Introduction
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To be well grounded
If you expect to read about mental alignment, chakra point, chi and the flow of soul centering energy, you have probably clicked on the wrong link. While being personally grounded, that is being balanced and sensible, is of great importance to individuals and to society, here we are concerned about the flow of electrons. Being well grounded or properly grounded is of paramount and practical importance.
An ungrounded circuit will not work. An improperly grounded circuit will function poorly, perhaps unpredictably. Because of the great importance of this subject, you would think that its concepts would be clear and that everyone would know and agree about them. That is not always the case. Let me direct your attention to figure 1.

Figure 1 Three types of ground.

I am sure you will agree that these symbols are used incorrectly quite often. I frequently see earth ground used as signal ground or used interchangeably with signal ground. And most of the time using chassis ground just confuses people.
Where on earth?
Earth ground indicates a connection to the earth. In the United States you commonly find 110VAC wall sockets that have three wires. The black wire carries the power. The white wire is the current return path. The green wire is earth ground. No current should flow in the ground wire. If you check the wiring at the meter on the side of your house you will find that the green wires actually have a physical connection to the earth.
The chassis, or box, is often connected to earth ground. For electrical appliances, a wash machine for example, the chassis is the metal outside of the machine and it is connected to the green ground wire. This is a safety measure. A common use of chassis ground is to connect PCB mounting holes to the chassis of the box, not just with a screw but electrically as well.
Not hula hoops but still loops
To prevent unwanted currents between unequal potentials, we do not connect grounds at multiple points along a line. This could cause ground loops and cause the circuit to malfunction. I was careful to say in a line because we commonly use a ground plane that has grounds connected all across it. In contrast the shield wire in a cable should only be connected to ground at one side.
You have likely heard the term star ground indicating a common point from which all system grounds are defined. This point is commonly close to the power supply or the input connector. Often two grounds will be connected using a ferrite bead to restrict the high frequency current spikes found on some grounds.
Not all grounds are created equal
We may think of all grounds as having the same potential, but this is a dangerous idea. Ground is an arbitrary and local potential level. While most grounds may be at a similar potential, that is not a requirement. I once had an animated discussion with a physics student who was disturbed that we had both positive and negative voltages on our board. He claimed that all voltages were positive. His ground clearly had a lower potential than ours did.
Grounds that are not connected to the earth are said to be floating. A flashlight has a local ground, the negative terminal of the battery. It is also, usually, floating because it is not connected to earth ground.
If I have two batteries stacked on top of each other I may call the lowest potential GND1 and the highest VCC1. There is nothing from preventing me from calling the potential in between the two batteries GND2. Now there is a potential difference between GND1 and GND2, fixed by the voltage of the battery.
You might claim that is silly and I should just have one ground and call the voltage level at the top of battery 1, VCC1 and the voltage at the top battery VCC2. And in many cases, that is the way it is done. But if we hade an ADC at voltage level 2 that was referenced to the middle ground (GND2). It would be very convenient to call that potential GND2 and live with the potential difference between the two grounds. Parenthetically, the voltage of GND1 relative to GND2 is now negative.

Figure 2 Grounds with different net names.

Analog and Digital
Two grounds that are usually at the same potential but are kept separate are analog ground and digital ground, see figure 2. Digital ground is very noisy because of the fast switching of digital circuits. This can interfere with sensitive analog circuits. That is why they are frequently connected only at the star point. It is important that if you have two ground planes, they do not overlap. Having an analog and a digital ground plane that overlap only ac couples the two planes through a capacitor (two plates separated by a dielectric).
Even analog circuits can have different noise levels. Circuits that push around large amounts of current or asynchronously turn on and off can create noise and my warrant their own grounds. It is not uncommon to have several analog grounds with different net names in a design.
Final thoughts
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