E-Stops in industrial control circuits
- Thread starter sluton
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- Illinois
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- retired electrician
Re: E-Stops in industrial control circuits
That is not covered by the NEC. In my opinion, a true e-stop should not use the PLC to shutdown the equipment. We use a hard wired relay for e-stops on PLC controled systems. The relay drops out the power to all of the output modules and to the motor starter coils. The relay is wired using only normally open contacts and the coil is normally energized. Any coil failure, e-stop circuit failure, or operation of the e-stop switches will cause a shut down.
Don
That is not covered by the NEC. In my opinion, a true e-stop should not use the PLC to shutdown the equipment. We use a hard wired relay for e-stops on PLC controled systems. The relay drops out the power to all of the output modules and to the motor starter coils. The relay is wired using only normally open contacts and the coil is normally energized. Any coil failure, e-stop circuit failure, or operation of the e-stop switches will cause a shut down.
Don
friebel
Senior Member
- Location
- Pennsville, N.J.
Re: E-Stops in industrial control circuits
To: sluton, and don_resqcapt19,
I totally agree with you Don, never use a PLC program for an emergency shutdown system.
I worked for a large Chemical company for many years, and we had a program, called a Process Hazards Review. It was our company policy, world-wide not to depend upon a software program for an emergency shutdown. It was always hardwired, as you said Don, with a hardwired relay etc.
To: sluton, and don_resqcapt19,
I totally agree with you Don, never use a PLC program for an emergency shutdown system.
I worked for a large Chemical company for many years, and we had a program, called a Process Hazards Review. It was our company policy, world-wide not to depend upon a software program for an emergency shutdown. It was always hardwired, as you said Don, with a hardwired relay etc.
gregoryelectricinc
Senior Member
Re: E-Stops in industrial control circuits
I agree, ALWAYS hardwire an E-stop. However, most PLC's will allow you to use the E-stop as an input also. This is sometimes useful for warning buzzers, flashing lights, etc. Usually these inputs are isolated from all the rest of the PLC's inputs so they stay active when the E-stop is pushed.
I agree, ALWAYS hardwire an E-stop. However, most PLC's will allow you to use the E-stop as an input also. This is sometimes useful for warning buzzers, flashing lights, etc. Usually these inputs are isolated from all the rest of the PLC's inputs so they stay active when the E-stop is pushed.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Re: E-Stops in industrial control circuits
I just read an article in the AB journal on estops and NFPA 79. the article stated that NFPA 79 now allows estops to be done with a PLC instead of a hard wired relay.
Here is a link to the article
http://www.ab.com/abjournal/sept2003/departments/technology_watch/index3.html
I still like hardwired, but PLCs are very reliable, but I have seen several develop fatal errors. I suppose you could have the estop drop out on an error
I just read an article in the AB journal on estops and NFPA 79. the article stated that NFPA 79 now allows estops to be done with a PLC instead of a hard wired relay.
Here is a link to the article
http://www.ab.com/abjournal/sept2003/departments/technology_watch/index3.html
I still like hardwired, but PLCs are very reliable, but I have seen several develop fatal errors. I suppose you could have the estop drop out on an error
Re: E-Stops in industrial control circuits
IMO Hard wired is the only way to go. A life may depend on that machine stopping. The E-Stop has to be accessible, simple, and foolproof. In the past I used detent type pushbuttons to drop out the holding contacts on the motor starters and run relays. The machine couldn't be re-started unless the detent button at the incident location was manually reset. This prevented a remote operator from re-starting a motor or process. Some of the process lines were up to 300' long with numerous motors and drive systems. I personally would never depend on a PC to provide E-Stop.
steve
IMO Hard wired is the only way to go. A life may depend on that machine stopping. The E-Stop has to be accessible, simple, and foolproof. In the past I used detent type pushbuttons to drop out the holding contacts on the motor starters and run relays. The machine couldn't be re-started unless the detent button at the incident location was manually reset. This prevented a remote operator from re-starting a motor or process. Some of the process lines were up to 300' long with numerous motors and drive systems. I personally would never depend on a PC to provide E-Stop.
steve
Re: E-Stops in industrial control circuits
i think there is a mis-understanding of what the article said.
you can replace the hardwired relay controls with a safety plc controller. this is different from a standard plc. the safety plc is specifically designed for e-stop circuits and can allow the user to run a coax type cable from the main processor to remote modules. this eliminates all the wires from the e-stops back to the central location. depending on the type safety plc module used, you can also assign safety matts, light curtains, and other safety items as the inputs, and a second module for the outputs to control the machine operation (such as conveyor motors)based on what safety device was tripped.
the safety plc can also communicate back the the machine control plc via its communication cable.
pilz makes several versions of e-stop controllers and safety plc's. we quoted a system which required 30 of their safety plc's.
i hope this information helps.
i think there is a mis-understanding of what the article said.
you can replace the hardwired relay controls with a safety plc controller. this is different from a standard plc. the safety plc is specifically designed for e-stop circuits and can allow the user to run a coax type cable from the main processor to remote modules. this eliminates all the wires from the e-stops back to the central location. depending on the type safety plc module used, you can also assign safety matts, light curtains, and other safety items as the inputs, and a second module for the outputs to control the machine operation (such as conveyor motors)based on what safety device was tripped.
the safety plc can also communicate back the the machine control plc via its communication cable.
pilz makes several versions of e-stop controllers and safety plc's. we quoted a system which required 30 of their safety plc's.
i hope this information helps.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Re: E-Stops in industrial control circuits
thanks Pwhite for clarifying that on the safety PLC. If this is a an area a reader is interested in, then the AB article should be read.
thanks Pwhite for clarifying that on the safety PLC. If this is a an area a reader is interested in, then the AB article should be read.
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