expression
except when; excluding the case when
Fixed expression used to state an exception or condition, often in formal or written contexts. Literally 'removing the case when...'.
緊急の場合を除き、このドアを使用しないでください。
Except in an emergency, please do not use this door.
雨天の場合を除き、試合は予定通り行われます。
The game will be held as scheduled, except in the case of rain.