10 PLC System Fault Causes and Solutions

10 PLC System Fault Causes and Solutions

In recent years, PLCs have become indispensable in industrial production. As their usage expands, ensuring stable system operation has become crucial. While PLCs themselves are highly reliable, improper operations can lead to issues. Here are 10 common fault causes and solutions:

1. Grounding Issues

PLC systems have stringent grounding requirements. An independent, dedicated grounding system is recommended, and all related equipment must be properly grounded. Improper grounding can cause unexpected currents, leading to logic errors or circuit damage. Grounding points should be close to each other. PLC systems typically use single - point grounding. For enhanced anti - common - mode interference capability, analog signals can employ shielded floating ground technology.

2. Dealing with Interference

Industrial sites are prone to high - and low - frequency interference, often introduced through cables connected to on - site equipment. In addition to proper grounding, the following anti - interference measures should be taken in cable design, selection, and installation:

For analog signals, use double - shielded cables.

For high - speed pulse signals, use shielded cables.

For PLC communication cables, use manufacturer - provided cables or shielded twisted - pair cables.

Do not route analog signal lines, DC signal lines, and AC signal lines in the same conduit.

Shielded cables introduced to or from control cabinets must be directly connected to devices without passing through terminals.

AC signals, DC signals, and analog signals should not share the same cable. Power cables and signal cables should be routed separately.

On - site maintenance tips to address interference include using shielded cables for affected lines and reinstalling them, as well as adding anti - interference filtering code to the program.

3. Eliminating Inter - Wire Capacitance to Prevent Misoperation

Cables have inherent capacitance between conductors. Even qualified cables can have excessive capacitance if their length exceeds recommended limits. When used for PLC inputs, this can cause misoperations, such as incorrect or missing input signals. Solutions include:

Using cables with twisted cores.

Minimizing cable length.

Separating interfering inputs into different cables.

Using shielded cables.

4. Selecting Output Modules

Output modules come in three types: transistor, triac, and relay:

Transistor - type modules offer the fastest switching speed (typically 0.2 ms) but have the lowest load capacity (0.2 - 0.3 A, 24 VDC). They are suitable for fast - switching devices and signal - related equipment, such as inverters and DC devices. Consider transistor leakage current effects on loads.

Triac - type modules are contact - less and suitable for AC loads but have limited load capacity.

Relay - type modules support AC and DC loads and have high load capacity. They are commonly used in conventional control but have a slower switching speed (around 10 ms), making them unsuitable for high - frequency applications.

5. Handling Inverter Over - Voltage and Over - Current

When reducing the given value to slow down the motor, it enters a regenerative braking state. The motor feeds energy back to the inverter, causing the filter capacitor voltage to rise and triggering over - voltage protection. Solution: Install an external braking resistor to dissipate regenerative energy.

When multiple small motors are connected to an inverter, a fault in one motor can cause the inverter to trip, stopping all motors. Solution: Install a 1:1 isolation transformer on the inverter output side to isolate fault currents from the inverter.

6. Labeling Inputs and Outputs for Easy Maintenance

PLC systems can be complex, with numerous input and output relay terminals. To facilitate troubleshooting:

Create a table based on the electrical schematic and place it on the control panel or cabinet. List each PLC input and output terminal number along with corresponding electrical symbols and Chinese names.

Develop a PLC input - output logic function table to illustrate the logical relationships between input and output circuits during operation. With these tables, experienced electricians can perform maintenance without blueprints.

7. Fault Diagnosis Using Program Logic

With various PLC types in use, ladder diagrams for high - end PLCs like the S7 - 300 are often written in mnemonic code. Effective ladder diagrams should include Chinese symbol annotations. For electrical fault analysis, the reverse - lookup method is commonly used. Start from the fault point, identify the corresponding PLC output relay, and trace back the logical relationships required for its activation. Experience shows that most faults stem from a single point.

8. Judging PLC Self - Faults

PLCs are highly reliable with a low failure rate. Hardware damage or software errors in PLCs and CPUs are rare. PLC input points are unlikely to fail unless subjected to high - voltage intrusion. PLC output relay contacts have a long lifespan unless overloaded due to external short circuits or poor design. When troubleshooting, focus on peripheral electrical components rather than suspecting PLC hardware or software issues. This approach speeds up repairs and minimizes production downtime.

9. Making Full Use of Software and Hardware Resources

Commands not involved in control loops or activated before the loop can be excluded from the PLC.

For multiple commands controlling a single task, connect them in parallel externally before linking to a single input point.

Utilize internal soft components of the PLC and intermediate states to enhance program continuity and ease development. This also reduces hardware costs.

Where possible, design each output independently for easier control, inspection, and protection of other circuits.

For outputs controlling forward and reverse loads, implement interlocking both in the PLC program and externally to prevent bidirectional load movement.

For emergency stops, use an external switch to cut power for safety.

10. Other Precautions

Never connect AC power lines to PLC input terminals to avoid damage.

Grounding terminals should be independently grounded, not connected in series with other equipment. Use a grounding wire with a cross - sectional area of at least 2 mm².

Auxiliary power supplies have limited capacity and should only power low - power devices like photoelectric sensors.

Do not connect wires to unused PLC address terminals.

If no protective devices are installed in the PLC output circuit, include fuses or other protective elements in the external circuit to prevent load short - circuits from damaging the system.