## 4 Conclusions  
In summary, this study establishes ZnSCu EL coating with potential as an on-line sensing strategy for zero-value insulator detection in high-voltage transmission systems. Critically, we demonstrate a quantitative relationship between applied voltage and EL intensity, characterized by an activation threshold at 2 kV and performance saturation above 6 kV, with blue-green luminescence characteristics within CIE coordinates of 0.19-0.32. Crucially, integrating this EL coating at the steel cap-shed interface preserves the insulator's core dielectric functionality, maintaining a stable permittivity of 5.5 at 50 Hz/ 20 \({}^{\circ}\)C and low dielectric loss tangent < 0.02 under ambient conditions. The coating enhances insulator flashover voltage from 35 kV to 37 kV, with a characteristic breakdown strength of 37 kV/mm, thereby meeting practical insulation requirements. Validation on 110 kV insulator strings robustly confirms the coating's diagnostic capability, where insulators of effective insulation performance show naked-eye-observable, voltage-proportional light emission, while zero-value insulators exhibit near-zero luminescence due to collapsed potential differences. Comparative analysis of simulated and experimental voltage distributions reveals <10% deviation, confirming the reliability of the strategy for real-time on-line monitoring proposed here. Consequently, this work establishes an intelligent paradigm for the on-line health monitoring of insulators by merging passive sensing with optical diagnostics, proposes an emerging strategy for the design of self-diagnostic power infrastructure, and potentially contributes to the maintenance strategies in next-generation smart power grids.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15147965/s1](https://www.mdpi.com/article/10.3390/app15147965/s1)]([https://www.mdpi.com/article/10.3390/app15147965/s1](https://www.mdpi.com/article/10.3390/app15147965/s1)), Figure S1: SEM image of the cross-section of the EL coating sample; Figure S2: EL spectra of the coating sample at varied voltages; Table S1: Performances of representative EL devices; Table S2: Luminance and CIE coordinate of the EL coating sample under 0-8 kV; Table S3: Luminance and CIE coordinate of the EL coating on the insulator under 0-25 kV and different light conditions.  
Figure 8: The EL result of the coating in the insulator string.  
**Author Contributions:** Conceptualization, Y.N. and Y.J.; data curation, D.C. and J.S.; formal analysis, Y.N., Y.J. and P.W.; funding acquisition, Y.N. and Y.H.; investigation, J.S.; methodology, P.W. and D.C.; project administration, Y.N. and Y.H.; resources, Y.H. and Y.Z.; software, P.W.; supervision, Y.H., Y.Z. and S.L.; validation, Y.H., Y.Z. and S.L.; visualization, D.C.; writing--original draft, Y.N., Y.J. and P.W.; writing--review and editing, Y.H., Y.Z. and S.L. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research is funded by China Southern Power Grid (YNKJXM20240497), the National Natural Science Foundation of China (52477028), the Key R&D Projects in Shaanxi Province (2025 CY-YBXM-133), the Youth Fund of State Key Laboratory of Electrical Insulation and Power Equipment (EIPE23407), and the Fundamental Research Funds for the Central Universities (sxzy012023180).  
**Institutional Review Board Statement:** Not applicable.  
**Indromed Consent Statement:** Not applicable.  
**Data Availability Statement:** The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author(s).  
**Conflicts of Interest:** Authors [PERSON] and [PERSON] were employed by the company Yunnan Power Gird Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.