IEC 60950-1 prescribes that if a surge suppressor is used in a primary circuit it "shall be a VDR". The document informs that a VDR is sometimes referred to as a "varistor or a metal oxide varistor (MOV)".
It further explicitly states that "gas discharge tubes, carbon blocks or semiconductor devices with non-linear voltage/current characteristics" are not allowed.
Why is this? From what I've read, I'd much rather use a high voltage TVS diode. MOV's seem to have a finite life when subjected to transients.
Edit: As was helpfully pointed out by Spehro, the word is Prescribe, not Proscribe as I initially wrote :-).
Best Answer
It prescribes VDRs, and proscribes the other stuff.
Speculation here- maybe VDRs, of the approved persuasion, are guaranteed to die a peaceful smouldering black death. Approved TVS parts are probably nonexistent and gas discharge tubes tend to arc (well they're supposed to).