## 3 Experimental Test Results
### Impulse Test Results  
#### 3.2.1 Voltage and Current Traces  
Figures 4 and 5 show the voltage and current traces of positive-impulse polarity for ground electrodes (i) and (ii), respectively. For all the ground electrodes (iii), the voltage and current traces are similar to those seen in (ii) for all added water content (WC) and voltage levels. Similar trends were also seen for negative-impulse polarity. As can be seen in both figures, the traces are fairly similar, but the voltage and current traces were found to discharge to zero for electrode (ii), whereas the traces for electrode (ii) and all the electrodes (iii)a-(iii)d swung towards the opposite polarity or overshot at the discharged times. This shows that the overshoot only occurs in low \(R_{DC}\) values. Careful measurements was carried out on the voltage and current magnitudes of the traces that swung to the opposite polarity, revealing that the higher the applied voltage, the higher the magnitude of voltage swung to the opposite polarity was. It was found that the impulse polarity had no effect on the magnitudes of the overshoot.  
A few studies did not explain the reason for the overshoot, whereas several studies [7; 8] have indicated that these overshoots result from the inductive effect of the test load, although no specific demonstration has been conducted to validate this claim. Hence, for the purpose of enhancing our understanding regarding these traces swinging to the opposite polarity, a simple impulse circuit was established, where the test load consisted of inductance in series with resistance, as shown in Figure 6. This equivalent  
Figure 4: Voltage and current traces of electrode (i) GEN at a charging voltage of 50 kV.  
Figure 5: Voltage and current traces of electrode (ii) GEM at a charging voltage of 50 kV.  
circuit has been used in several publications [18] to represent grounding systems under high-impulse currents. However, in their study [18], the parameters that affect the swing of voltage and current traces to the opposite polarity were not highlighted. Their results were mainly related to the ionisation effect, which mainly occurs during the front or crest times [16; 17; 18; 19; 20]. In this work, the test circuit was simulated using the PSPICE circuit transient analysis program. Switch t_open represents the impulse generator spark gap switch. Some inductance (L1), capacitance (C1) and resistance (R1) elements were used to generate the test circuit so that a standard of 1.2/50 \(\upmu\)s was achieved. The equivalent circuit of the test circuit is represented by the inductance (L7) in series with the parallel branch of capacitance (C6) and resistance (R2). The R2 changed from as low as 5 \(\Omega\) to as high as 50,000 \(\Omega\), demonstrating the effect of resistance on the overshoot of voltage and current at discharged times. The value of L7 was purposely set in the milli-Henry range to establish a clear relationship when R2 was changed, whereas C6 was found to have little effect on the results.  
Figures 7 and 8 show the voltage and current traces, respectively, for the changes in resistance values while keeping the inductance value constant. As can be seen, similar to the experimental results, the lower the ground resistance value (i.e., with the addition of EM and water content), which relatively increases the current magnitude, the higher the overshoot. This is because as the resistance values are reduced, the inductance component, which is in series with the resistance, becomes significant, leading to higher overshoot levels