EMC Question of the Week: August 18, 2025
Which of the following phrases best describes a requirement for an EMC ground?
- conductor of relatively large extent
- small relative to a wavelength
- return current path
- connection to the earth
Answer
The best answer is “a.” Whether you're designing circuit boards, modules or entire systems, identifying and utilizing the EMC ground is an important part of ensuring that your product will meet all of its electromagnetic compatibility requirements. The EMC ground is your local zero-volt reference. It's one-part of any unintentional antenna that might be responsible for radiated emissions or immunity problems. By design, it's also the intended local destination for the injected noise currents in transient immunity tests. The EMC ground can't be chosen arbitrarily, and it can't be a conductor that is small relative to other nearby conductors. It generally must be a conductor that is relatively large compared to the other conductors in the board, module or system.
Occasionally you may hear somebody claim that EMC ground is an equipotential surface and therefore must be small relative to a wavelength. That misconception has led some people to conclude that ground is nonexistent (and therefore unimportant) at high frequencies. And while it's true that the EMC ground structure is the local zero-volt reference, it is NOT an equipotential surface at high frequencies. [At least not by any conventional definition of "potential."]
EMC ground structures can sometimes serve as a current return path for DC or low-frequency power currents, but this is not a "requirement." EMC ground structures cannot carry intentional high-frequency signal currents. Conductors that serve as the return path for high-frequency signals are often labeled ground, but these are signal grounds. Signal ground (a current-return function) and EMC ground (a local voltage reference) are very different. Getting these two concepts confused often results in bad design decisions.
EMC grounds do not require a connection to the earth. In fact, the presence or absence of a connection to earth rarely has an impact on any EMC design requirement other than lightning immunity.
Note: The words "conducting body of relatively large extent" appear in the ANSI C63.14-2023 standard definition of grounding. These words describe both EMC and safety ground structures.
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