E stop and PLC with HOLD or Pause

This has been started in another post
http://www.plctalk.net/qanda/showthread.php?p=361488#post361488

A referance was made in that post (# 18) by Mordred
http://cableorganizer.com/osha-regulations/?p=14

The referance to OSHA by article brought up the idea of tool changes and how to deal with them on equipment controlled by PLC.

I am probably out of date and for sure confused but generically as I understand it

Pushing the stop stops the PLC where it is but may allow timers etc to run out depending on how programmed. This is not an acceptible lock out tag out practice.

Pushing a HOLD or PAUSE stops the PLC and teh machine and allows an operator to do a tool change. The PLC is not deenergized. Inputs are not deenergized and only some outputs may or may not be deenergized. This does not satisfy lockout tagout and I would be leery of this. IF I was changing a tool I for sure would want some positive means of making sure that motor for the chuck could NOT start in any condition ie a disconnect switch - with that there may have to be a contact in disconnect telling PLC it is open and not to run until disconnect is shut.

Pushing E stop.
In most places I have worked for many machines resetting E stop would restart machine. Basically the E stop swithc was used as start stop. I have never liked this and it is now getting recognized as dangerous.

As I understand it in a low risk machine pulling the E stop will
deenergize all relays to all motors cylinders etc
EXCEPT those that must run to place machine in safest position.
Deenergize power to inputs
Deenergize power to outputs
both above may have the exception to allow running to get machine to a safe position then all is shut down.
Pulling an E stop will not satisfy lock out tagout although it is common to rely on E stop for lockout tagout. This is bad practice.

IN more dangerous machines with higher risk then comes the need for safety relays and I am not proficient in these because of lack of exposure. As I understand it these will protect against welded and or dust jammed contacts. I will have to study up on these.

I have worked with E stop switches where the contact block was attached to an acuator which would mechanically latch in UP or LOWER position where LOWER is E stop actuated and UPPER is machine run. What bugged me about these is the contacts in the screw on contact block were NC
IF the contact block became unattached from the actuater then the function of the switch was completely defeated. I always made sure the attach screws were tight and never saw a block fall off - YET,,,
Is there a better way or better build E stop switch?

Dan Bentler

I never came across equipment restarting when an E-Stop is pulled out until I came across the pond and seen in the US and Canada.

In Europe its generally a few buttons.

1. Pull out the E-Stop
2. Reset the E-Stop cct with a RESET button
3. Re-Start the System

Also pressing an E-Stop is something that should rarely be done, only in an Emergency (hence the name).

Finally an E-Stop should stop whatever you can see, if you can see someone in trouble 20m away, you should be able to press the nearest E-Stop to save them.



I was so shocked when I first came over here, people pressing E-Stops, so they can climb on conveyors to release jams, jumping off and pulling the E-Stop out and voila the line restarts.

Zonal E-Stops that stop a handful of conveyors, so you can press an E-Stop between 2 conveyor lines and only 1 stops (and then perhaps 5 or 6 conveyors only).

It appears that on the American continent, production is more important than lives.



Using a PLC to isolate for someone to get their hands in should also be a no-no.

By the way, if it has machine tools then surely there will be a guard to open to kill the equipment.

In general any equipment i.e. motors cylinders or rotating machinery that could cause injury must be stopped imediately.
an example may be a traveling car, if someone was hit or trapped in such a manner then a person pressing a e-stop button, the equipment should stop imediately.
The only safe way with motors to guarantee that a welded contactor can still release the potential from a motor is to use a main contactor for the supply to all other motor contactors in that group, this should be driven from a safety relay that has positive guided contacts that also monitor the feedback from all contactors in that group, if on an e-stop one of the contactors welds in then this will be detected by the safety relay thus disabling a re-start until all contactors are proved to be open.
In general it is not required to shut down the plc but rather remove supplies to the outputs that control any equipment that could cause injury (again in some instances it may well be that by removing the potential instantly would couse more harm so this has to be taken into consideration).
Each country has it's own requirements on safety e-stop circuits & all designs should be done by people that understand the regulations of that country and indeed are qualified to a degree of competence in that field.
To design a control system one of the first essentials is to assess the risks of potential harm etc. then where possible design out hazzards or put in place practices to reduce the risk of injury.
I think that anybody working on control systems be it software only or hardware should fully understand the concept of risk assesment & the processes to design out hazzards or put in place safe systems of work.

Peter

It appears that on the American continent, production is more important than lives.
REPLY I fully agree with you on this.


Using a PLC to isolate for someone to get their hands in should also be a no-no.
REPLY I agree but it seems to be very common especially with tool changes.

By the way, if it has machine tools then surely there will be a guard to open to kill the equipment.
REPLY If it is only a door switch then this is relying on an interlock for safety which dating clear back to my Navy training is an absolute no no. This is why my comment that I would want some means to disconnect power to the motor driving the chuck so that it does not turn my hand into "ground chuck"

Dan Bentler

As a participant in the original thread that has now carried over to this one, i agree with most of the comments that have been made here thus far. An ESD button should immediately stop the process and any / all moving parts, machinery, motors, etc. Once the emergency situation has been rectified, the ESD system should be reset and then the process restarted. An ESD button should stop the entire process, not just part of the process. If individual shutdowns or stop functions are required for indivual pieces of equipment, the 'stop' button should be different from the ESD buttons (either a different color or different style) and should also be labeled according (local shutdown, etc).

mordred said:

http://cableorganizer.com/osha-regulations/?p=14
or this page

Click to expand...

PeterW said:

It appears that on the American continent, production is more important than lives.

Click to expand...

parky said:

To design a control system one of the first essentials is to assess the risks of potential harm etc. then where possible design out hazzards or put in place practices to reduce the risk of injury. I think that anybody working on control systems be it software only or hardware should fully understand the concept of risk assesment & the processes to design out hazzards or put in place safe systems of work.

Click to expand...

You've got some interesting set ups Lietmotif any systems we have in our plant have 3 main schemes on Emergency stops.
1) the estop deenergizes all power including the PLC
2) PLC is still on but all outputs are immediately turned off except on controlled stops or where the same position must be maintained such as on a ram based system. Or holding position power such as on chain based elevator systems In our case the Pork Hog incline conveyers.
3) All systems require a minimum of disnegaging the E-stop and pressing either start or Disingaging the E-stop, then reset then start.

We have zero systems where merely disengaging the E-stop allows the system to start back up

My only concern where I work is on the categories of emergency stop circuits
some equipment we have here according to some of the literature I've studied should I feel be a category 3 circuit as opposed to a category 1 these are machines that were developed a decade or more ago so are not on par with current codes in that regard however any newer machines we have been getting are with the correct categories thus far. Including X- ray machines, Ram systems and systems with blades ginders etc