Need basic MC/MCR question answered-explained

[jeffellis1 0]

Can someone explain to me if hard wired e-stops shown on electrical prints can be entered into ladder logic as a MC or MCR? I tried to learn more about this reading the AB help files but its not entirely clear to me.

[TimWilborne 108]

Absolutely NOT. Emergency Stops should not go through the PLC under any circumstances. Safety PLCs are a different story but I'm not completely sold on them yet. REMEMBER PLCs ARE NOT A SAFETY DEVICE!!!

[jeffellis1 0]

Ok, I hear you,, even though the e-stops will still stop or shut everything down dead in its tracks, what type of device is MC/MCR input when I see an XIC instruction in ladder logic? See what I am asking? and if plcs are not a safety device, why does Osha require two for redundancy on presses?

[Been Around 0]

jfls41 TWControls is exactly right. Years ago when I started it was industry standard to use MCR in program to stop all machine functions. It was practiced that as long as the plc did not see e-stop, or did see, people relied on plc not to energize any real word outputs. Unfortunately some people had to die before laws were enacted to guard from this. You could input contacts from estop as insurance to stop plc from execututing any real world functions or for data acquisition. I don't use MCRs much anymore. I just looked at some of my old plc/2 files and confirmed that we did use to use MCRs as e-stops. Not just us but alot of different programs by different vendors. MCRs works good to: stop section of program so you can look at registers later. ( brfore good and inexpensive data acqusition software became available). Keep sections from performing math functions On some batch files When memory is tight. ( that's not the case these days) And as a backup safety device. ( IF ONLY IN THE MIND) AS TWCONTROLS SAID,, PLC IS NOT A SAFETY DEVICE !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! I don't believe you will have the need for many MCRs. ( unless it's a vintage instructor stuck on logic in there period) As for presses, I have never needed a contact off a MCR, just looked at logic making MCR true. If you try to monitor state of MCR in progam you will not get any results as to the MCR pausing program. I have and still do work on a few presses. What I have found with redundant PLC is to check yourself. You know even safety PLCs are electronic devices. What else could you install on it that is not electronics. OSHA has no choice but to come up with a central ground to try to make different situtations as safe a possible. With a machine that has only hard wire or simple controls its easy to design components for fail safe. Presses are different. You have to monitor pressure, what with? Electronics. What else could be relied on to stop pressure from building? Or determing pressure drop.? Good luck with project. I hope this has helped some. I'm sure theres alot of discussion you can get on this subject. I look foward to reading replies. Also, MRC Means Master Control Relay If only our PLC founding fathers would have known that safety has become the forefront of controls, do you think they would have put so much time on MCRs to stop sections of logic? Is that not we use program files for ? Wow.

[TimWilborne 108]

Been Around pretty much covered everything. Great job. Just to add a few things. I really with they would just take the MCR instruction out of the software. I guess it is kept for compatibility with older processors that are replaced. The only place I hear of it used now is by instructors in classes

[Been Around 0]

Allen Bradley Bulletin 1882 copyright 1982 WARNING: A PC system should not be operated without a hard-wired Master Con- trol Relay and Emergency Stop Switches to provide emergency I/O power shut down. Emergency Stops can br moni- tored but should NOT be controlled by the User Program. These devices should be wired as describe in tle PLC-2/20 As- sembly and Instruction Manual (Publica- tion 1772-801). This is direct quote from manual page 7-1 CAPITALS and all Being said it was ignored for years. One of my fathers quotes: If carpenters build houses the way some programmers wrote programs, the first woodpecker that came along, would destroy civilization as we know it I wish I could type as good as I reed!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!1

[TimWilborne 108]

LOL! Now that's a good saying. Would give you 5 points for it but already gave them for you first post on this thead

[Mickey 71]

More about Safety: Safety_Guidelines.pdf

[Been Around 0]

Dosen't this refer to: basically The MCR instruction is not a substitute for a hard-wired master control relay that provides emergency-stop capability. You should still install a hard-wired master control relay to provide emergency I/O power shutdown. and Allen Bradley Bulletin 1882 copyright 1982 WARNING: A PC system should not be operated without a hard-wired Master Con- trol Relay and Emergency Stop Switches to provide emergency I/O power shut down. Emergency Stops can br moni- tored but should NOT be controlled by the User Program. These devices should be wired as describe in tle PLC-2/20 As- sembly and Instruction Manual (Publica- tion 1772-801). This is direct quote from manual page 7-1 CAPITALS and all Being said it was ignored for years.

[TimWilborne 108]

Well Been Around, you haven't quite been around here long enough to meet Mickey. He is our link person. Whenever there is a thread that needs a little clarification from documentation you can usually count on him chiming in with the link we need. He may not give us the proper page, but points us in the right direction. Take the link Mickey gave and search for safety. It has some very good information on what is necessary for using solid state controls in place of a hardwired safety system. Mainly lots of redundancy and fail safe wiring Mickey - Where were you on THIS topic. I needed you

[BobLfoot 302]

Let me add one thing to this discussion. There was a time, it was early 1990 when I was working for Toshiba that we received a machine from our parent company on which all the emergency stops were wired into a Toshiba T3 PLC. They even had it set up dual channel with tie down and contact weld detection. Of course this did not meet ANSI / OSHA guidelines and was not NEC. I asked my supervising engineer and he said I quote "Japenese Industrial Standard". Needless to say we American engineers prevailed and the Estops and MCR's where converted to hardwire. Just curious how some of our none USA members interpret EStop and MCR?

[Mickey 71]

Well,Hmmm "I am not an electrican". That is my standard answer to that kind of question. I was an instrutment technican for 30 years that did PLC programming for 20 of those years, but always thru the eyes of an instrutment tech. Although worked on many motor starters in my time I know my limitation and had a very knowledgeable electrican to turn to.

[TimWilborne 108]

Yes but you are our link person. You are suppose to be able to find links to everything Just kidding of course

[Been Around 0]

Well I hope jfls41 has satisfied his curiosity. Now look at programming, we just spent all this time on hardwire and safety. No programming at all with PLC. But are we not talking about process controls? Great replies, just what I expected. Thanks everbody. How about this. What kind of safety device works on a output that cannot fail open or closed. Maybe OGK or a redundant system. One of the first systems I installed was a plc-2/30 in 1982 at a Monsanto plant with over 3000 I/O just as a redundant system to Fisher Provox. I look at old program and it just dosent make since. It really wasn't redundant but seperate. They called it redundant and got it passed. what i'm trying to say, is that you can never design a system safe enough. but to much safety reduces production and can cause harm or prevent equipment from being moved to release somebody. There has to be a line where someone has to trust enough judgement to make a product without harm. You will always have human error. (just look at my typing) You have to look at each system. Where will we be in 20 years, another standard ? Electronics will always fail at some point anyone do not believe this, get back to me in twenty years and tell me your stories of I don't know how that happened. As with most of OSHA and MSHA, these laws have been written in blood. Everyone keep preaching safety. I don't care what laws and publications they come out with, I will always design a system with safety in mind. I don't lay at night and wonder what if someone does this? But I still do. I lay and wonder how could I have designed this better and make it safer!!!!!!!!!!!. Anyway Happy stories.