About Emergency Switch...

[Tom749 0]

I have one question about emergency switch. When I sketch the Emergency switch on the control panel drawing, the emergency switch is drawn as normal close(B contact). At the normal situation the line is connected by the normal close(B contact). But when someone push the emergency button at the emergency situation, the switch is cut off as a normal open(A contact) for stopping of whole system. At this point I have one question. That is how I can program the PLC code by the ladder. For instance normal open? or normal close? Waiting for your advices. For more understanding I attach the drawing. (p.s If someone give me the sample of the emergency switch drawing and PLC program I appreciate that.) Emergency Switch(DWG).bmp Edited 10 Oct 2010 by Tom749

[bartolim 0]

Typically it is not considered good practice to use an E-stop switch to a PLC input to control a machine or process. Use the NC contact of the E-stop to directly control a motor starter, etc., with an auxiliary NO contact on the E-stop switch to signal the PLC that the E-stop has been pressed. That way, you are not depending on PLC logic, which may or not be running, to stop a macnie or process that could possbly cause injury to personnel or damage to equipment. Hope this helps. Edited 10 Oct 2010 by bartolim

[Leitmotif 0]

A lot has been written here and in other places referring to E stops. This is not an easy question and there can be different answers adn opinions. The purpose of an E stop is to completely shut down a machine in emergency and is intended to protect PEOPLE not the machine. All parts of the machine should go to zero potential mechanical state. In some cases components must be allowed to move to aquire zero mechanical state. Yes you are correct they are open contact (in pushed in position ie E stop activate) and normally closed when pulled out ie OK to run. I prefer to use two contacts in series to ensure activation - same as I do on the STOP switch. I care twice about being able to STOP the machine than abilty to start it. I noted with Cutler Hammer you can remove the contact body from the actuator and if this were done on a machine the E stop will not activate no matter how many times you push the red button. Reactivating an E stop should (must??) NOT start the machine by itself. This is quite common in my experience and I believe can be dangerous especially on machines with multiple E stops in multiple locations. In general an E stop (MUST ??) should kill all contol power to deactivate all relays thus shutting down all motors etc. You can wire the E stop into the PLC (configure it in same method as the stop button) - HOWEVER - the average PLC cannot be relied on to shut down machne components - that is why you kill control power. You can get PLCs that are safety related and are designed and tested to provide personnel protection. To conclude When designing you have to evaluate the risk and degree of hazard to people the machie imposes THEN decide on control mechanisms algorythms to achieve this protection. I woud recommend you 1. come up with the evaluation of risk 2. Come up with preliminary design of controls 3. Have someone check the design (we all make errors, do not think of everything etc) 4. IF machine is a high risk machine ie punch press etc have an independent agency review and approve. Dan Bentler

[ssommers 20]

I agree with everyone here. The NC portion of an emergency stop belongs in the hardwired portion of the circuit, not the PLC logic. However, you can monitor the e-stop in the PLC circuit and for that I usually use a NO contact on the e-stop button (pull cord, guard, etc) to signal a light (HMI, etc). This can be especially helpful when you have several different places with e-stops so the operator can tell that all of them have been reset before trying to restart the machine. Definitely evaluate the risk for each point where you want to put an e-stop. Remember, it's for the operator's safety, not the machine's.