Multiple power supply potentials
Here is what I have learned:

Every decent AC to DC Power supply uses a Transformer of some kind. This means that the Difference in electric potential (Voltage) to a known reference point e.g. earth will be undefined. See diagram 1)

As soon as a reference point is set, the Voltage to a known point is defined. See diagram 2) and 3)

There are also other unisolated ways to make DC-Voltage from AC. For example by usind a capacitive dropper (See this Video by DiodeGoneWild). Or directly using a rectifier of some sort.

Also as Unknown123 has mentioned:


  One more thing is that it doesn't have to be a crappy capacitive dropper circuit, take a look at here and here. They have a transformer, but the lack of: PCB isolation length, transformer winding insulation between primary and secondary, safety capacitor interference class, inductive spiking protection, et cetera, making it to be the worst of the worst. So don't assume every supply that use transformer are 100% safe.


This is problematic and possibly dangerous because the potential of one DC output pins will be either at 0V (Neutral) to earth or 230V to earth. Danger of electrocution!







simulate this circuit &ndash; Schematic created using CircuitLab

So to answer my own questions:


Connection the grounds of two DC power supplies is generally not problematic if at least on of the supplies is floating.


The Voltage between the Ground is the diagram is undefined.
Any proper power supply will have a "floating" DC Output. (Any thing else is a serious hazard)
Yes. As there is no reference.
probably not



Thanks to everyone commenting on my post and taking the time to explain!
Especially Unknown123.