Commom cause failure model of redundant structure

Common cause failure is the main reason resulting in the failure of redundant system. When calculating PFD_avg of redundant structure, the common cause failure must be considered. The single β-factor model refered in IEC61508 can not differentiate the different redundant structure affectting on common cause failure factor β. The Muti-ply β-Factor Model, a more exact model, is put forward in this paper. By going deep into analyzing the common cause failures of two redundant equipments and three redundant equipments, the modification factor for voting configrations C_MOON is ascertained. The significance of C_MOON and its effect on the PFD_avg are more clear by analyzing the PFD_avg for the 1oo2 voting and 2oo3 voting and performing a mathnatical calculation.