PLC Training Video Transcription 1
PLC Wiring Tutorial
NEMA has developed a set of standards for Programmable Logic Controllers. This standard recommends that serious consideration be given to the use of an emergency stop or safety circuit.
This circuit should be independent of the operation of the Programmable Logic Controller. A portion of this standard says, when the operator is exposed to machinery, such as loading or unloading a machine tool or where the machine cycles automatically, consideration should be given to the use of an electromechanical override or other redundant means, independent of the controller for starting or interrupting the cycle.
The graphics shown in PLC wiring tutorial video depicts a control relay that will interrupt the power to the output module when the emergency stop button is pressed. This will not cycle the power to the processor or interrupt its scanning of the program. It will however, disable all of the output devices that's causing a safe condition. Many will recommend the use of a push-pull emergency stop button or even the use of a single-pole switch to operate the control relay. This circuit it will require operator intervention to restart. When the operator controls the restarting of the equipment, a safe restart is more likely to occur.
(The software used in video is at PLC Programming Basics Course )
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