In-situ EMC assessment of a naval shipboard system

Client Context

A UK-based defence contractor developed a large electronic system intended for installation aboard a naval vessel for the French Navy. The system measures approximately 2.6 m × 1.0 m × 2.5 m and weighs around 2,500 kg, making conventional laboratory-based EMC testing impractical.

The system was assessed against UK Defence Standard DEF STAN 59-411 Part 3, with particular emphasis on:

DRE 01.B – Sea service use (below decks)

DRE 02.B – Land, sea, and air use

An accredited EMC test laboratory (also acting as a notified body) was initially engaged to perform on-site/in-situ radiated emissions testing. However, high ambient electromagnetic noise within the manufacturing facility prevented a clear pass against the DRE 01.B limits. As a result, the laboratory issued a statement declaring that compliance could not be confirmed, without providing technical justification or mitigation guidance.

This outcome was unacceptable to both the client and their French naval customer, prompting them to engage us to perform a technically defensible EMC assessment.

Technical Approach

Our on-site EMC investigation revealed that ambient electromagnetic conditions varied significantly with time. Measurements conducted on Friday evening, 10 June, showed noticeably lower background noise levels compared with those recorded during the original assessment.

Despite this improvement, low-frequency ambient noise around 840 kHz still exceeded the applicable limit line. This ambient contribution was the primary reason the accredited laboratory could not declare conformity.

Through systematic investigation, the low-frequency interference source was identified as the factory power distribution cable network. To assess the equipment under test (EUT) independently of this interference, we used current probe monitoring on the mains supply. When combined with historical test data, this demonstrated that emissions from the EUT itself were not of sufficient magnitude to constitute a failure in the low-frequency range.

To further isolate the EUT, we recommended:

• Shutting down the factory’s three-phase power distribution system

• Turning off all lighting, particularly compact fluorescent lamps

• Powering the EUT from a separate, clean AC source, ideally supplied via battery-backed power

This approach significantly reduced low-frequency ambient noise but did not affect emissions measured within the FM and DAB broadcast bands, where measured levels still exceeded the test limits. These signals are governed by transmitter power levels set by broadcast authorities and cannot be controlled locally.

Key Actions

• Identified and eliminated dominant low-frequency ambient noise sources within the facility

• Supplied the EUT using a clean AC power source to eliminate coupling from the site power network

• Performed radiated emissions measurements using an active rod antenna

• Analysed FM and DAB spectrum content in detail, including demodulation and audio monitoring, to confirm that observed signals originated from local broadcast transmitters rather than the EUT

Results & Validation

All findings were documented in a comprehensive technical report, clearly distinguishing ambient electromagnetic interference from emissions generated by the system under test.

The report provided robust technical evidence demonstrating that the system met the relevant requirements of DEF STAN 59-411 Part 3, including DRE 01.B and DRE 02.B. This documentation was accepted by the client and used successfully to support discussions with their French naval customer.

Conclusion

This project illustrates the importance of engineering judgement and evidence-based analysis in complex, in-situ EMC assessments. Unlike standard test-house reporting, we went beyond a simple pass/fail declaration to identify root causes, eliminate external interference, and present technically defensible conclusions.

By doing so, we enabled the client to demonstrate compliance with demanding defence EMC standards and avoid costly redesigns or programme delays.