3. Comparison of Fault Occurrence and Fault Existence
The term Fault Occurrence refers to the fact that an undesired event has taken place and may or may not still exist. Fault Existence, however, implies that the fault has occurred and continues to exist. Therefore, the fault can be described as being either transient or permanent.
During the construction of the fault tree, all systems analysts should use Fault Occurrence, rather than Fault Existence, as the focus of interest.
4. Comparison of Failure Causes and Fault Effects
A failure is considered to be an inability to perform a normal function. Example: Valve does not open. A fault is a higher level Occurrence which is usually preceded by a lower-level failure, such as a casing cracking due to overheating because of a lack of coolant induction due to an inoperable valve (lower level of failure). However, a fault may also occur when no failure is present. Example: Coolant valve operates properly, but the signal to operate it encounters a delay. A fault has occurred, but there is no valve failure. Because of this, it can be stated that any failure causes a fault, but not every fault is caused by a failure.
Failure Categories: a. Component, b. Environment, c. Human, d. Software.