Myths About Earth Grounding
As electronics developed, the common return paths of various circuits were also referred to as “ground,” regardless of whether or not they were eventually connected to earth. In addition, a single ground circuit most often serves, either intentionally or accidentally, more than one purpose.
Thus, the very meaning of the term ground has become vague, ambiguous, and often quite fanciful. Some engineers have a strong urge to reduce these unwanted voltage differences by “shorting them out” with massive conductors – the results are most often disappointing.
Other engineers think that system noise can be improved experimentally by simply finding a “better” or “quieter” ground. Many indulge in wishful thinking that noise currents can somehow be skillfully directed to an earth ground, where they will disappear forever!
Here are some common myths about grounding:
Earth grounds are all at zero volts – presumably with respect to each other and to some “mystical absolute” reference point. This leads to whimsical ideas about lots of ground rods making system noises disappear! In fact, the soil resistance between ground rods is much higher (often tens of ohms) than a wire between them.
Impedance – symbolized as “Z,” it’s the apparent AC resistance of a circuit containing capacitance and/or inductance in addition to pure resistance. Wires have zero impedance, and, therefore, can extend a zero-voltage reference to many locations in a system, eliminating voltage differences. In fact, wires are quite limited:
The DC Resistance of a wire applies only at very low frequencies and is directly proportional to its length. For example, the resistance of 10 feet of 12-gauge wire is about 0.015 Ohms.
The inductance of a wire is nearly independent of its diameter (gauge) but is directly proportional to its length and increases at bends or loops.
Our 10 feet of 12-gauge wire has an impedance of 30 Ohms at 1 MHz (AM broadcast band) as shown in the Figure 1.
Substituting a 1/2-inch diameter solid copper rod lowers the impedance only slightly to about 25 Ohms.
A wire resonates (becomes an antenna) when its physical length is a quarter wavelength. For a 10-foot wire, this means it will essentially become an open circuit at about 25 MHz.
Are earth grounds really necessary for low-noise system operation? Think about all the electronics in an airplane!
Under fortuitous conditions, systems may be acceptably quiet in spite of poor techniques. But physics will ultimately rule and noises may appear for no apparent reason!