First steps after a failed EMC test
Don't rush into hasty action. The most effective sequence:
- Read the test report: which test (emissions/immunity), which frequency range, how large is the exceedance?
- Reproduce: does the effect occur in your own (pre-compliance) setup? Only what is measurable is solvable.
- Localize the cause instead of blindly stacking up countermeasures.
- Prioritize: the one or two most effective levers first.
Typical causes
In practice, a few causes dominate:
- Grounding concept & return-current paths — the most common root of emissions.
- Shield termination — e.g. “pigtails” instead of a 360° connection.
- Filtering & decoupling in the wrong place or undersized.
- Cable routing & installation — coupling paths in the real-world setup.
- Clocked software / switching edges — software behavior as an unexpected source of interference.
More on the terms: Immunity vs. emissions.
Frequently asked questions
Why does my device fail the EMC test?
Usually it comes down to grounding, shield termination, filtering or cable routing — rarely a single component. A targeted root-cause analysis in a pre-compliance setup brings clarity.
EMC limit value exceeded — what to do?
First reproduce and localize, then prioritize the most effective measure (often grounding/shielding). Only then implement it in a targeted way and verify it in the lab.
Is external help worthwhile?
When a deadline or start of series production is under pressure, a fast, experienced root-cause analysis usually saves more than it costs. See EMC troubleshooting.