This question has came up a few times in the recent past, and I seemingly cannot find a definative answer. I was trained that an E-Stop circuit on a machine should always be hard-wired with redundant failsafes (i.e. safety relays, etc) and this makes perfect sense, as you do not want the E-stop circuit to fail, if possible. Lately though, we have been seeing machines where all the E-Stop circuits are ran through the PLC. There is *no* physical hard-wired E-Stop circuit, even the E-stop button is just wired to inputs on the PLC. IMHO, this is bad practice. I have already seen an instance where this design failed in practical application. In these designs, the PLC is responsible for shutting down everything else, which works fine, as long as the PLC is actually RUNNING. When the PLC 'locks up', or has other glitches (RAM problems, etc) this could lead to bad things. My question is: Is it mandated anywhere (OSHA, etc) that an E-stop circuit should be hard-wired? I was hoping that someone here could provide a link to information that specifically addresses this question.
Thanks in advance.