A safety circuit is an emergency circuit that must be provided to partially or completely stop the operation of a controller or controlled machine or process. These circuits should be hard wired outside the controller so that they can be independently and quickly shut down in the event of a complete controller failure.
Safety requirements for installation:
The main isolation switch is installed on the incoming power line as a way to power off the entire programmable controller system. The main power cut-off switch should be located in a position that is easily accessible to operators and maintenance personnel. Ideally, the isolation switch should be installed in the external Programmable Logic Controller casing so that it can be operated without opening the casing.
The isolation transformer is used to isolate the controller from the main distribution system and reduce the voltage to 120 VAC.
Hardwired main control relay 34 provides a convenient way for emergency controller shutdown. Because the main control relay allows several emergency stop switches to be placed in different positions, its installation is important from a safety perspective.
The overtravel limit switch or mushroom head emergency stop button is connected in series, so when one of the switches is turned on, the main controller loses power. This will cut off the power supply to the input and output device circuits. Continue to supply power to the controller so that any diagnostic indicators on the processor module can still be observed.
