I have an adjustable power supply (TENMA - two channels 30V-3A and one 5V-3A I think) and I want to get a +-12V rail from it. By putting the supply into series it says that it has internal relays so I assume I don't need to bridge something (right??). When in series mode I can seem to understand how to get positive and negative 12V potential in respect to the ground... What am I doing wrong?
You should wire the power supply like above picture. Put both 30V channels to 12V. Then connect them in series. The wire connecting the - to the + is your reference.
I think you are confusing the ground with your reference voltage. Ground is attached to the wall plug of the power supply which is not the same as your reference voltage.