I have been wrestling with the philosophy behind galvanic isolation, and whether or not it makes sense to employ it in an application I am working on.
I understand galvanic isolation is required for wired communications between two circuits operating at different ground references (floating w.r.t. one another). The need for galvanic isolation in this case is self-explanatory (comms won't work, stuff gets fried, etc).
I also understand that the concept behind galvanic isolation for safety is to eliminate the conductive path between a power source and a point in the circuit that the user might touch (e.g. a jack or other wire-to-board connector). In this case, if there is a problem with a circuit that the user touches, no current will flow through the user to earth ground.
My question is this:
At what DC and AC voltages is galvanic isolation required from a safety perspective?
For example, if I want to measure AC and DC signals, at what point do I need to isolate the ADC section of my board from the section that supports user interface peripherals (see below diagram)?
In other words, how big does V1 or V2 have to be before I need galvanic isolation between the ADC section and the rest of the board?