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Indeed, they can be! Especially if your goal is to certify your electronic product, and EMC keeps failing test after test.
I think this issue is one of the most common and yet underestimated.
Rightly so, mostly because we don't have a way to visualize what is happening with our product in terms of currents.
In this picture, I wanted to provide you with a way to visualize what is happening at the circuit level and how these common-mode currents are able to sneak into our product and jeopardize the outcome of our much-needed EMC tests.
Two principles to keep in mind here:
Current always flows in a loop. So does the common-mode current.
Parasitic effects (especially at higher frequencies) such as stray capacitance are phenomena that we need to take into account. For example, at the Conducted emissions test (typically 150kHz to 30 MHz), these effects start to get in our ways.
The question now is, how do I get rid of them?
I hope this helps,
Dario