Question What is the correct measure of the performance of a grounding grid under a transient surge?
Answer The only accepted measure of performance of a grounding grid under a transient surge is its surge impedance. Chapter 6 of the 1999 CDEGS User's Group Meeting Manual shows how to compute the surge impedance for several configurations.
The full surge impedance (i.e. the surge impedance as a function of time) offers a complete characterization of the ground grid's response to any transient energization signal. As such, it is equivalent to the unmodulated frequency response of the grid that you can obtain directly using HIFREQ and FFTSES. The time variation of the surge impedance is usually too complex to handle, however, and researchers often resort to a simple approximation, often using a constant value (the maximum) to characterize the entire response.