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When not to trust the electronics

Modern electronics are extremely reliable, especially if well designed and used correctly. Nevertheless, silicon chips contain features smaller than the wavelength of light, and control programs contain many thousands of lines of program code. Electronics can (and do!) fail, be it due to electrical failure or imperfect programming.
 
Would you bet your life on the electronics not failing? I wouldn't, and I make the stuff!
 
Any industrial machine that can hurt people should be fitted with safety cutouts and interlocks. The most common safety measure is the emergency stop, sometimes called E-stop or mushroom switch. On conveyors it would be a rope switch. It could also be a fence or physical guard barrier with a safety switch. These are wired so that if activated they immediately remove power from the machine or otherwise inhibit it from making a dangerous move.
 
Do not ever even think of wiring the safety switch to the controller and having the controller stop the machine. Always wire the safety switch directly into the machine in accordance with safety design rules in your location. If the safety switch has an auxiliary contact, by all means wire that into the controller so it can know what has happened and not get upset, but do not expect the controller to make safe.
Do not treat this as expert advice. If you are in the least bit of doubt, consult an expert.
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