According to Section 3.1 of Institute of Electrical and Electronics Engineers (IEEE) Standard 1570-2002, safety-critical is “a term applied to system or function, … the incorrect performance of which may result in an unacceptable risk of a hazard.” Safety-critical functions are closely tied to vital functions. IEEE 1570-2002 defines a vital function as “a function in a safety-critical system that is required to be implemented in a fail-safe manner” and fail-safe as “a design philosophy applied to safety-critical systems such that the result of a hardware failure or the effect of software error shall either prohibit the system from assuming or maintaining an unsafe state, or shall cause the system to assume
a state known to be safe”