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Suppose we have a circuit that measures some external voltage like so:

schematic

simulate this circuit – Schematic created using CircuitLab

We will assume that the circuit is powered by batteries, and the circuit is well insulated and isolated from coupling to other circuits except through the \$V_{in+}\$ and \$V_{in-}\$ nodes. The "ground" in this circuit is just a common reference point in this circuit, and is not connected to any external "ground". Because this measuring circuit is isolated in this way, it can be used to measure potential differences (differential voltages) that themselves are at a potential difference from an earth ground. (Think of a battery powered volt-meter).

Note that \$V_{in-}\$ is tied to the "ground" (or common) for this circuit.

Now let us suppose that the above circuit is used to test a voltage in another circuit which has it's own ground, let's say a ground connected to the earth. However, the ground in the circuit under test is not where \$V_{in-}\$ is connected. Like so:

schematic

simulate this circuit

My question is whether there is an accepted terminology to explain this situation. I am tempted to call the "ground" in my testing circuit a "local ground" or a "local common" or a "floating common". But I would like to use terminology that others might readily understand as referring to a situation like that described.

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  • \$\begingroup\$ Don't know of a common name, but I just use different ground symbols and net names like GNDA and GNDD. If it's truly earth ground I use the earth ground symbol and call it "earth ground". Local ground works fine too, just don't use the same symbol for different grounds on the same schematic. \$\endgroup\$ – John D Dec 18 '20 at 16:55
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    \$\begingroup\$ I believe that even if there is a "right" way to call these, no one actually knows and follows it :) What is definitely not right is to use the same symbol for different "grounds" \$\endgroup\$ – Eugene Sh. Dec 18 '20 at 16:55
  • \$\begingroup\$ Note that the output of your circuit will be relative to its (internal) Ground - this may cause problems in your example measurement circuit. \$\endgroup\$ – Peter Bennett Dec 18 '20 at 17:03
  • \$\begingroup\$ @PeterBennett The circuit is isolated except through the \$V_{in+}\$ and \$V_{in-}\$ nodes. \$\endgroup\$ – Math Keeps Me Busy Dec 18 '20 at 17:09
  • \$\begingroup\$ I think "local ground" is reasonable and not difficult to understand : I might prefer "remote ground" for some purposes like point-point comms rather than "global ground". \$\endgroup\$ – user_1818839 Dec 18 '20 at 17:13
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There is no accepted terminology, but most people use similar terms.

I've never heard the term local ground, usually a more descriptive term is used, like what the ground is attached to. The thing I usually do is differentiate grounds if they need to be physically separated, if they don't then just label them ground. Usually chassis ground or mains ground and analog and digital grounds need to be separated. (although many designs do not need separate analog and digital grounds when designers incorrectly think they do)

Grounds fall into two categories as follows: (1) safety grounds and (2) signal grounds. The second category probably should not be called grounds at all, but rather returns, and it could be further subdivided into either signal or power returns. If they are called grounds, they should be referred to as ‘‘signal grounds’’ or ‘‘power grounds’’ to define the type of current they carry and to distinguish them from ‘‘safety grounds.’’ Common usage, however, often just refers to them all as grounds.

Source: Chapter 3: Grounds Electromagnetic Compatibility Engineering

In isolated designs, I have referred to grounds as GNDA GNDB GNDC (to keep them separated in the design)

Another common one is GND and GNDE to differentiate the PCB ground and earth ground or mains/chassis ground. Mains and chassis grounds typically use different symbols.

schematic

simulate this circuit – Schematic created using CircuitLab

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