'fail-safe' definitions:

Definition of 'fail-safe'

(from WordNet)
adjective
Guaranteed not to fail; "a fail-safe recipe for cheese souffle"
adjective
Eliminating 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"
noun
A mechanism capable of returning to a safe state in case there is a failure or malfunction