Validation and Testing of an Analytical Formulation to Compute the Reduction Factor in MV Grids

Global Earthing Systems (GESs) are defined by International Standards IEC 61936-1 and EN 50522 as an equivalent Earthing System (ES) created by the interconnection of local ESs. Thanks to this interconnection, just a percentage of the total fault current is injected to ground in a single ES, reducing the risk of electrocution. However, even if several experiments and models proved this effect, the identification and official certification is still a difficult task. Dangerous scenarios caused by a single-line-to-ground fault can be easily evaluated for a specific MV feeder by measurement or analytic models (quite cumbersome to use), but operative procedures valid for all the scenarios are still not available. In this work, a simplified formula to compute the reduction factor is presented, as well as its rationale. The proposed formula is easy to use and the results provided are sufficiently accurate, taking into account a desired safety margin. For this reason, it could be a valid tool for Distributor System Operators (DSO) and Certification Bodies and a step forward for the GES identification. The proposed formula is finally tested on three study cases.