Tree Unavailability calculation

Basic method for Fault Tree unavailability calculation is Minimal Cut Sets generation and then unavailability calculation using Esary-Proschan formula.

Two main types of tree result can be calculated:

•	Mean Unavailability Q (long-term steady-state average unavailability) or

•	Unavailability Q(t) (probability of failure at a given time point). To choose which result to calculate and display on the diagram, see paragraph "Using FTA module", FTA properties.

RAM Commander also supports Frequency (unconditional failure intensity) at time t W(t) calculation - see Frequency W(t) Calculation paragraph for more information.

In order to perform the MCS Calculation, algorithm performs bottop-up fault tree analysis, building set of Minimal Cut Sets and performing Boolean transformations of the cut sets according to Boolean algebra laws. Then probability of each cut set is calculated, and then the top gate probability is calculated.

For large fault trees, with a large number of identical events, the number of MCS may be very large increasing the calculation time immensely. In this case, MCS Cut-off is used. During the Cut-Off, some minimal cut sets, which barely affect the final result, can be deleted. Three Cut-Off parameters are the Max. MCS number for Cut-Off, Relative cut-off value and Relative Cut-Off order (see FTA properties). When the MCS count in calculated tree increases the Max. MCS number, the Cut Off process is performed. If the relative affect on the calculation result for the MCS is less than the Relative cut-off value, the MCS is deleted.