## 3 Experimental Results
### Liquid Penetration Condition  
The typical results are presented as follows. In the experiment, saline with a conductivity of 3 mS/cm was used, and the grounding resistance was set to 30 \(\Omega\). During the experimental process, first, the discharge arc can be observed occasionally. As the discharge process was going on, the insulation defect expanded due to erosion, allowing more liquid to gradually penetrate the defect. Figure 5 illustrates the change in the size of an insulation defect in a sample cable, increasing from the initially set 1.1 mm to 2.4 mm. Intermittent, distinct arc flashes were clearly observed at the insulation defect, accompanied by sharp sounds resulting from the rapid expansion of gas, as well as small amounts of liquid ejected from the defect. The arc generation phenomenon is shown in Figure 6.  
Based on changes in the insulation defect and the characteristics of voltage and current waveforms, the discharge evolution process during liquid penetration into the insulation defect can be divided into three stages. Taking a sample cable that experienced intermittent discharge leading to a sustained fault as an example, the typical waveforms corresponding to these three stages are presented in Figure 7.  
Figure 5: Variation in insulation defect size.  
In the first stage, when the insulation defect is relatively narrow, the voltage waveform exhibits a temporary drop at the voltage peak during sub-cycle arc generation, lasting approximately 1/4 of a cycle. Simultaneously, the current appears in a pulsed form with an amplitude of 6-8 A. The arc voltage waveform is relatively smooth and approximately semi-arc shaped. During discharge, the voltage between the conductor core and the metal layer drops instantaneously, while the current rises sharply. After a certain period, the arc extinguishes, causing the voltage to recover and the current to return to nearly zero until the next discharge occurs. In this stage, the interval between two intermittent arcs is relatively long, ranging from several tens of minutes to several hours.  
In the second stage, the degree of insulation degradation intensifies due to continuous erosion, leading to an increase in defect width and allowing more liquid to penetrate. At this stage, the discharge remains primarily characterized by intermittent sub-cycle arcs, with current pulses maintaining the same amplitude of 6-8 A and waveform characteristics as observed in the first stage. The frequency of arc occurrences increases significantly, with the time interval between two arcs reducing from tens of minutes to several cycles or a few seconds. Compared with the first stage, the widening of the defect mainly affects the frequency of arc occurrences, while the amplitude and the waveform of the pulse current show no significant changes.  
In the third stage, as the defect continues to expand and becomes gradually filled with sufficient liquid, the conductor core and the metallic layer can establish brief direct conduction. Compared to the previous stages, the discharge waveform undergoes significant  
Figure 6: Observed arc flash at the defect location.  
Figure 7: Typical voltage and current waveforms at different stages: (**a**) arcs are generated at a low frequency in 1 st stage; (**b**) arcs are generated at a high frequency in the 2 nd stage; (**c**) a stable periodic current is generated in the 3 rd stage.  
changes, exhibiting a periodic current with an amplitude of approximately 0.4 A and an absence of voltage disturbances. Meanwhile, the frequency of intermittent arc occurrences dramatically decreases. This is because the entire defect area remains filled with liquid, turning the intermittent discharge status into a stable insulation leakage status.