High-voltage (HV) post insulators are vulnerable components of the electrical substations during strong earthquakes. It is rather hard to repair them in service. There are two controversial approaches to increase their seismic safety. They are application of the base stiffening techniques and utilization of the base isolation systems. A new low-cost seismic isolation device that is mounted underneath of HV post insulator is being proposed here to provide period elongation and supplementary damping. The seismic isolator consists of two steel plates, four polyurethane springs and a steel rod. Its cost is insignificant compared with price of HV post insulator. Quasi-static and shake table tests were performed on fixed base and isolated specimens. A set of historical acceleration records selected according to IEEE-693 were utilized in the dynamic tests. The supplementary damping provided by the isolation system was determined between 4 and 8%. The accelerations as well as base shear and base moment of HV post insulator were reduced about 20-70% thanks to the isolation system. The generated numerical models reproduced the experimental results. Consequently, the isolation system is capable to enhance the seismic safety of HV post insulators.