Abstract: The external safety circuit system of PLC is a necessary protection circuit to ensure that the entire system can work reliably in a safe state and avoid serious economic losses and personal injury accidents caused by external power supply problems, PLC malfunctions, misoperation and erroneous output.
ABBPLC
1. Overload Protection. If the PLC power supply malfunctions and the interruption time is less than 10 seconds, the PLC operation will not be affected. If the power interruption exceeds 10 seconds or the power drop exceeds the allowable value, the PLC will stop working, and all output points will be disconnected simultaneously. When the power is restored, if the RUN input is connected, the operation will automatically stop. Therefore, necessary current-limiting protection circuits should be installed for some input devices that are prone to overload.
2. Alarm and protection against serious accidents. In locations prone to serious accidents, to ensure reliable alarm and protection of the control system in the event of a serious accident, signals related to serious accidents should be output through an external circuit so that the control system can operate safely.
3. Emergency Stop Circuit. For loads that pose a risk of injury to users, in addition to consideration within the control sequence, an external emergency stop circuit should be designed to reliably disconnect the power supply to the load causing the injury when the PLC malfunctions.
4. Maintenance circuits. Control systems with reversible operation, such as forward and reverse rotation, must be equipped with external electrical interlock protection; control systems with reciprocating operation and lifting/moving functions must be equipped with external limit protection circuits.
5. Programmable controllers (PLCs) have self-testing functions such as monitor timers, and all outputs are shut down when an abnormality is detected. However, when the PLC CPU malfunctions, it cannot control the output. Therefore, for high-risk loads that could harm users, external circuitry must be designed for protection to ensure the equipment operates safely.
