Initiation of Electrical Breakdown of Soil by Water Vaporization

Field tests: Performances of practical earthing systems under lightning impulses

“…6 shows the relationship between the time-to-current peak, t 1 and t 2 and the amplitude of the voltages at its corresponding time, V at t 1 and V at t 2 for three earthing systems consisting of rod electrodes (2, 3 and 4 rods). This pattern of time delays is different than that is described in the literature [1,2,19,20], where in these references, the time delays were found to decrease with increasing currents/voltages. The difference in the pattern of time delays could be due to uncontrolled effects of thermal and ionisation processes, non-uniform water settling process, drying and heating in wet sand, and of course, the statistical nature of air breakdown inside the soil.…”

“…[18] found the current with two current peaks, whereas the rest of the field work [6][7][8][9] found with one current peak only. However, the trend of the resistances with current magnitudes is not discussed in previous paper [19]. In this present study, when impulse tests were conducted on the earthing systems at field site, two current peaks were observed, where the voltage shapes decrease smoothly.…”

“…In many studies [1,2,19,20], time delays are estimated to describe the thermal and ionisation processes occurring in soil under high impulse current conditions. Time delays are generally defined as the time to peak currents.…”

Effects of moisture content, impulse polarity and earth electrode's dimension on dry and wet sand under high voltage conditions

“…However, the published work [5][6][7][8][9] found that t D increases with increasing soil resistivity. In this present study, the higher time delay t D in wet sand could be due to the time taken to vaporise the water content in the sand, as suggested by Snowden and Erler [11] and van Lint and Erler [12]. However, t D can also be statistical in nature, which could be due to mainly uncontrollable water settling process in wet sand and the statistical nature of air breakdown in dry sand.…”

A critical review on soil ionisation modelling for grounding electrodes

“…In the thermal heating process, the discharged current increases the temperature of the existing water filling among the soil grains. Due to the heating process, the resistivity of the heated water decreases, which in turn causes the resistivity of the bulk soil and consequently reduces the grounding electrode resistance [6][7][8]. In the soil ionisation process, the electric field enhancement in air voids enclosure among the soil grains, causing the soil breakdown occurrence [5,[9][10][11].…”