fail-safe
adjective uk/ˈfeɪ.l.seɪf/ us/ˈfeɪ.l.seɪf/
万无一失的，万全的
very unlikely to fail
a fail-safe plan 万无一失的计划
可自动防止故障危害的；有自动保险装置的
If something is fail-safe, it has been designed so that if one part of it does not work, the whole thing does not become dangerous.
a fail-safe device 故障保护装置