Fault Tree OR Gates

With an OR gate, when any of the inputs are true, then the gate output will be true or, as we have indicated before, active. Quantitative evaluation requires summation of the probabilities. For non-mutually exclusive events (the correct case for independent failures)  

Note that this is similar to Equation 5-2 again, showing the relationship between the fault tree and the equivalent RBD. 

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Consider the case of a pressure switch and a process connection for that pressure switch (Figure C-6. Pressure Sensor Subsystem). If the pressure switch has a PFD of 0.005 and the process connection has a PFD of 0.02, the PFD of the system could be modeled with a fault tree OR gate. 

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