We have developed a promising method for suppressing moisture condensation that prevents insulation failures in surge protectors. By analyzing surge protectors retrieved from the field, we found that electrolytic corrosion had occurred due to the encroachment of sea salt, the application of bias voltages, and the condensation of moisture. To suppress moisture condensation, a key factor in insulation failure, we applied a previously developed humidity-control package containing water-absorbent polymer. We experimentally optimized the design and functionality of the polymer package. We found that sealing the feed-through apertures alone was not enough to suppress moisture inflow and that a relatively large amount of water-absorbent polymer was needed to prevent water condensation in environments with extremely high humidity for extended periods of time. Laboratory experiments and field tests demonstrated that our optimized package minimized humidity fluctuation and thus moisture condensation in surge protectors, thereby preventing insulation failure. Application of this method to installed surge protectors greatly reduced the insulation failure rate.