EMC Question of the Week: January 6, 2025
When designing for EMC compliance, it is important to realize that the EMC ground is generally
- a path for returning currents to their source
- a conducting body of relatively large extent
- non-existent above 100 kHz
- a place where carrots and potatoes thrive
Answer
The best answer is “b.” Proper grounding plays an important role in meeting EMC requirements. The EMC ground is the reference for conducted emissions requirements. It is potentially half of the unintentional transmitting antenna in a radiated emissions test. And, for most immunity tests, it is the local destination for injected noise currents. Identifying and utilizing the EMC ground is an essential part of guaranteeing EMC compliance.
The EMC ground is never a path for intentionally returning high-frequency currents to their source. Yes, it's important to pay attention to the paths of power and signal currents. Yes, those conductors are sometimes labeled "ground". However, it's important to differentiate between conductors whose main function is returning currents and conductors whose main function is grounding.
Once in a while you may hear someone claim that "ground doesn't exist at high frequencies." In reality, proper grounding becomes even more important at high frequencies. An EMC ground is a conductor of relatively large extent. It does not need to be small relative to a wavelength at the frequency of interest.
Finally, many EMC engineers are familiar with the phrase "ground is a place where carrots and potatoes thrive." The idea this phrase is meant to convey is that the word "ground" can be interpreted in many different ways. Design rules that refer to "ground" are ambiguous unless the meaning of the word "ground" is explained. Is it the EMC ground, the safety ground, or a current return? These grounds serve very different purposes and are governed by very different rules.
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