In 1999 the Italian Standard CEI 11-1, based on the CENELEC HD 637 S1 Standard, introduced, with reference to MV distribution systems, the notion of "global grounding systems", i.e. large interconnected grounding systems, able to keep touch voltages below safety limits, through lowering the equivalent grounding resistance and making the ground surface nearly equipotential. This can typically happen in urban areas. However, so far, it is not clear which features an interconnected grounding system must have to achieve this goal. During this work a numerical model has been implemented to describe the behavior of an arbitrary MV electric system during a ground fault. Therefore, it can constitute a useful tool in judging about the actual effectiveness of a possible global grounding system.