when working on live i always use an isolation transformer
Let's not get too carried away with the use of the word "live". The OP is not necessarily talking about measuring mains voltages. He is talking about a circuit that has power applied - which could be anything from a coin cell up. He is simply asking one of the most basic questions possible - and there's only one person so far that has come close to answering it!
At a very fundamental level, any voltage you will measure is at a potential with reference to some other point of the circuit...
THIS is where there is a start to answering the question!
Rephrasing the above, I would say it this way.....
When measuring voltage with a DMM, once you select the appropriate range, you place the red probe on one point and the black probe on another. As I see it, what is being asked is:
Where are these two points?Now, the point for the red probe is pretty straightforward, it will be the place where you want to measure the voltage - but this is absolutely useless unless you place the black probe on the right spot. It is the
reference point for taking measurements, but knowing where this spot is can sometimes be tricky.
Usually 0V, or often that’s tied to GROUND, but [WARNING] you can’t guarantee that.
Yes. The usual reference point can be labelled 0V - or it can be implied by use of a symbol to indicate a chassis, ground or "earth" connection. (NOTE: The terms "earth" and "ground" do not always mean a connection to the soil under your feet through the earth pin of your mains socket - and thinking too hard around this idea can get you into some very confusing territory. In most cases when it comes to voltage measurements, it is best to forget about that detail, as it usually doesn't help.)
Once you've got the basics sorted, the following are just some of the other points you will need to understand:
Consider that some voltages may be AC, so your readings, method and range of measurement will be different from DC measurements.
And finally, do your research on WHAT you’re probing. mains-level voltages (AC or DC), can kill you in a dozen different ways.