EMC testing for components and subsystems has evolved greatly over the last 40 years since MIL-STD-461/461A was released. This design and test-procedures document was a credible attempt at standardizing EMC tests and has formed the basis for many other government and industrial procedures for equipment and subsystem tests. Unfortunately, it was not designed for system-level testing, and any attempt to use the procedure for in situ operational tests is futile. Unfortunately, duplicating the operational EME is impossible. Even coming close is very expensive because of the continually worsening adverse RF and physical environment such as shock and vibration, ESD, lightning, salt fog, or rain. These changing environmental conditions directly affect the performance of the EUT device. Since the EME is continually evolving, it is not possible to guarantee future performance of a new EUT based on the performance of the same or similar devices that may have been fielded in the past. This is especially true if an EMCSM was not established. Without previous EMCSM data or current EMC test data that provides the susceptibility profile, it can only be assumed that the equipment is at the threshold of failure with respect to the original EMC test environment. If the EMC test standard used was MIL-STD-461C, that standard defines a 20-year-old environment that is not adequate to protect today's equipment. Equipment often fails in its operational environment, and manufacturers chalk it up to unknown causes when, in many cases, the problem is an EMC susceptibility problem. All such failures should be investigated and a post-mortem reporting procedure developed to determine the cause of the failure so future systems won't have the same problem. When it's not possible to test like you fly and provide reasonable assurance that the system will work in an adverse environment, use a modern standard tailored to the most flight-like conditions possible and determine an EMCSM during the performance of the test. The recommended minimum EMCSM is 6 dB, but 12 dB would be better.
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