jbelectric777
Member
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 