## 1 Introduction
### Electrical Properties  
In this research, a salt-fog flashover test was performed in a testing hall of Chongqing University. The test power supply was provided by a YDJ 5 kVA/50 kV AC transformer, which could meet the power requirements of the salt-fog flashover test. Three samples of the same aging time were cut from silicone rubber, with a size of 6 cm \(\times\) 4 cm, as shown in Figure 5. The sample was placed between high-voltage and ground electrodes, as also shown in Figure 5, and flashover was produced between the two electrodes. The wiring diagram of the test principle is shown in Figure 6. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Samples** & **Si** & **C** & **O** & **Al** & **Fe** & **Pt** \\ \hline A\({}_{1}\) & 22.51 & 41.85 & 30.77 & 4.54 & 0.06 & 0.27 \\ A\({}_{2}\) & 21.75 & 40.19 & 33.72 & 4.15 & 0.06 & 0.13 \\ A\({}_{3}\) & 20.88 & 38.62 & 35.36 & 4.83 & 0.05 & 0.26 \\ A\({}_{4}\) & 19.64 & 36.44 & 38.48 & 5.06 & 0.13 & 0.25 \\ A\({}_{5}\) & 18.62 & 36.23 & 40.22 & 4.62 & 0.07 & 0.24 \\ A\({}_{6}\) & 17.51 & 35.36 & 42.12 & 4.71 & 0.09 & 0.21 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Main element contents (%) of composite insulators of Factory A. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Samples** & **Si-(CH\({}_{3}\))\({}_{2}\)** & **Si-O-Si** & **H** \\ \hline A\({}_{1}\) & 0.2228 & 0.2361 & 0.9437 \\ A\({}_{2}\) & 0.2198 & 0.2337 & 0.9405 \\ A\({}_{3}\) & 0.2176 & 0.2324 & 0.9363 \\ A\({}_{4}\) & 0.2139 & 0.2293 & 0.9328 \\ A\({}_{5}\) & 0.203 & 0.2188 & 0.9278 \\ A\({}_{6}\) & 0.1649 & 0.1 In Figure 6, T and B are AC test transformers, R\({}_{0}\) is a current limiting resistor, L represents the artificial air fog chamber, P is the salt-fog generator, and DAS (Data Acquisition System) is a voltage measuring system where flashover voltage can be displayed directly. To prepare the test, the samples were washed with anhydrous ethanol, rinsed with water and dried in the shade for 24 h.  
The salt-fog flashover test is in accordance with IEC 60060-1989 [24]. According to this standard, the concentration of salt-fog in this study was 1000 \(\upmu\)S/cm. In order to ensure the accuracy and rationality of the test, three samples were selected under the same conditions for six repeated tests. The average flashover voltage was obtained for comparing the electrical properties between samples aged for different years, and the standard error between average flashover voltage and each flashover voltage should be less than 10%. The equations for calculating the average flashover voltage \(U_{f}\) and deviation error \(\sigma\)% are expressed in Equations (2) and (3):  
\[U_{f}=\frac{\sum\limits_{i=1}^{N}U_{i}}{N} \tag{2}\]  
\[\sigma\%=\sqrt{\frac{\sum\limits_{i=1}^{N}\left(U_{i}-U_{f}\right)^{2}}{N-1} }.\frac{100\%}{U_{f}} \tag{3}\]  
\(\sigma\%\) is the standard error of the flashover voltage. Test results of the salt-fog flashover test are listed in Table 7.  
Table 7 shows the following:  
Figure 5: Schematic diagram of samples and the electrodes in the electrical test. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Samples**} & \multirow{2}{*}{**Aging Time/Year**} & \multicolumn{3}{c}{**1000 \(\upmu\)S/cm**} \\ \cline{3-4}  & & \(U_{f}\) (\(\upmu\)V) & \(\sigma\%\) \\ \hline A\({}_{1}\) & 1 & 13.87 & 3.27 \\ A\({}_{2}\) & 3.5 & 13.30 & 4.89 \\ A\({}_{3}\) & 6 & 12.76 & 2.27 \\ A\({}_{4}\) & 8 & 11.21 & 2.13 \\ A\({}_{5}\) & 10 & 9.99 & 4.76 \\ A\({}_{6}\) & 12 & 8.11 & 3.28 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: The results of salt-fog flashover voltage of samples of Factory A.  
Figure 6: Test circuit. (1) With the increase of aging time, the salt-fog flashover voltage of samples gradually decreases. For example, _Uf_ of sample A\({}_{1}\) aged for 1 year is 13.87 kV. When the aging time reaches 10 years, _Uf_ of sample A\({}_{5}\) declines to 9.99 kV. (2) The increasing rate of salt-fog flashover voltage accelerates gradually with an increase of aging time from 1 to 12 years.