Critical Dangerous Failure
dangerous failure leading to the complete inhibition of the safety action (i.e. leading to a dangerous
situation for the protected system)
Note 1 to entry: This is a systemic failure in relationship with a given safety action performed by the safety
system. Therefore this concept is irrelevant for an individual item on the shelves.
Note 2 to entry: The same failure of a component belonging to a safety system with internal redundancy may be
dangerous or critical dangerous depending on the system state from which it occurs.
Note 3 to entry: The critical dangerous failures that are undetected (e.g. those revealed by periodic tests) are
sometimes called safety critical failures (cf. ISO 14224). The equipment subject to such potential failures can
be identified within a plant and monitored, and the ratio between the number of safety critical failures detected
by periodic tests and the corresponding number of tests performed (commonly called “ failure fraction”) is being
used for that purpose. This indicator of the average unavailability (PFDavg) due to dangerous undetected failures
is established by using test reports. It is important not to mix such failure fraction with other reliability terms.
Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global Standards