Risk indexes for studying interconnected earthing systems inside MV networks

The paper presents a risk-evaluation-based approach to the issue of interconnection between the earth electrodes of the MV/LV substations within MV networks. After a short introduction on the interconnection between earthing systems, the paper proposes four risk indexes defined starting from the safety requirements given by CENELEC HD 63 7 S1. The indexes allow one to put in evidence, in a clear way, the benefits of the interconnection in terms of reduction of risk of critical electric shock for persons and of stress on the insulation of the LV devices in the presence of single line to earth faults and double earth faults inside MV/LV substations and single line to earth faults inside HV/MV stations. Moreover they can be used to identify in a simple way a Global Earthing System.