2. Collect and estimate the data required for the calculation

2.1 The data you need

To evaluate the PFD _average by means of a fault tree or the application of formulas, it is necessary to know a minimum of information about the various items of equipment that make up the safety instrumented function:

• their hazardous and non-hazardous failure rates (λ _DU and λ _DD );

• type of redundancy (if any) ;

• their test periods (TI) ;

• their repair times (MTTR) ;

• the percentage of undetected common-cause failures (β).