Failure Tree Analysis

Safety and reliability analyses for technical systems of all kind can be performed via fault tree analyses, including common mode failures and human errors. These deductive analyses are based on Boolean Algebra. Thereby, logical combinations of component or partial system failures are determined, which lead to an unwanted event (Top Event). The results of the analyses allow for a system assessment regarding reliability, availability and safety.


Fault Tree Analysis Example: brake system

The following illustration depicts a series system and a parallel system in a reliability block diagram and a fault tree. Concerning the parallel system, basic event 1 AND basic event 2 must occur in order for the top event to happen. With regard to the series system, however, basic event 1 OR basic event 2 initiate the top event.


parallel and serial systems

Aims of a Fault Tree Analysis:

–          Systematic identification of all possible causes of failure and failure combinations, which lead to an unwanted event

–          Determination of reliability parameters (e.g., frequency of failure combinations, frequency of the unwanted event or the lack of availability of the system when required)

–          Preparation of a graphic presentation of failure combinations as a kind of tree structure including input and output variables

–          Comparing different design proposals, detecting weaknesses and proving required reliability and safety related demands analytically through probabilistic reliability and safety prognoses

Fault tree analyses are especially qualified for the demonstration and analysis of complex systems in terms of their reliability and safety characteristics, which in general can comprise thousands of cut sets (combinations of events, which lead to an unwanted top event). The procedures of constructing and analyzing are therefore performed computer-aided.

Further findings about the system under investigation in an FTA can be generated through importance measures. By including these evaluation parameters it is possible to determine the impact of single basic events (often components) on the reliability or safety of a system in order to, for example, objectify and quantify issues in terms of system optimization, weak point analysis, fault detection or maintenance strategy.

The team of the IQZ uses qualitative and quantitative FTAs (amongst others) in the field of functional safety to gain first estimates about the probability of a defined top event at a very early stage in the development process of a product. Different system configurations can be compared by software-aided construction and evaluation of a fault tree in order to achieve an optimum level of safety, reliability and economic efficiency.

The IQZ has access to all common databases of failure rates. In addition, the deployment of the Reliability Prognosis Model of Wuppertal allows for an integration of the failure rates of our partners and customers.