Earth is an infinite sink for noise current
This is often ranked as number 1 EMC misconception in the official EMC misconception chart (well, I invented this chart).
This problem is particularly true with industrial application, where often we see a high power machinary such as a variable speed drive (VSD) is installed. The problem often is worse when a new machinery is installed in a premises with outdated wiring and grounding system.
The technician often told us that an earth cable was provided and the noise should now be guided to earth, where the impedance is 0. Well, there are two misunderstandings here.
First misunderstanding is earth has 0 impedance. Earth’s impedance depends on the surface structure, the soil type, the moisture level of the soil and many other things. When high frequency current travels on earth, due to skin effect, the impedance of earth also increases. Also there’s earth connection impedance and resistance. To give a very rough estimation, the resistance value of earth connections often is in the range of 10 ohms and 20 ohms.
Even though the ground is done by using a strap such as the one shown in Figure 1. The connection introduces inductance. Again, very rough estimation, a 10 cm-long strap will probably introduce a loop area of 20cm, using 20nH/inch rule, that will result in an inductance of 150 nH. At 20 MHz, the impedance of this connection is then close to 20 ohms.
Second misunderstanding is that current sinks into earth. No it is not. Current flows in a loop and if you are struggling to understand this, I would suggest grab a physics book and revisit. If current does not flow in a loop, it will then topple our base understanding of the universe. Therefore, if current does sink into the earthing cable provided, it must come back to the noise source from the other end (via parasitic capacitance).
The problem that this misconception is so profound is that in the 80s and 90s, this theory did work. Because everything then was switched so slowly. When we have systems switching at sub 5 kHz as a switching frequency, then earth might have been a low impedance path. When things are getting faster, this rule collapsed. But somehow, the idea of “earth sinking noise” has passed through generations of Engineers and even found its way in today’s world. Quite shocking if you think about it!