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I am probably out of date and for sure confused but generically as I understand it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