Why Use a Safety PLC Instead of Safety Monitoring Relays

Hello this is Ray Marquiss, Senior Application Engineer with Valin Corporation, and this video is going to discuss why you would use a safety PLC instead of using safety monitoring relays for your machine safeguarding. Let's get started.

During this presentation we’ll discuss these reasons for using the safety PLC instead of using the safety monitoring relay:
• Safety PLC will simplify the wiring.
• Make machine maintenance easier.
• It can make troubleshooting easier.
• It gives you more flexibility in case your safety system changes.
• Integrating to the upstream master controller or machine controller is easier
• And when it comes to the end of the line, total cost of ownership. That means design, hardware, construction and operation are all going to be less expensive.

Here’s a picture of what we would consider a simple safety circuit. There is dual channel wiring for redundancy that goes through the E-stop and the safety door interlock. You have several force-guided relays to kill power to the hazard. The hazard could be a motor or a pinch hazard or any moving part or or something that can affect the operator. There is external device monitoring wires for the force guided relays so that you can confirm the operation of those relays. You have your hazard here on the bottom with the H, and then there are some additional monitoring relays. You have these additional monitoring relays down here that monitor the hazard and give an indication when it's safe even after you shut the power. That's in the case of something like a motor that has to run down or something along those lines. That will monitor that movement and then give a signal back to the safety monitoring relay to let you know that it's safe.

Many safeguarding applications start out very simple. This has just a E-stop and a door interlock. So in the design phase this makes sense to have a safety monitoring relay and some force guided relays to control the hazard. But during the commissioning phase, if there's a wiring error, it might be tricky to find because these this E-stop and the safety interlock are wired in series, so it's difficult to tell if there's a wiring error with the safety interlock or what the E-stop. Same thing for the force guided relays. Since they’re wired in series, if one of them is working and one of them is not, it's a little more difficult to figure out where the wiring problem is. Once you overcome that hurdle and you're actually in operation, the system will work well. But during the maintenance phase, they have to do maintenance on the machine while it operates, even if it's at a lower speed that they have to do work on it or maintenance to it. And so a lot of times what you'll see is they'll take the key or have an extra key for this safety door interlock and they'll stick it in there so they can operate the machine with the safeguard disabled. Sometimes the operators of the machine will get the idea that that's a good plan is to just defeat this door interlock so that they can work more of, what they would say, more efficiently. And so that shows another weakness here is that we don't have the ability to use the safety monitoring relay to go into another mode; all we have is the safety mode.

With the safety PLC, the wiring is simplified. Each safety device is wired to an input point on the safety PLC. Just like on a normal PLC, each input device would be wired to an input, so our safety door interlock, even though it's dual channel, it'll go to two inputs on the safety PLC and they'll be monitored as if it's one input [Actually, monitored as one device, but the inputs are evaluated individually]. Same thing for the E-stop. Then we can have each of the force guided relays, even though it doesn't show it in this graphic, we could have each force guided relay on its own output, controlled by the safety PLC. And the green line, symbolizing the feedback from those relays, could go into Inputs on the safety PLC as well. And then our external monitoring devices for the hazard here, those can also go back into inputs on the PLC, so each device has its own input and can be monitored independently of each other.

In addition to simplifying the wiring, with this series wiring, if there's a problem where the safety circuit is engaged and the machine won't operate, the users have to figure out is it the e-stop or the safety door interlock. This is not a big deal with just two devices. They can check those two, but I've seen where operators have maybe 10, 12 or fourteen safety door interlocks on their machine and the machine won't operate.

If you have any questions or are just looking for some help, we're happy to discuss your application with you. Reach out to us at (855) 737-4716 or visit https://www.valin.com.

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