Distribution of Impulse Current in Earth Grid Electrodes

This paper reports the results of an experimental study to measure the current distribution in the horizontal branches of (i) a reduced-scale earth grid buried in real soil and (ii) the grid with bonded vertical rods at its corners. Low-magnitude lightning-type impulse currents were injected at the centre and at the corner of the grid. The results show that there are significant differences in current distribution between the centre and corner injection cases. Asymmetries between horizontal branches also exist when the grid in fed at its centre point, indicating that non-uniform localized soil conditions may significantly affect current dissipation. Such asymmetries, which may also exist in real grids, may have implications on the localized earth potential rise during lightning strikes. Vertical rods connected at the corners of the grid have limited effect on the current distribution in peripheral branches but contribute to some extend in dissipating current to ground. For the current injection at the grid corner, not only the current distribution varies when vertical rods are added, but also there is little benefit from rods connected at the corners far from the injection point. This may have practical implications as to the application of "high-frequency"rods for improved transient performance.