# Evaluation of Pressure Effect on Accelerated Aging Tests of Polymer-Insulated Aircraft Wires
## 5 Experimental Results
### Comparative Analysis of the Three Proposed Methods  
After each degradation cycle, surface discharges were generated on the samples. Three indicators were measured, the surface discharge inception voltage (SDIV), the intensity of the images captured by the CMOS sensor, and the intensity captured by the solar blind UV sensor when the reference voltage, i.e., the voltage in the first degradation cycle, was applied. This process was repeated at four different pressure levels, namely 100, 72, 44, and 16 kPa. The reference voltage corresponded to the first CIV value recorded during cycle zero for each pressure level and sample. To study the degradation of the insulated wires over time, Figure 6 shows the evolution of the three indicators during the degradation process of eight independent samples. Figure 6 shows the averages of the four pressure levels, allowing the overall trend of insulation degradation to be seen. Figure 5: Progressive degradation of XL-ETFE wire sample #4. Figure 6: Experimental degradation results for the 8 XL-ETFE insulated wires. Average values for 16, 44, 72, and 100 kPa pressure levels. (**a**) SDIV evolution. (**b**) Image intensity determined with the CMOS sensor. (**c**) Solar blind UV sensor output. The results presented in Figure 6 show that each of the eight twin wire samples analyzed underwent a unique degradation process, and thus some samples underwent more degradation cycles to complete failure than the others. Although the results are highly scattered due to the nature of the samples and the degradation process itself, it can be observed that the method based on SDIV gave the most consistent results for all the samples analyzed. Among the three indicators, the slope of the SDIV showed a consistent negative evolution with the degradation cycle. This trend indicated a decrease in the material's resistance to surface discharges over time. Conversely, the solar blind UV sensor showed a consistently positive slope, indicating an increase in UV light emission as the degradation process progressed when the same voltage level was applied after each cycle. However, the behavior of the intensity of the images captured by the CMOS sensor did not follow a clear trend as the samples aged. Depending on the sample, the slope of the image intensity may have been positive or negative (see Figure 6b). Figure 7 attempts to explain the origin of this variability. The images in Figure 7 show that in some cases, the discharge area decreased, so the number of pixels with discharge activity also decreased. This could be due to the erosion of the insulation by the tree activity, which reduced the effective area of the insulation. As a result, for a given voltage, although the electric field was enhanced at some points of the insulation, resulting in brighter light being captured by the CMOS sensor, the total area of discharge activity was reduced, thus reducing the number of bright image pixels. In addition, the inherent randomness in the extent of the discharge area introduced further uncertainty into the image intensity measurements. Other factors that could explain the variability of the image intensity with sample degradation are the inherent stochasticity in the discharge phenomenon due to various factors such as the presence of electrolyte droplets and traces, the accumulation of decomposition products, defects in the samples, or temperature variations and random surface changes caused by the arcing effect during the previous degradation cycles. While the CMOS sensor focuses on the near UV and visible regions, the solar blind UV sensor is sensitive to the more energetic solar blind region of the UV spectrum. Thus, it is possible that only the brighter regions were able to emit photons falling in the solar blind region, making the solar blind UV sensor less affected by the randomness of the extent of the discharge area.