System Safety for the 21st Century, 2nd Edition

CHAPTER 11Analytical Trees

Analytical trees can be used in a variety of ways in the system safety effort. The most common application of analytical trees in current system safety programs is probably the use of fault trees for fault tree analysis (FTA). However, analytical trees can also be used as planning tools, project description documents, status charts, and feeder documents for several hazard analysis techniques (including fault tree analysis). Analytical trees can be multipurpose, life cycle documents and represent one of the most useful tools available to managers, engineers, and safety professionals.

Analytical trees are nothing more than graphic representations (Fig. 11‐1). They are literally pictures of a project. Analytical trees use deductive reasoning; that is, they start with a general top event or output event and develop down through the branches to specific input events that must occur in order for the output to be generated.

Trees are called trees because, basically, they have a structure that resembles a tree, that is, narrow at the top with a single event symbol and then branching out as the tree is developed.

There are two basic types of analytical trees. The positive tree, or objective tree, which is developed to ensure that a system works properly, and the negative tree, or fault tree, which is generally used for troubleshooting and investigating system failures (Fig. 11‐2).

Positive or objective trees are extremely useful planning tools. In the early ...