Download the FNR-25K820 datasheet PDF.
This datasheet also covers the FNR-05K180 variant, as both devices belong to the same zinc oxide leaded varistors family and are provided as variant models within a single manufacturer datasheet.
Note: The manufacturer provides a single datasheet file (FNR-05K180-JohansonDielectrics.pdf) that lists specifications for multiple related part numbers.