This is the most usually reported version of the fault trees.

1.6.2.1 Logic Symbols

The shorthand symbols which are used to define a fault tree are very few. They consist of a set of logic symbols which interconnect an undesirable event with its causes and at the same time define the relationship between the event and its causal events. As we shall see they are Boolean operators.

Figure 1.23 shows the three operators:

(a) AND gate: This describes the connection by which an event takes place only if all the input events occur.

Basic component fault which needs no further development

EVENT Fault event not developed to its causes

I Fault event of probability one

xransfer symbol

Fig. 1.23. Fault tree symbols.

(b) OR gate: This describes the connection by which an event takes place even if only one of the input events occurs.

(c) INHIBIT gate: This is a causal relationship which states that the output event is caused by the input event if the indicated condition is satisfied. It is a conditional connection.

In addition a set of symbols defines events in terms of their state of analysis within the fault tree. Figure 1.23 shows the fault event which is expanded from its causes to its consequences, the fault event which, as a basic component fault, requires no further analysis, and the fault event which, although it could be developed further, is not.

Two special symbols define a transfer of one part of the tree to another branch and an event which has unity probability of occurring (that is, a certainty).

These 8 symbols form the basic blocks from which a fault tree is construc­ted. All that remains is to connect the different events in a particular system.