章節分配表
4.4.3 Top-down methods
At first, the undersirable single event or system success at the highest level of interest (the top event) should be definded. The contributory causes of that event at all levels are then identified and analyzed.
The starting point of the top-down approach is to proceed from the highest level of interest, that is, the system or sub-system level, to successively lower levels in order to identify undesirable system operations.
The analysis is performed at the next lowest system level to identify any failure and its associated failure mode, which could result in the failure effect as originally identified. Foe each of these second level failures, the analysis is repeated by tracing back along the functional paths and relationships to the next lowest level. This process is continued as far as the lowest level desired.
The top-down approach is used for evaluating multiple failures including sequentially related failures, the existence of faults due to a common cause, or wherever system complexity makes it more convenient to begin by listing system failures.