**5. Conclusions**

The proposed experimental procedures of partial discharge degradation of silicone rubber are evaluated by experimental results using five kinds of silicone rubber samples. Also the di fference in resistance to partial discharge among samples are discussed. The results are summarized as follows.

1. The procedures proposed in this study is considered suitable for investigation of partial discharge degradation of hydrophobic material like silicone rubber. It has advantages of no arc discharge occurrence, recovery of hydrophobicity during the resting time, acceptable repeatability of results, and possible acceleration of degradation by adjusting gap spacing and applied voltage.


**Author Contributions:** Conceptualization, K.K. and Y.M.; Methodology, K.K., H.L. and M.S.; Validation, K.K.; Formal Analysis, K.K., H.L. and M.S.; Investigation, K.K. and H.L.; Resources, Y.M.; Data Curation, K.K.; Writing-Original Draft Preparation, K.K.; Writing-Review & Editing, Y.M.; Visualization, K.K. and M.S.; Supervision, Y.M.; Project Administration, K.K. and Y.M.; Funding Acquisition, Y.M.

**Funding:** This research received no external funding.

**Acknowledgments:** The silicone rubber samples were supplied in connection with activities of an Investigating R & D Committee of the Institute of Electrical Engineers of Japan. The authors would like to thank the members of the committee.

**Conflicts of Interest:** The authors declare no conflict of interest.
