Someone has tried to tell me that the free-air distance needed to prevent arcing varies inversely with frequency; i.e. a 10 KV RMS 60Hz conductor will need less spacing than a 10 KV RMS 25Hz one.
I'm no power distribution engineer, but I can't recall anything close to that. I asked my MIT-grad friend as well, but outside of the obvious end case [0 Hz, where peak=RMS] we both came up blank.
Obviously, OAT, %RH, etc alter the result, but frequency?