Safety Circuits Electrical

We have touched briefly in the past on machine safety and it might be a good opportunity to have a look at some of the things we need to consider when setting up a control circuit on machinery.

When we start with the circuit design, we need to know what category the machine falls into as this will determine the type of safety circuit we need to incorporate into the design. The one thing no one tells us is how and/or where we need to place some of the controls. To explain this, consider a machine where we need to incorporate a reset button on the safety circuit.

Take a bit of time to follow what this means:
We have a machine that has a safety gate to prevent people entering the hazardous area whilst the machine is operating. This gate has a dual channel safety switch to monitor the gate and to shut down the machine when the gate is opened. Now comes the interesting part. You have wired the safety switch to a safety relay and set the control circuit up. Does this satisfy requirements?
Actually, it does NOT! What we need to ensure is that the safety circuit is wired in such a manner that the machine will not be able to start until such time as the circuit has been reset. Think about what could happen. A person opens the gate, the machine stops and he/she walks into the protected area. The gate closes behind him/her and as such the safety circuit is “healthy” and the operator will be able to start the machine.
So, how do we avoid this? We wire the circuit in such a way that when the gate is closed and the safety circuit “healthy”, we have to push a reset button before the machine can be started. The question now is where to place this reset button? Can we position that at the operator station? Again the answer is NO. The reset has to be placed in such a position that the person that is going to reset the circuit must have a clear view of the area.
Again, there are different methods available here. You may even need to install a dual reset function. This works on a specific time between the two reset operations. Take the same scenario. The person entered the protected area and has now finished what they were doing. When leaving the area they push the first reset button. They now have a certain time to get to the second button and push that to complete the reset action. Please note that you must use approved safety relays and timers when wiring the safety circuits.
I suppose the best thing you can do is to stand in front of the machine, look at what you are planning to install to satisfy the requirements and then try to find scenarios that could make it fail. Forget your technical background. Look at it as if you are going to walk up to the machine and use it, and what you would do if something goes wrong on the machine. This is typically where you continue to ask yourself “what if”
You may argue that the person that entered the protected area did not follow procedure because the machine was not locked out. You are correct but that is another issue. From our perspective, we have to take into account that this could happen and that we ensure the person will still be protected.
Hope this short article helps a bit and until next time, stay safe and remain passionate about your trade

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