Background - Using Event-B for Critical Device Software Systems - page 19

Fig. 2.3 FTA tree

level event. The fault trees can also be used in a confirmatory role where they are
particularly useful in showing that a probability requirement for a hazardous fail-
ure mode has been met by the system. Figure 2.3 depicts a basic architecture of the
FTA. In this figure, highest level event (hazard) is traced backward to identify the
source of errors or faults. Events and gates in fault tree analysis are represented by
symbols. The source of errors or faults are known as the base events (errors) [ 73 ].

Failure Mode Effects Analysis (FMEA)

Failure modes and effects analysis (FMEA) is a step-by-step approach [ 31 ] to iden-
tify the possible hazards in a complex system that facilitates the identification of
potential problems in the design or process by examining the effects of lower level
failures. The FTA safety analysis technique is based on top-down approach, while
the FMEA is used a bottom-up approach. In this bottom-up analysis, the technique
determines possible failures of a system and produces a list of probable failures ac-
cording to the degree of severity. The feed-forward technique of the FMEA is used
to discover possible failures or errors through forward tracing (see Fig. 2.4 ). FMEA
is useful for evaluating a new process prior to implementation, and for assessing the
impact of proposed changes on the existing processes. The output of FMEA presents
in a tabular form that describes the failure modes, in which something might fail,
and the consequences of those failures.