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David J. Thomas:
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. [8]
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! [9] Here are some
common myths about grounding:

Kevin Graf:
David, your link to the 'grounding myths' didn't stick,  but here are some from Henry W. Ott's new book:

3.1.7  Grounding  Myths

More myths exist relating to the field of grounding than any other area of electrical engineering.  The more common of these are as follows:

1.   The earth is a low-impedance path for ground current.  False,  the impedance of the earth is orders of magnitude greater than the impedance of a copper conductor.
2.   The earth is an equipotential.  False,  this is clearly not true by the result of (1 above).
3.   The impedance of a conductor is determined by its resistance.  False,  what happened to the concept of inductive reactance?
4.   To operate with low noise,  a circuit or system must be connected to an earth ground.  False,  because airplanes, satellites, cars and battery powered laptop computers all operate fine without a ground connection.  As a mater of fact,  an earth ground is more likely to be the cause of noise problem.  More electronic system noise problems are resolved by removing (or isolating) a circuit from earth ground that by connecting it to earth ground.
5.   To reduce noise,  an electronic system should be connected to a separate “quiet ground” by using a separate, isolated ground rod.  False,  in addition to being untrue,  this approach is dangerous and violates the requirements of the NEC (electrical code/rules).
6.   An earth ground is unidirectional,  with current only flowing into the ground.  False,  because current must flow in loops,  any current that flows into  the ground must also flow out of the ground somewhere else.
7.   An isolated AC power receptacle is not grounded.  False,  the term “isolated” refers only to the method by which a receptacle is grounded,  not if it is grounded.
8.   A system designer can name ground conductors by the type of the current that they should carry (i.e., signal, power, lightning, digital, analog, quiet, noisy, etc.),  and the electrons will comply and only flow in the appropriately designated conductors.  Obviously false.

Henry W. Ott

Jonathan Johnson:

--- Quote from: Kevin Graf on October 16, 2013, 11:29:28 AM ---The earth is a low-impedance path for ground current.  False,  the impedance of the earth is orders of magnitude greater than the impedance of a copper conductor.

--- End quote ---

How I learned this:

First, some background. I grew up on a farm. An electric fence charger has one lead connected to the fence wire which is supported on insulators, and the other lead is connected to a ground rod. The fence charger provides a high voltage (typically 3kV-7kV), high impedance pulsed direct current energy energy source to the fence wire. An animal coming in contact with the electric wire completes the circuit between the wire and earth ground, and receives a nasty shock. Because of the high impedance of the charger, the amperage is too low to cause harm.

Anyway, one day during a rather dry spell, I happened to touch the ground rod and received a shock. Because the soil was dry, the connection between the rod and the soil was high resistance. My body provided an alternate, complementary current path between earth ground and the "ground" terminal of the charger.

If the earth were a low-impedance current path, there might be a lot less lightning and nearby lightning strikes could be less deadly due to lower "step potential."

(By the way, when an electric fence has a short circuit, the arc generates an RF signal. I've used a de-tuned AM radio to detect and find faults in electric fences. As you approach the fault, the pulse becomes louder in the radio.)


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