A system that I am using a gate valve to restrict flow leaving a tank - details of what are going on in the tank are not important though it does involve a motor. I have an E-Stop on the control panel that will kill power to the motor(s). However, if I kill power to the gate valve actuator I could, potentially have an overflow condition that the E-Stop should prevent, not make worse. I have been told that E-Stops should always be hard-wired. Does my gate valve require special circumstances where, when an E-Stop signal is received, the PLC will power the contactors to open the gate valve actuator? What are your thoughts or are mine too confusing? 1) E-Stop means every put into a safe state, powered off when appropriate. This is what I subscribe to. Don't just kill power to everything, because of the exact scenario you describe. Some things are BETTER OFF being powered into a safe condition. Think of a Nuclear reactor. If an E-Stop killed power to everything, including the motors for the control rods, the reactor goes critical! In that industry, they use the term SCRAM to denote a rapid shutdown to a safe condition (SCRAM stood for Safety Control Rod Axe Man; based on a guy who had the job of holing a fireman's axe to cut the manila rope holding the control rods if everything else failed). That to me sounds more like your situation. 2) E-Stop means immediately remove all potential sources of energy from everything. The Automotive industry seems to like this a lot but my argument against it, aside from the above, is that it also denotes a false sense of security. I have seen people hit an E-Stop button and then go into electrical equipment assuming it was "safe". The only true method of removing power is a visible blade disconnect device in my opinion (I'm even a little distrustful of circuit breakers).

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