One way to hold this word: it assumes the thing will break one day, and asks which way it should fall when it does. A fail-safe machine falls towards stopping, towards the closed valve, towards the locked brake — the direction where nobody gets hurt. Read the two halves in order and the word is its own instruction: fail, then safe.
Two branches
壞掉的時候往安全那邊倒（機器與設計）
這一組就是核心本身，只是換了詞性：性質、做到這個性質的裝置、以及做這件事的動作。
adjective · B2
Used about a machine, system or piece of equipment that is built so that if one part stops working, the whole thing stops or switches itself off instead of causing damage or danger.
故障自動防護的
從「不會出事」到「不會失敗」（計畫與方法）
核心講的是「壞掉也不會出事」。當這個字從機器移到計畫上，被保留下來的只剩「你可以放心」這一層，於是整句話變成「一定會成功」。
adjective · B2
Used about a plan, method or way of doing something that is certain to work, with no chance of going wrong.
萬無一失的