How to select PLC external safety circuit?

The external safety circuit system of PLC is to ensure that the whole system can operate reliably in a safe state, and to prevent serious economic losses and casualties caused by external power problems, abnormal PLC, misoperation and misoutput, and the necessary protection circuit of equipment. 1. Protection against power overload. If there is a problem with the PLC power supply and the infixing time is less than 10 seconds, the PLC operation will not be affected. If the infixing of the power supply exceeds 10 seconds or the power supply drops beyond the allowed value, the PLC will stop the operation and all output points will be disconnected at the same time; When the power is restored, if the RUN input is connected, the operation will stop automatically. Therefore, necessary current limiting protection circuit shall be set for some input equipment that are easy to overload. 2. Alarm and protection of serious problems. For places prone to severe accidents, in order to ensure the reliable alarm and protection of the control system in case of severe accidents, the signals connected with severe problems shall be output through the external circuit, so that the control system can work under safe conditions. 3. Emergency stop circuit. As for the dangerous load that can transport users to cause damage, in addition to consideration in the control sequence, the external emergency parking circuit should also be planned, so that when PLC problems occur, the power supply of the load causing damage can be reliably blocked. 4. Protect the circuit. The control system with reversible operation such as forward and reverse operation shall be provided with external electrical interlocking protection; External limit protection circuit shall be set for the control system of reciprocating operation and lifting movement. 5. The programmable controller has the function of self checking such as monitoring timer. When abnormal conditions are detected, all outputs are closed. However, when the CPU of the programmable controller is in trouble, it cannot control the output. Therefore, in order to ensure that the equipment works in a safe state, it is necessary to plan an external circuit to protect the dangerous load that can transport users and cause damage.