Whenever we say that there should be separate grounds between the power circuit and control circuit we connect an optocoupler between power the driving controller and the driven MOSFETs (lets say we are driving MOSFETs) but both the grounds (the ground for power electronic circuit and control circuit) remain same, then how we have separated the grounds.
Secondly, if we seperate the grounds of control circuit and power electronic circuit then on what convention the potential will be considered as voltage measured is the potential difference between two points.