Nouns

(n)fail-safea mechanism capable of returning to a safe state in case there is a failure or malfunction

Adjectives

(s)fail-safeguaranteed not to fail“a fail-safe recipe for cheese souffle”
(s)fail-safeeliminating danger by compensating automatically for a failure or malfunction“a fail-safe device in a nuclear weapon to deactivate it automatically in the event of accident”