PLC , Programming question

[jbelectric777]

Hi All, I have a question about my actual work. I am getting back into PLC programming on an old account and I am brushing up on the software. My question is: In the software to make a magnetic contactor or relay latch you use an OTL and to unlatch you use an OTU respective elements and use a N.O momentary contact for both stop and start, which is fine. In the old days when we wired the controls in the field we would use a N.C momentary contact and an auxilary holding contact which would drop out on open or a normally closed overload relay and shut the control circuit down until it was manually reset. Now with the PLC we can latch in the program and the output connected to the load will remain energized and de-energize when the stop input was energized. Now my concern is safety in the plant, the PLC does not seem to have any normally closed elements that when opened will drop out the motor, so my question is : When wiring limits and E-stop switches in the field, should I use standard electro-mechanical safety with a closed loop or do they now just use normally open momentary contact pushbuttons for start and stop as well as limits and safety ? I'm old school with controls and my priority is safety for all personell and I always used a closed loop safety system. What do you think ? jb

 

[jbelectric777]

Ammend: PLC question

actually, If I use a N.O contact and field wire it N.C it does open the circuit, so I guess the best way is to use an output to the motor starter and field auxilary holding contacts to open during an unsafe condition. Is that the proper application for todays modern PLC's ? Thanks again, jb

 

[Leftyretro]

Hi All, I have a question about my actual work. I am getting back into PLC programming on an old account and I am brushing up on the software. My question is: In the software to make a magnetic contactor or relay latch you use an OTL and to unlatch you use an OTU respective elements and use a N.O momentary contact for both stop and start, which is fine. In the old days when we wired the controls in the field we would use a N.C momentary contact and an auxilary holding contact which would drop out on open or a normally closed overload relay and shut the control circuit down until it was manually reset. Now with the PLC we can latch in the program and the output connected to the load will remain energized and de-energize when the stop input was energized. Now my concern is safety in the plant, the PLC does not seem to have any normally closed elements that when opened will drop out the motor, so my question is : When wiring limits and E-stop switches in the field, should I use standard electro-mechanical safety with a closed loop or do they now just use normally open momentary contact pushbuttons for start and stop as well as limits and safety ? I'm old school with controls and my priority is safety for all personell and I always used a closed loop safety system. What do you think ? jb

At our refinery we still 'hardwire' the normally closed stop pushbutton that will drop out the 'M' coil of the motor starter, as you say it's a safety factor. However the start push button is normally just a input to the PLC so that any other permissive can be added to the logic. Also a aux contact from the 'M' coil is usually wired as an input to the PLC logic so that it will always know the status of the motor starter and it can latch the starter on.

Good luck

 

[jbelectric777]

Thanks thats what I wanted to hear

I totally agree, the input for start is a control issue only, as the drop coil is a safety issue. I'll use the other input from a set of contacts off the motor as a proofing switch so it can be seen as true or false on the panel. Thats what I wanted to know (meaning accepted industry standard) But how are you dropping the coil on momentary N.C ? Are you using a control relay (ice cube type) on the output because the PLC wont unlatch it or are you using a toggle type E Stop in conjunction with a set of N.O contacts to send to input stop/unlatch ? I know where we are going, I just dont want to over complicate things for our other maintenance mechanics, I'm the only formal electrician, the remaining shifts just " cover " until the problems can be addressed. Lefty , I also wanted to ask you if at your facility, do you use the program for general use lighting, intrusion alarms etc... ? It can be right ? Thanks much, jb