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Is there any way you can provide a definition for each of the grounds to which you are referring? Are these earth, wires, building frame, or something else? Harmonics from where?
If I have an EGC with small wire diameter, and wound into a coil, then it will have a higher impedance compared to a 16" wide sheet of 1/16" thick copper sheet.
If I have a single EGC in a circuit connected to many instruments distributed along the circuit, and the last instrument on the circuit creates lots of noise on the EGC, then other instruments close to the noise source on the EGC will see lots of noise. This is relative to some point like the neutral bus in the main panel.
Change to two separate EGCs, then the second instrument won't see the noise of the first if we still use the main panel as the reference point.
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