mettelus
400Hz is used for high precision AC gear (SONAR suites in submarines run on them as well). Sorta sucks about the DC aspect of computers (but for the sake of the "3 millionth post" this is definitely debatable!)
Submarines ... that's where my experience with 400hz was. Also Beryllium and "mass unbalance modulation", IIRC.
Improperly aligned 400hz synchros were sure whiney