Spark gaps are widely used for protection. The most common is the ceramic gas tube protectors for Telecom. They come in 2 and 3 lead versions. 3 lead used for balanced lines.
In the old days, the gas could have radioactive isotope added but that is very rare today except for certain military and industrial applications. Waveguide Radar receive protectors back in the day were filled with a radioactive isotope gas and a HV priming voltage.
One should never use a gas tube protector across an AC or DC line. Once they fire they will not shut off unless the voltage/current drop below the sustaining level.?
