Common grounding system

Current European Standards (HD 631 S1-1999 E) have fostered the so-called "common grounding system" (CGS) concept. It consists in the interconnection of the grounding systems of all the MV/LV substations of a MV network together to the HV/MV substation grounding grid, by means of the MV cables' metal sheaths and/or of dedicated ground conductors buried along the MV cable lines. In continental Europe, MV (15-20 kV) public distribution networks are generally operated in radial configuration; neutral grounding varies, with ungrounded neutral in Italy, resistance grounding in Spain and France, resonant grounding in Germany and Northern Europe in general, i.e. all choices except solid grounding. The interconnection of grounding systems involves a significant reduction of the apparent grounding resistance at the MV/LV substations, by creating a metallic ground-fault current path in parallel with the ground which drains away most of the fault current. It must be noted that even with ungrounded neutral, in extensively "cabled" 20 kV networks the single-line-to-ground (SLG) fault current may attain several hundreds A. Due to the mentioned ground resistance reduction, the Standards allow simplified, relaxed design criteria (touch/step voltage) for the individual grounding systems. Although technically justified, the unconditional acceptance of the Standard's provisions requires a thorough investigation of the possible operation/fault conditions of the system. In fact, in case of two simultaneous SLG faults at different locations, double earth fault (DEF), a large ground current has to be drained by the faulted grounding systems. Moreover, end-of-line MV/LV substations in a purely radial system, though normally operating, are liable to be actually disconnected from the global grounding system. The paper shows that the double earth fault may constitute a severe design criterion even in the presence of the CGS. On the other hand, given the strong reliance of the safety Standards on the soundness of the individual ground systems and, even more, of their interconnection, the task of verifying and maintaining the integrity of the whole common grounding system should be regarded as an utility service. Finally, the above remarks are also extended to MV networks with resistance-grounded neutral