In the event of 是一个预防措施的告示用的.即是用在一上随时都准备有可能发生的事.
In case of 则是准备一些未必会发生的事,即万一的意思.
例:
in the event of fire this door must be shut.
若发生火警时,此门一定要保持紧闭.
In the event of flooding,people shoot head for the higher ground.
若遇洪水,大家要跳向高处.
In the event of storm windows should be shut securely
遇有风暴,窗户定要紧闭.
In case you can make it home on time,I shall record it for you.
预防万一你不能赶回家的话,我会把它录下给你看.
In case you don't have enough money take my wallet with you.
预防万一你不够钱,拿我的钱包去吧.
In case of the weather get wet suddenly you better carry an umbrella.
预防万一会下雨,你都是携把雨伞去罢.