Nature of Failures

In the very complex issue of defining the kind and types of failures, two types appear—those that are physical failures and those that are functional failures. Both physical and functional failures have three attributes that are important to the analysis of control system reliability: that is, a failure source, an effect on the controller function, and some time-dependent behavior. Many different things individually or in combination can cause failure. Failure sources are both internal and external to the system. Design faults can cause system-wide failures; in fact, they are the major source of functional failures. Manufacturing faults occur when some step in the process of manufacturing component or module or system is done incorrectly. The number of failures due to manufacturing faults typically decreases with time. External environmental failures frequently are related to temperature. Operational faults or maintenance faults can be responsible for common-cause failures.