Background: PLCs, Communication Boards and Ethernet Switches

The customer in this case builds automation cabinets that combine PLC control systems, remote I/O modules, custom control boards and at least one industrial ethernet switch for communication with higher-level systems. Several DIN-rail power supplies provide 24 V rails for logic and I/O, while additional DC-DC converters feed local loads on some boards.

In the field, these cabinets are installed close to motors, drives and other noisy equipment. The control and communication boards must keep running even when the environment is full of conducted and radiated interference. In previous projects, the customer had seen occasional resets and communication glitches but did not have a clear picture of the underlying EMC behaviour.

For a new generation of cabinets, they decided to treat emc testing for plc control systems and communication boards as a dedicated task instead of just “one more test” at the end of certification. They contacted TPS to organise a pre-compliance project focused on these electronics inside a realistic cabinet.

EMC Challenge: Random Resets and Lost Messages

During initial tests at a third-party lab, the cabinet failed several immunity tests. The symptoms were subtle but serious:

• PLC and controller boards that occasionally reset during EFT and surge tests on supply and I/O lines.
• Short communication dropouts on ethernet and fieldbus links when RF fields were applied or when ESD strikes occurred on nearby metalwork.
• Inconsistent behaviour across cabinets, suggesting that wiring and mounting differences also played a role.

The lab report made it clear that the electronics were near the edge of acceptable performance, but time in the certification lab was too limited for deep debug. The customer needed a way to reproduce and investigate these problems in an engineering setting.

TPS EMC Test Plan for Control and Communication Boards

TPS worked with the customer to design an EMC test plan that focused on the control and communication boards, without losing connection to the real cabinet environment:

• Representative cabinet wiring. We replicated typical I/O and communication cable lengths, shields and routing, based on drawings of the industrial control panel.
• Combined emissions and immunity view. While the main pain points were immunity-related, we also checked emissions from switching regulators and digital buses to ensure they stayed within limits.
• Functional monitoring. During ESD, EFT, surge and RF immunity tests, we logged PLC status, I/O values and network activity to see the exact impact of disturbances.

The plan followed the principles in TPS’s EMC pre-compliance guide but adapted to the needs of control and communication boards inside cabinets.

Debug: Layouts, Cabling and Shielding

Once the EMC behaviour was visible in a controlled setting, the TPS and customer teams started to experiment with improvements. The focus was on practical changes that could be applied across future cabinets.

• Board-level layout refinements. On the control and communication boards, we improved return paths for high-speed signals and switching regulators, reduced loop areas and clarified where and how shields and references connected.
• Cable and shield treatment. We tried different ways of routing I/O and ethernet cables, terminating shields and bonding the industrial ethernet switch to the cabinet. Some small changes gave clear improvements in immunity behaviour.
• Power supply and reset handling. We added local decoupling and clarified reset thresholds so that PLCs and controllers would not spontaneously restart during ESD and fast transients.

Each change was directly verified in the EMC lab, so the team quickly learned which ideas were worth adopting as standard practice and which could be dropped.

Results and Lessons Learned

After implementing the agreed changes, the cabinet returned to the certification house for a full test schedule. This time, the results were solid:

• Control and communication boards stayed operational during ESD, EFT, surge and RF tests.
• Emissions from the boards remained within limits, with no unexpected peaks in the key frequency ranges.
• Cabinet-to-cabinet consistency improved, because wiring and bonding rules were now clearly defined.

The customer documented the updated layouts, cabling and shielding guidelines as part of their internal design standard. EMC testing for control and communication boards is now a regular part of how they qualify new PLC platforms and communication hardware, not an afterthought.

How to Prepare Your Cabinet for EMC Testing

If you are planning EMC testing for PLC control systems, communication boards or ethernet switches, you can prepare for a smoother project with TPS by collecting a few items in advance:

• A diagram of the industrial control panel showing PLCs, communication boards, switches and I/O modules.
• Information about I/O and network cabling: types, lengths, shielding and how they enter and leave the cabinet.
• Supply voltages and power distribution for the control and communication boards.
• Any known field issues or previous EMC test results relating to resets, lost messages or sensor errors.

With this information, we can design a focused EMC test plan for your control and communication boards, combine emissions and immunity tests and help your cabinet pass EMC certification with fewer surprises.

Talk to an EMC Engineer About Your Cabinet   Back to “Typical Devices We Test”

Key questions this case answers

This case study responds to the core search questions behind EMC testing for PLC control systems and communication boards:

• Why control and communication boards inside industrial cabinets are so exposed to EMC issues.
• Which EMC tests are most relevant for PLCs and industrial ethernet switches in real projects.
• What design changes in layouts, cabling, shielding and resets most effectively improve EMC robustness.
• How to prepare data and hardware so an EMC pre-compliance session at TPS gives fast, clear results.