Safety performance evaluation of typical grounding configurations of MV/LV distribution substations

Abstract

Typical grounding configurations are used in MV/LV distribution substations often without evaluating their safety performance against the danger of critical electric shock due to touch and step voltages arising in case of a ground fault. A method for the straightforward safety assessment of typical grounding configurations of MV/LV substations is introduced on the basis of simple calculations. A safety performance curve is constructed by using proportionality factors, specific to each typical grounding configuration, and the time–current characteristic of the installed protective device. The safety performance curve relates ground fault current to upper limits of soil resistivity, thus also ground resistance, below which safety is ensured. Thus, safety of an existing or new MV/LV substation can be easily evaluated through the associated safety performance curve. In a similar approach, a method is introduced to determine the shortest separation distance between MV/LV substation and LV neutral grounding configurations ensuring safety against transferred potentials where common grounding is not applied. The use of the proposed methods is demonstrated through an application to typical 20/0.4kV distribution substations.