A novel seismic base isolation system has been developed for high-voltage (HV) porcelain post insulators. The seismic isolation device consists of two steel plates, four polyurethane springs, and a steel rod. These are low-cost components compared to the post insulators. Two alternative designs of the device are experimentally and numerically assessed in this paper. A simple and robust numerical model consisting of linear line elements and non-linear springs was generated and subsequently validated using experimental results. Incremental dynamic analyses (IDA) were then performed to obtain fragility curves. Ten historical earthquake profiles scaled to intensities between 0.1 to 2.0 g were then applied to the numerical models. The fragility curves were generated according to the latest version of IEEE-693 and demonstrate that the seismic isolation devices are highly effective in diminishing the base moment of the porcelain insulator. Relatively large displacements at the top of the pole must be accounted for by ensuring adequate slackness in the flexible conductors.