ABSTRACT
The purpose of accident analysis is to look for the events and conditions that led to the final outcome, which is the same as ftnding the set of probable causes (Woods et at, 1994). The outcome of an accident analysis is usually a description of one or more combinations of causes that together constitute a satisfactory explanation, cf. the cynical deftnition of causes in Chapter 1. Complementary to that, an accident can also be described as one or more barriers that have failed, even though the failure of a barrier only rarely is a cause in itself. A barrier is, generally speaking, an obstacle, an obstruction, or a hindrance that may either: (1) prevent an event from taking place, or (2) thwart or lessen the impact of the consequences if it happens nonetheless. In the former case the purpose of the barrier is to make it impossible for a speciftc action or event to occur. In the latter case the barrier serves, for instance, to slow down uncontrolled releases of matter and energy, to limit the reach of the consequences, or to weaken them in other ways. These simple considerations suggest that it is useful to make a basic distinction between barriers that prevent and barriers that protect.
