Troubleshooting EFT/Burst EMC Failures in Biomedical Equipment

Troubleshooting EFT/Burst EMC Failures in Biomedical Equipment

Client Context

The client is a London-based biomedical electronics company developing complex equipment for the biomedical industry.

Problem Description

During an on-site EMC assessment, a system consisting of two interconnected cabinets failed the EFT/Burst immunity test at a 2 kV test level.

The failure mode was severe:

1. The system shut down during testing

2. Multiple internal sensors were permanently damaged

This indicated a critical EMC immunity issue, requiring immediate investigation.

Troubleshooting

EFT/Burst EMC Test Set-up Considerations

From experience, the EFT/Burst test set-up plays a crucial role, as results can vary significantly depending on grounding and configuration.

As testing was carried out on-site, a Tekbox roll-up ground plane was used to establish a controlled EMC test environment. Both cabinets were placed on the ground plane, and the EFT/Burst generator was bonded to the same reference plane.

The failure was successfully reproduced under controlled conditions.

It is important to note that a high-quality EMI filter was already installed at the AC mains input. The filter was correctly bonded to the cabinet frame. Despite this, the system still failed the EFT/Burst test.

Initial Troubleshooting

As a diagnostic step, a short braided grounding strap was temporarily used to bond the cabinet frame directly to the test ground plane. Although not a production-feasible solution, this is a common EMC troubleshooting technique. With this additional grounding applied, the system passed the EFT/Burst test. This immediately indicated that the issue was related to grounding and return current paths, rather than insufficient filtering.

Root Cause Analysis: Common-Mode Current Loop

Further investigation focused on identifying how EFT/Burst noise coupled into the system.

With Module 2 disconnected, testing focused on Module 1. It was observed that the system passed when the Ethernet cable was unplugged. The failure reappeared when the Ethernet cable was connected.

Two matched RF current probes (Tekbox TBCP2-500) were used to measure current:

• One on the mains cable

• One on the Ethernet cable

Using an oscilloscope for time-domain measurement, significantly higher common-mode currents were observed when the Ethernet cable was connected. This confirmed that a low-impedance current loop was formed during the EFT/Burst event.

The injected transient energy coupled into the system. The EMI filter was bypassed via alternative return paths(in this case, most likely, the earth wire for safety). The Ethernet cable provided a path for common-mode current circulation as parasitic capacitance enabled noise coupling between modules and the test ground. When the Ethernet cable was removed, the loop was broken, and the system became significantly more immune to EFT/Burst disturbances.

Simulation results supported this mechanism.

Solution: Improving Grounding and Bonding

Several mitigation strategies were evaluated.

1. PCB Grounding Improvement

Connecting PCB 0 V(PCBs that sit inside the cabinet, as shown in the drawing) to the chassis improved EMC immunity, even with imperfect chassis bonding.

2. Additional Bonding Between Cabinets

The final solution involved adding a braided bonding strap between the two cabinets.

With this modification:

• The system passed the EFT/Burst immunity test

• Common-mode current was significantly reduced

Important Design Insight

The location of the bonding strap is critical.

When placed close to the ground plane → system passed

When moved higher in the cabinet → system failed

This highlights a key EMC principle:

Loop inductance (and impedance) is determined primarily by loop area, not just conductor length.

Key Lessons for EMC Design Engineers

• EFT/Burst EMC failures are often caused by grounding and return path issues, not just filtering

• Common-mode current loops can bypass EMI filters

• Interconnecting cables (e.g. Ethernet) can form unintended noise paths

• Temporary grounding is a powerful EMC debugging technique

• Bonding strategy and physical layout are critical for EMC immunity performance

Conclusion

This case study highlights the importance of understanding current paths and system-level EMC behaviour when troubleshooting EFT/Burst failures.

In complex systems, EMC performance is not determined solely by component selection, but by how the entire system is grounded, bonded, and interconnected.