A system failure is an situation in which the system does not function as expected. Typically, it means undesired reduction of the functionality, thereof, reduced profit ( operational loss).
How can we tell if the system functions as expected? People's expectations vary, and are contaminated by interests.
A practical definition of system failure is using the concept of incidence. The guide defines an incidence as a deviation from the operational rules. In a design implementing the principle of self control ..., the system can identify deviation from the operational rules. Therefore, in this guide, system failure is defined as an instance of incidence.
A unit failure is an situation in which the unit does not function as expected, resulting in system failure.
A critical unit failure is a unit failure ending up in an incidence.
The damage to a valve used to control the entry of coolant water in a chemical process is a critical unit failure.
Updated on 30 Mar 2017.