Grounding impedance based on the current distribution for the horizontal ground electrode installed in two-layer soil structure

Large grounding systems such as ground grid and long horizontal ground electrodes are typically installed at non-uniform soil structure. In case of the ground resistance for these grounding systems, it is enough to reflect the average value of soil resistivity. However, the transient response of grounding system is much different with the ground resistance when the surge caused by lightning strikes is injected into the grounding system. The grounding system in lightning protection systems is designed and installed with considering the grounding impedance. Unlike the ground resistance, the grounding impedance is affected by the soil structure or locally different soil resistivity. This paper presents the grounding impedance and current rate dispersed into the soil as functions of the frequency of ground current and the fronted time of impulse current for the horizontal ground electrode of 40 m long when it is installed in two layer soil structure. As a result, the conventional and transient grounding impedance measured at the injection point with low soil resistivity shows low value even though the length of ground electrode is same since the fast fronted impulse current are much dispersed near the current injection point. As the effective length of the horizontal ground electrode is getting short with decreasing the fronted time of impulse current, the location of ground lead is important factor to decrease the grounding impedance. These results show the optimized technique for the location and condition of the ground electrode and the ground lead to decrease the grounding impedance.