Impulse impedance tests on laboratory model ground electrodes

Impulse impedance of vertical rods embedded in several soil types was determined by laboratory tests under four temperatures and four soil moisture contents with three different waveshapes of the applied impulse voltage. To avoid errors due to current spikes caused by soil ionisation, it is proposed that the impulse impedance be defined as the ratio between the average voltage and average current during the application of the impulse current. With this definition, the impulse impedance is a nonlinear function of the injected impulse current Z_avg=kI_P^α where k and α are parameters that are functions of the soil characteristics and the grounding electrode configuration. It was also found that the impulse impedance is a function of the waveshape of the applied impulse voltage, higher for steeper wavefront of the applied impulse voltage.