## 3 Results and Discussion
### Experiment of the Insulators with EL Coating  
The luminescent intensity and spectral stability of EL materials under high electric fields are pivotal for their application in EL-voltage sensing systems. To quantitatively assess the relationship between electric field strength, luminous intensity, and chromaticity characteristics, the EL behavior of ZnS:Cu-epoxy composite films was systematically characterized using a custom-designed ball-sphere electrode configuration, as schematically illustrated in Figure 6a. Experimental observations reveal that a distinct EL response emerges at an applied voltage of approximately 2 kV, marking the onset of visible light emission from the composite film. As the applied voltage increases, the luminescent intensity progressively enhances, and at voltages exceeding 5 kV, the entire film exhibits a pronounced and uniform blue-green glow. The corresponding chromaticity coordinates are shown in Figure 6b, confirming the spectral stability of the emission across varying field intensities. Further analysis of the voltage-dependent luminous response, as presented in Figure 6c, indicates a clear saturation behavior beyond 6 kV. In this regime, the luminous intensity plateaus, suggesting that most of tge ZnS:Cu particles have reached excitation equilibrium, and the charge injection processes governing radiative recombination have stabilized. Overall, these results demonstrate that the ZnS:Cu-epoxy composite not only exhibits robust electroluminescence under high-voltage excitation but also maintains chromatic stability and intensity reproducibility, reinforcing its potential in advanced optoelectronic and energy-autonomous sensing platforms. In order to evaluate the feasibility of EL coating sensing and indicating the voltage magnitude endured by insulators, different voltages were applied to both ends of EL-coated insulators with a surface coating thickness of 120 \(\upmu\)m, and the luminescence phenomenon of EL coating under different voltages was observed. Under daytime indoor conditions  
Figure 6: Electroluminescence performances of the EL material. (**a**) Demonstration of electroluminescence under voltages of 0–6 kV. (**b**) CIE chromaticity diagrams of the light-emitting layer under voltages of 0–6 kV. (**c**) The luminous intensity of the light-emitting layer under voltages of 0–8 kV. (Figure 7a), the coating exhibited a clear blue-green EL response when the applied voltage exceeded a threshold of 5 kV. The luminous intensity increased progressively with higher voltages, demonstrating a strong voltage dependence. Nighttime experiments (Figure 7b) revealed a similar trend, with luminescent intensity enhancing as voltage increased. Notably, the chromaticity coordinates of the emitted light remained stable at (0.26, 0.31) across all tested voltages (Figure 7c), confirming the consistency and reproducibility of the luminescent color--a critical feature for practical sensing applications. The specific numerical relationship between voltage and luminous intensity (Figure 7d) displayed a linear increase within the 5-20 kV range. In particular, when the applied voltage exceeds 22 kV, the intensity saturated, suggesting that carrier injection had reached equilibrium, and further voltage increases did not significantly enhance light output. Interestingly, nighttime measurements recorded a 10% higher luminous intensity than daytime observations at identical voltages. This discrepancy is attributed to the absence of ambient stray light at night, allowing more accurate detection of the coating's intrinsic luminescence. The coating's metrological performance was rigorously validated through comparisons between experimental measurements and finite element simulations. The developed model successfully predicted voltage distributions across test insulators using luminescence intensity as input, achieving a mean error of 10% (Table 2).