Something that is fail-safe is designed or made in such a way that nothing dangerous can happen if a part of it goes wrong.
There was a fail-safe mechanism in place to override the controls in case of a malfunction.
fail-safe in British English
adjective
1.
designed to return to a safe condition in the event of a failure or malfunction
2.
(of a nuclear weapon) capable of being deactivated in the event of a failure or accident
3.
unlikely to fail; foolproof
verb
4. (intransitive)
to return to a safe condition in the event of a failure or malfunction
fail-safe in American English
(ˈfeɪlˌseɪf)
adjective
designating, of, or involving a procedure designed to prevent malfunctioning or unintentional operation, often specif. of nuclear-armed aircraft, through an intricate system of protective devices