In the commissioning of high-voltage substations, the determination of the grounding grid resistance stands out. In real-life procedures, usually the involved measurements are carried out considering the guard wires of the transmission lines connected to the substation grounding grid. This issue can lead to an overestimation of the ground current, which may indicate an erroneous unsafe grid condition, in terms of touch and step voltages exposed to human beings. Considering the practical difficulty or impossibility of measuring the portion of the total fault current flowing through the ground, this paper deals with the proposition of a formulation (equation that govern the phenomenon) to estimate the ground current. The proposed simplified approach is based on an optimization method subject to adequate constraints, in which the determination of the substation grid resistance and the equivalent impedance of the guard wires allow estimating the most important parameter: the portion of the total ground-fault current that produces the touch and step voltages. The methodology was compared with results available in the literature and with field tests performed in a real substation.