Fundamental Considerations and Impluse Impedance of Grounding Grids

The impulse impedance of a grounding grid must be known for determining its performance while discharging impulse currents to ground if problems concerning lightning protection and transients under ground faults are to be accurately analysed. This paper out-lines the basic concepts required for this study, analyses the equivalent circuit ,21]of a grounding grid for its response to practical impulses like ramp (i.e. idealised lightning stroke) and double exponential wave inputs and establishes expressions for the impulse impedance. Its value is shown to be higher than the grid´s power-frequency grounding resistance, decaying to this latter value with passage of time. Inductance of the grid is shown to be the governing factor contributing to its impulse impedance. Only those values of current which do not cause breakdown of the soil are considered