Due to the complexity of EMC test set-ups, sometimes combined with the complex operation modes of a product, mistakes can occur, even in accredited EMC test labs with experienced engineers. Incorrect test set-ups can result in two consequences: either under-testing or over-testing the product. Under-testing may risk product compatibility and performance reliability, while over-testing can lead to over-engineering, increasing cost, weight, and size, contributing to longer time-to-market. Therefore, a fundamental understanding of EMC tests, especially coupling mechanisms, is essential. Most mistakes in EMC tests relate to grounding or layout. Here are a few stories collected in the field to illustrate the point:

Story 1 (from Oliver Green):

During a radiated immunity test, the cables of the supporting equipment were routed outside of the anechoic chamber through a hole without any filtering or waveguide. Considering the radiated field inside the room could reach 100V/m or more, the cables likely brought the radiated field outside of the chamber to the control room, potentially exposing people in the control room to dangerous levels of radiation.

Story 2 (from Doug Smith's paper "Analysis of the IEC 61000-4-4 Capacitive Clamp Using Current Measurements"):

The author discussed subtle details in this test, highlighting that the results can vary significantly depending on the point to which the EFT generator is connected to the clamp. "The Capacitive Clamp used in IEC 61000-4-4 delivers significantly more current to the auxiliary support equipment than the equipment under test. It appears that the Capacitive Clamp connections may be specified backwards in the standard. A common error I have seen made by test labs is to connect the generator to the wrong end of the clamp. This error turns the EUT into the AUX equipment and over tests the EUT. Such a  condition can potentially fail a compliant EUT."

Story 3 (from Min Zhang):

During the EFT/Burst test of a unit under test, the DUT was placed on the test ground plane on a wooden table. The DUT was grounded to a different point close to the chamber wall, with a cable length of about 1 meter. This setup resulted in the failure of the DUT. Upon examination of the mains filter, which was a 2-stage filter from a well-known Swiss manufacturer and properly grounded to the DUT, it became evident that the DUT should not have failed the test. The root cause of the failure was traced to the long grounding wire.

In this scenario, the transient noise, in the form of current, needed a path during the test. Some of the current landed on the test ground plane through capacitive coupling between the DUT and the ground, but a portion of it travelled through the lengthy grounding wire. When current flows through a long wire, it can lead to the development of a significant voltage due to the Ldi/dt effect, especially when the inductance (L) is in the hundreds of microhenries. The noise through the ground wire can also radiate during the test, coupled to other signal ports and cause issues. The following simulation demonstrates the ground difference during the transient test in this particular set-up compared with what the voltage should be if the ground wire is short.

Story 4 (from Min Zhang):

I was once asked by a client whose product failed the flicker test. It was a peculiar case to me because there was no periodic mains switching in their product. I went to the lab and stood there, observing the test. During the test, their equipment was repeatedly stopped and powered back on. When I asked my client why the product was behaving like this, the client's response was, "I thought this was a consequence of the test generator; our machine was never designed this way." It turns out that the issue was related to the clean power supply's inability to handle enough current during the test. However, the test engineer believed the product was exhibiting unusual behaviour and recorded it as a flicker failure. This experience taught me that sometimes you need a third person to identify and resolve such issues!

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