Common Causes and Troubleshooting Methods for PLC Failures

Failures of Peripheral Circuit Components

This type of fault frequently occurs in failures that arise after the PLC has been operating for a certain period. If components in the PLC control circuit fail, 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 load‑carrying capacity of a PLC’s output terminals is limited. If this specified limit is exceeded, an external relay or contactor must be used for 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.

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Poor Contact in Terminal Wiring

Such faults occur after the PLC has been in operation for a certain period, becoming more frequent as the equipment’s operating cycle increases. Due to wiring defects in the control cabinet, increased vibration during use, or the natural wear of mechanical components, connection terminals or component terminals may loosen, leading to poor contact.

The troubleshooting method for this type of fault involves using a multimeter and referring to the control system schematic or the PLC ladder diagram to diagnose and repair the issue.

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.