In short. If both Your DVM and Power supply 0V/(-) are connected to Protective Earth(PE), for some reason. Connecting -30V to the DVM (-) would short that voltage through the DVM to PE, and possibly burn the PCB out.
Some of the mains based instruments I have seen, have had a link that makes it possible to Connect 0V to PE.
You should also know that we are only trying to make hypothetical solutions to Your problem, since we have not seen any pictures or schematics of Your setup or Your Equipment. You have all the answers, we can only ask questions.