Wikipedia states that a 30-hour clock is used in written communications by businesses whose hours straddle midnight and in late night schedules for TV and radio stations:
https://en.wikipedia.org/wiki/Date_a...ation_in_Japan

I have a feeling that referring to hours beyond 24 is just a thing that came about randomly / casually without a formal standard. But perhaps Wikipedia is right that there's an established convention that limits it to 30.