John wrote, "...?because it would probably explode due to excessive current if it ever reached striking voltage."

Interesting point.

But I think it would depend on the impedance of the source of the overvoltage.? If it's a transient overvoltage, presumably the source of that transient does not have the same low impedance as the mains supply itself.? (Unless it was something like the HV wire falling and shorting to the regular mains wire.)

I'm not trying to advocate putting a gas discharge tube directly across an AC mains line without a fuse or breaker.? I'm just playing the "what-if" and wanting to understand what goes on.

Andy