Jeff, not sure what you are missing or wanting.
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If I have an isolated 5 volt supply, I can treat the negative as ground, zero volts, if I like and the positive as +5 if that suits me.
If I have an isolated 12 volt supply, I can do the same.
If I now connect the negativbe of the 12 volt supply to the positive of the 5 volt, then I can't usually sensibly have two grounds with a fixed voltage between them, so I have to change my mind about where I consider earth/ground.
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As above, it is common to connect the earths/grounds form multiple parts of circuits or wiring to one point so there are no loops in the earth paths for current. Finite to-ground resistances means that eart currents frm one circuit will tends to induce voiltages across any shared resitance and therefore into the other circuits.
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So, if I have three supplies at different voltages, each isolated with respect to each other, it would be common to bring one side of each (say the negative) to a common point and call this the ciruit earth/ground. Often this point would be connected to true ground,
THis would mean the ground return path form any circuit could end up in the same place, that common point for all three supplies.
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Sometimes what seems a simple circuit is not so simple without details as experienced people have seen many variants of similar and need to work out exactly what you have. If you'd like to explain in more detail about the connections you have and what qustions you have, people may be able to answer in a way that best suits your situation.