I am applying surge to the EUT signal line through a varistor from SURGE OUT in lightning surge testing. What kind of varistor should be used?

I am applying surge to the EUT signal line through a varistor from SURGE OUT in lightning surge testing. What kind of varistor should be used?

If there are no specifications in the reference standard, selection is made based on the test level and impact on signal lines. Please confirm the following with multiple varistors and make your selection.
(1) Confirm signal connectivity and waveform in the connected state during testing, and that transmission quality is within acceptable limits.
(2) Confirm surge waveform at the test level with varistor alone, and that waveform degradation is minimal.