Correlation between laboratory and field tests on the impulse impedance of rod-type ground electrodes

A previous study on laboratory-model ground rods showed that the impulse impedance of these ground electrodes is a function of the injected impulse current and fits the relationship: Z=kI_p^α, where k and α are two parameters which depend upon the electrode configuration, soil properties and the impulse waveshape. The power-frequency and impulse impedances of two vertical rods embedded in the soil of a field near the university campus are measured. It is found that the ground electrodes in the field also have a similar relationship with the injected current. In complementary experiments, the same soil is tested in the laboratory with a laboratory model electrode system, consisting of a vertical rod embedded in the axis of a cylindrical soil holder with controlled moisture content in the soil sample. The objective is to correlate the laboratory data on the impulse impedance to estimate the impulse impedance of the ground electrodes tested in the field and compare the results with that of the actual field tests. It is possible to estimate the impulse impedance of the ground electrodes in the field from tests performed on laboratory-model electrodes. In correlating the laboratory data to the field data, the critical electric field of soil ionisation plays a critical role, which depends upon the soil properties, e.g. soil resistivity. Future research should be directed to find a more precise relationship between the critical electric field and the soil properties.