jshiepe
Member
Well folks, I've been lurking here for a while, occasionally responding to a few threads, but now ask the assistance of the great minds out there, specifically the OEM's and SI's.
For the longest time, and even still today, I have used the practice of terminating power to PLC outputs when the E-stop is pressed, while leaving the input power energized to maintain the state of the program. To me, this is generally a safe practice, especially in today's world of remote tech support where the tech should have complete confidence that if the e-stop is pressed, nothing on the machine should energize or actuate.
We recently ordered some new equipment from two different vendors on which I found out that this practice is no longer observed. Both are reputable vendors, so naturally I thought that this was some coincidence. Is this now an accepted practice?
The scenario that makes me nervous is the fact that a programmer could potentially force an output and energize something while the machine is e-stopped. If this is an air-driven machine, and the e-stop kills the air to the valve bank, but the outut power for the solenoids is still live, is this an acceptable e-stop state?
I've reviewed NFPA79 and found a couple of arguable points (9.4.3), but I'm just curious if there was some sweeping change in the way e-stop circuits are now allowed to be run through PLC's rather than hard-wired, and output power does not need to be removed.
Thanks,
Jeff
For the longest time, and even still today, I have used the practice of terminating power to PLC outputs when the E-stop is pressed, while leaving the input power energized to maintain the state of the program. To me, this is generally a safe practice, especially in today's world of remote tech support where the tech should have complete confidence that if the e-stop is pressed, nothing on the machine should energize or actuate.
We recently ordered some new equipment from two different vendors on which I found out that this practice is no longer observed. Both are reputable vendors, so naturally I thought that this was some coincidence. Is this now an accepted practice?
The scenario that makes me nervous is the fact that a programmer could potentially force an output and energize something while the machine is e-stopped. If this is an air-driven machine, and the e-stop kills the air to the valve bank, but the outut power for the solenoids is still live, is this an acceptable e-stop state?
I've reviewed NFPA79 and found a couple of arguable points (9.4.3), but I'm just curious if there was some sweeping change in the way e-stop circuits are now allowed to be run through PLC's rather than hard-wired, and output power does not need to be removed.
Thanks,
Jeff