## 2 Experimental Platform
### Experimental Platform and Setup  
An experimental setup for testing these faults was established, as shown in Figure 3. This setup is designed to observe the fault voltage \(u_{t}\) and leakage current \(i_{t}\) of insulation-defective cables under various conditions. The experiment employs an Anmutake ATA20003 adjustable AC power supply (manufactured by Anmutake, Shenzhen, China) as a voltage source, providing a single-phase power frequency voltage with an RMS value adjustable from 0 to 300 V for the cable samples. Voltage and current are measured  
Figure 1: Structure of low-voltage three-core cable.  
Figure 2: Leakage current (**a**) through the cable metal layer or (**b**) directly through the surrounding dielectric medium; (**c**) equivalent circuit diagram.  
using a Tektronix TPP0051 medium-voltage probe (manufactured by Tektronix, Oregon, USA) and a Tektronix A622 current clamp (manufactured by Tektronix, Oregon, USA), respectively. The voltage and current waveforms are observed and recorded using a Tektronix DPO3014 oscilloscope (manufactured by Tektronix, Oregon, USA), with a waveform recording sampling rate of 25 kHz. The experimental platform is illustrated in Figure 4.  
The cable sample used in the experiment is one phase from a commonly used YJV22-0.6/1 kV four-core low-voltage XLEE cable (manufactured by Buke, Nanjing, China). A 35 cm length cable is extracted for the experiment, with 5 cm of insulation stripped from both ends of the sample to expose the conductor for connection to the main circuit. A 1-2 mm notch is made at the center of the cable sample using a cutter to simulate an insulation defect. One end of the cable sample is connected to the power supply, and the other end is connected to a 200 \(\Omega\) current-limiting resistor. A copper metal layer is wrapped around the defect area of the cable, and a corresponding notch is made. A grounding resistance is connected to form a discharge path to ground at the cable's insulation defect, with the grounding resistance set to 10, 30, 70, and 120 \(\Omega\).  
The following methods are used to simulate different wet environments:  
1. Liquid penetration condition: saltwater is added dropwise at a rate of 1 drop per 10 s around the defect to simulate the slow penetration of liquid into the cable's insulation defect. This is to mimic the situation in which the cable is surrounded by shallow water, such as rainwater.  
Figure 4: The experimental platform.  
Figure 3: Experimental setup for incipient fault tests.  
2. Humid air exposure condition: the cable is placed in a sealed glass box, and saltwater mist is used to control the humidity within the box, simulating the cable being exposed to a humid air environment.
3. Liquid immersion condition: the cable insulation defect is fully submerged in water to simulate the situation where the cable is in standing water.  
The conductivity of rainwater generally does not exceed 0.5 mS/cm, while polluted rainwater can reach up to 2 mS/cm. Considering the potential accumulation of contaminants in cable ducts or trenches, the conductivity of mixed contaminated liquids is even higher. Therefore, salt solutions with conductivities of 0.2, 1, 3, and 5 mS/cm are selected to simulate the wet environment the cable is exposed to.  
The experimental steps are as follows:  
4. Place the prepared cable sample under the preset different wet conditions for the experiment.
5. After powering on, monitor and record the voltage between the conductor and the metal layer in real-time using the oscilloscope, and the current flowing into the ground through the preset grounding resistance. Continue observing until the cable experiences a permanent fault or the discharge phenomenon completely disappears.
6. Use multiple cable samples to obtain multiple sets of data, ensuring the reliability and repeatability of the experimental results.