Numerical estimation of mutual grounding impedance between building structure and loop electrode buried around building base

An equivalent circuit of a grounding system for a building base with a loop electrode is proposed in this paper. The loop electrode is buried around the building base for bypassing a current due to a direct-lightning struck to a building. Self and mutual grounding impedance between a building structure and a grounding loop electrode is calculated by means of a numerical electro-magnetic field analysis method, which can take into account the complex configuration of the grounding system. The voltage and current transient responses to a lightning current are transformed into the frequency domain. The ratio gives the impedance as a function of frequency. The configuration of the equivalent circuit and its parameters are obtained from the impedance. Once the equivalent circuit is obtained, the transient simulation can be quickly carried out by a circuit analysis method. The grounding impedance of the loop-electrode should be reduced as low as possible from a viewpoint of voltage rise. The ratio of the self impedance to the mutual impedance determines the ratio of the current flowing into the building structure to that into the loop electrode. The current flowing into the building structure cannot be reduced in proportion to the reduction of the self grounding impedance in most cases. In addition, the slope of the mutual impedance versus distance between the base and the loop-electrode characteristic becomes less steep as decreasing the self impedance. It is difficult to improve the current bypassing effect by increasing the distance between the building base and the loop-electrode.