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jhughes

Estop Position

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I have a small control panel with a Horner HMI/PLC combo that has a limited power source (120vac, 10 amp max). The panel runs a small torque limited stepper motor that takes a direct 120vac 5 amp supply. Is an EStop switch with the 120volt supply sufficient as a means of disconnect for this purpose? Do I need a disconnect switch on the device as well? It seems redundant on a panel that is 12" x 12". When the Estop is pressed power to the remainder of the cabinet is disconnected.

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Really, really bad idea for a couple of reasons. First off, you need a separate function to reset things according to NFPA 79. Second, unless you have some sort of power-on routine in the PLC, then the system will go immediately back into motion doing whatever it was doing before it lost power, which is a blatant violation of NFPA 79 (the reset function should leave it in a position where you have to restart things). Third, it's a really crappy idea from a troubleshooting point of view since now nobody can diagnose what was going on at the time that things died...all troubleshooting state has been wiped out. Although its legal. Fourth, it is a blatant safety code violation under all current safety codes to just throw in an E-Stop willy nilly like that without doing a proper risk assessment. Operators are only good for a SIL 1 rating so no matter how good your E-Stop is, that's all it gets for a rating. If there's an actual safety concern that is higher than a SIL 1, then an E-Stop is useless because it doesn't meet the requirements of the risk assessment. About the only time that you have to have an E-Stop is under OSHA when you have a press application because they have that written into the federal regulations, and that's the only place it's required. NFPA 79 doesn't require E-Stops, only describes what's required to implement one. Fifth, E-Stops are a very bad idea from a human behavioral point of view. When there is an emergency, humans tend to have one of two instincts: flee, or freeze. Neither one gets the E-Stop pressed. The only practical way to use E-Stops is to have constant and routine and regular drills that enforce E-Stop usage to the point that it becomes an automatic reaction, akin to military-style training. Since no plant I've been in does that, they are mostly just there as nuisances or to get abused by production for a thousand things that are not safety related. If it's not an E-Stop and you're not going to implement it like one, then it should be labelled "stop" or "process stop". Sixth, using a push button as a power disconnect device even if it can handle the current is a bad idea. For instance, the contacts could fuse together. It's meant as a safety grade device (if it's meant as a true E-Stop) as an input, not to carry power and to disconnect under load, or worse yet, to disconnect under overload.

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WHERE IN A PROGRAM WOULD YOU PUT AN ESTOP TO JUST TURN OFF OR UNLATCH THE MOTOR CONTACTS SAY FOR EXAMPLE THE PHOTO EYE WENT BAD AND THE MOTOR/PUMP KEPT RUNNING SO YOU HAVE TO HIT THE ESTOP IN ORDER TO STOP THE MOTOR/PUMP .... and sorry about the caps i just saw it was on here is my code i`m trying to write and estop and a man/auto switch is kicking my butt SCISSOR LIFTS.RSS

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