relay which will then have dry contacts that make the robots safety circuit true or not....dual channel mostly that have to actuate at the same time.

    Depending on the PLC - if it is not a safety PLC, then you are simply either going to send a status output from the robot control or the safety relay itself to the PLC.

    If it is a safety PLC and you are doing the logic of the safety circuit within the ladder instead of with safety relays, then you are sending dual OSSD signals to a safety-rated input, and handling the function blocks and ladder within the code. This is inherently different, and depending on the robot and PLC you will probably still need a safety-rated output with dry contacts, or a safety relay.

    Any questions feel free to PM me.

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    Quote Originally Posted by JL83 View Post
    Hi guys

    I am currently in the planning/design stage of upgrading a machine which has a lot of redundant relays and other such controls. The main panel itself is a mess of wires and connectors everywhere and not a single drawing exists. I will be installing a PLC and removing a lot of the mess. The thing is this machine has 16 doors on it all with their own safety switch (obviously) which are controlled and monitored through 2 safety relays. My question is how do i incorporate the relay into the PLC i know the relay has an output that connects back to the PLC for monitoring but im not sure how to write this piece of software code into the program. Is it just a matter of a NC contact and if it opens the PLC will not run? Also, if i put the doors and emergency circuit into the relay do i just include the normal stop buttons into the PLC software?

    Thanks in advance guys
    What outputs will the PLC be controlling?


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