I am having trouble trying to get my head around using safety relays in PLC controlled machinery. I want to install a PLC into a machine that needs upgrading, it is straight forward enough, well i thought so until i ran into two safety relays im just not sure how or if i should tie them in to the PLC. The relays are controlling 4 estops and 12 guard door sensors, which i thought the PLC would be capable of doing. So how do i keep the two seperate or do they get hardwired to each other? The net seems to be very scarce on this info
It is very important for safety, regulatory and legal reasons that you get this right.
- First you must assume that your PLC could fail with an output on, off, or switching due to software or hardware issues.
- You should, of course, put normal safety interlocks into your code but you do not rely on these for personnel safety.
- You then hardwire (* see note) the safety circuits so that even if the PLC goes berserk the e-stop will always render the machine safe.
Your safety relay will have auxiliary contacts one of which can drive an input to the PLC so that the PLC knows the status of the safety circuit.
Safety circuits now almost all use dual channels and monitoring. E-stops have two contacts that are forced apart (not sprung apart) when the e-stop button is pressed. Similarly guard door switches have two contacts and again the act of opening the door will force the contacts apart. If one channel fails to open on e-stop the other will make the system safe but the monitoring will prevent a reset. This forces the user to investigate the problem.
See my answer to Symbol or marking on safety relay for an in-depth of how a safety relay works.
- The situation has changed rapidly over the last few years with safety rated logic controllers. These are beyond the scope of this answer.
The relays are controlling 4 estops and 12 guard door sensors ...
No. The e-stops and guard door switches are controlling the safety relay or, if you prefer, the safety relay is monitoring the switches. The safety relay is controlling the load to ensure energy (electrical, pneumatic, hydraulic, etc.) is de-energised when required.
Related: my answers to the questions below may be helpful.
There are also "Safety PLCs" that can perform the safety functions and the normal logic functions in the same device, eliminating the need for the Safety Relays. If you are already going to have a PLC, that's often a better way to go, keeps everything together. Safety Relays are more useful when you have no PLC in the machine already, or are retrofitting a machine with and older PLC in it and cannot afford to replace the entire PLC.