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Gamble

Plantwide E-Stop Scheme

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I'm looking for a point of reference (preferable some established standard) to establish e-stop zones in a plant. One of the specific systems (which was installed and ran for 10 years or so with major e-stop gaps) has equipment that spans two floors. Some of the equipment in the basement is tied into other parts of the plant, such as some oil heaters and water pumps. It seems like it may be in appropriate that if someone upstairs hits and e-stop it has to shut down all the equipment in the basement. I know this is going to be very application specific, but like I said I'm just looking for a point of reference so that I can better work with the plant personnel to establish a solid e-stop scheme. I've tried searching OSHA, ANSI, NSA, and NFPA with no luck.

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It is a common fallacy and very dangerous to "just add E-Stops" and assume that all your troubles are solved. First off, an E-Stop is actually a very POOR safety mechanism when used for the appropriate purpose and not something else (emergency stopping). Why? Think about the standard response of an operator in the event of a life threatening emergency. Usually the responses are either to freeze or run. It takes some thought process to actually hit an E-Stop. Unless you run regular and routine drills (think military training methods), the automatic response to hit the E-Stop usually doesn't happen until AFTER either the machine has destroyed itself or someone is hurt. I have witnessed this happen time and again, and seen operators get disciplined because they did the normal human reaction (freeze or run) in an emergency situation. Second, you won't find a "right way to do it". You need to do a risk assessment of your machine/process. This risk assessment will indicate exactly what sorts of safety mechanisms you need and where they need to be placed. This includes E-Stops, if they turn out to be appropriate after all. You need to pick a risk assessment procedure. I recommend the RIA (Robot Industries Association, www.robotics.org) risk assessment procedure which is an ANSI standard. Out of the ones out there, it is the most concrete, leaving nothing to interpretation, and simple to implement. And in answer to your specific question, what you are describing is probably a "multiple zone" system. Usually you can achieve this by placing E-Stop relays in series with each other, forming a hierarchy. AB also makes a nice 3-zone system (Minotaur 300 series) which is relatively inexpensive for what all it does. Beyond these designs you'll have to cough up for a safety PLC which is much more expensive ($2-3K+ for a simple one that doesn't even let you do any non-safety functions).

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I agree with paulengr in that the Estop is not always a good answer. Human nature is unpredictable to the extent that you have to rely on somebody being there (and paying attention) to hit an Estop if they don't "run or freeze". I've seen a high speed wire winder torn to heck as the operator watched (while standing next to the Estop). I have always done everything I could to build safety mechanisms into the equipment as much as possible. My rule of thumb is do what ever I can to make sure safety (machine and human) is a mandatory function and not option. If it is optional then sooner or later someone is going to opt not to use it. Allen Bradley used to have some good information in their safety products catalog. It had a nice spreadsheet to determine what schemes would be appropriate and I think it also listed some governing body sources. I don't currently have one and haven't seen one in a while but you might check it out.

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Hey Gamble; Sorry for the late reply, haven't been here in a while. I hope this is both helpful and in time to be of use. If there is something you think needs to be added I'd appreciate the input. The standard I've arrived at is from my own experience, as I don't know of any particular 'standard' for planning this. Our OSHA regs just require an e-stop to be within a certain distance to each operating location of a machine, and every so often around it. I think most installers go overboard around here, to err on the side of caution. I also find that so-called e-stops are habitually used as sustained stop commands, not for emergencies, so that figures into my strategy. Operators look upon them as a general tool that is on their panel to be used. I have worked on a system where we had a separate global e-stop that shut down every piece of equipment in the room, which was never hit, and separate individual e-stops for each zone of equipment, which constantly were. The way we do it is that an e-stop button will affect any output or machine with controls in the same panel, and be, or cause, a physical electrical disconnect. If a control panel is large and has buttons for more than one zone of equipment, then I have an individual e-stop located above the controls for each zone. This is coupled with a strategy whereby no control button can affect a piece of equipment unless there is a clear line of sight from that panel. Depending on how interconnected the equipment is and how product flows from zone to zone, satellite e-stops will either be in series with the one on the panel, or be specific to the machine they are on. I let the overall process flow determine that - if the machinery is a station with internal processes then individual buttons are better. If it is all interconnected and mostly product transfer, then one button could shut off several zones. It depends on the complexity of the individual station, and whether or not you want, say, a press system to be interrupted when a nearby conveyor has a problem. For HMI systems, operators are always controlling equipment they can't see, and that is different. My e-stop plan for that is case by case, but physical e-stop locations around the machinery are easier - line of sight. I have upgraded wiring and e-stop plans where buttons would operate something in another room entirely - a weird and very unsafe situation. Following this rather simple concept simplifies my choices when planning the manual interface. Seems simple, even obvious I guess, but hopefully this will be of use. Happy programming! speakerman.

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