Common Cause Analysis
There are several models for quantifying systems subject to common cause failures. Some of the popular models are:
• Beta Factor model.
• Multiple Greek Letter (MGL) model.
• Alpha model.
• Beta Binomial Failure Rate (BFR) model.
Example
The following example is provided as an aid in understanding the mechanism of the handling of common cause failure (CCF) events in a fault tree. Assume that there are four basic events A, B, C and D belonging to a CCF group. When an analyst does the minimal cut set analysis of the fault tree, the following CCF events should be created corresponding to the basic events:
AB, AC, AD, BC, BD, CD, ABC, ABD, ACD, BCD and ABCD
For calculation purposes, each of the four original basic events (A, B, C or D) is replaced with an OR gate. The inputs to the OR gate include the individual basic event and CCF events that contain that basic event. For example, event A is replaced by an OR gate with A (individual failure), AB, AC, AD, ABC, ABD, ACD and ABCD as its inputs.
The following parameters are used in calculating CCF events:
Qt=Total unavailability of each basic event in the CCF group.
Qk=Unavailability of the CCF event of order k, which is a common cause failure involving k components.
n=Number of basic events in the CCF group.