Faults fall into two basic categories: operational and component.
The operational fault is one which occurs when a component is operating as it was designed to, but at an inappropriate time or place. An example is a failure of a control valve to close or to interrupt the introduction of a reactant into a chemical process due to an inappropriate signal from another device.
The component fault can be further divided into two sub-categories: primary and secondary. A Primary component fault occurs when a component fails to function in its intended environment. Example: A radar unit designed for use in aircraft which fails due to vibration. A Secondary component failure occurs when a component fails to function in an environment other than the environment for which it is intended. Example: A radar unit designed for a cargo aircraft fails in a fighter aircraft due to vibration.