I have already asked this question at electronics.stackexchange but I still don't get it. I thought I understood it in terms of a water analogy but when I went back to electrons it still didn't make sense to me. My first reaction when encountering the idea of common grounding 2 separate power supplies was that it would alter the voltages of the supplies, but that doesn't seem to happen. So I thought of the simplest example I could (using capacitors instead of batteries etc.). Here are two capacitors acting as power supplies in circuits with very high resistance loads, one at twice the voltage of the other and with twice as much charge on its plates : http://i.imgur.com/46rPk.png if I common ground them : http://i.imgur.com/RGWKK.png I realise that this does not create an additional closed circuit. However the bottom plates were at different potentials yet they must now be at the same potential. That means the potential of one has increased and the potential of the other has decreased. Meanwhile the potentials of the top plates are unchanged so to my mind the voltages across both capacitors must have changed. Could someone please explain where I am going wrong in terms of potentials and / or electrons, before and after.