Common Causes and Troubleshooting Methods for PLC Failures

(1) Peripheral Circuit Component Failure

This type of fault frequently occurs after the PLC has been operating for a certain period of time. If a component failure occurs in the PLC control circuit,the PLC control system will immediately shut down automatically.

The input circuit is the port through which the PLC receives discrete and analog input signals; the quality of its components, the wiring method, and the reliability of the connections are all critical factors affecting the reliability of the control system.

For discrete output, PLC outputs come in three types: relay output, thyristor output, and transistor output. The specific type to choose should be determined based on the load requirements; an improper selection can reduce system reliability and, in severe cases, prevent the system from functioning properly.

Furthermore, the PLC’s output terminals have a limited load‑carrying capacity. If the specified limit is exceeded, external relays or contactors must be usedfor proper operation.

The quality of external components such as relays, contactors, and solenoid valves is a key factor affecting system reliability. Common faults include coil short circuits and mechanical issues that cause contacts to fail to operate or result in poor contact.

(II) Poor contact at terminal connections

Such faults tend to occur after the PLC has been in operation for a certain period, as the frequency of equipment movements increases. Due to wiring defects in the control cabinet, increased vibration during operation, or the end of mechanical life, connection terminals or component terminals may loosen, leading to poor contact. To troubleshoot these issues, use a multimeter and refer to the control system schematic or the PLC’s ladder logic diagram for fault diagnosis and repair.

For the wiring of certain critical peripheral device terminals, to ensure reliable connections, it is common practice to use soldered cold-pressed terminals or cold-pressed pins.

(III) Functional failures caused by interference affecting the PLC

The various types of PLCs used in automation systems are control devices specifically designed for industrial production environments. During their design and manufacturing, multi-level anti-interference measures and component-level safeguards are implemented, giving them strong adaptability to harsh industrial conditions, stable operation, and high reliability. Therefore, they can typically be deployed directly in industrial settings without requiring any special precautions. Interferences affecting PLCs can be categorized into external interference and internal interference.

In real-world production environments, external interference is random, independent of the system architecture, and originates from sources that cannot be eliminated; it can only be mitigated on a case-by-case basis. Internal interference, on the other hand, is related to the system’s structure.