Example of PFDAvg Calculation

Last edit: 07/08/2023

We want to calculate the PFDavg for a low demand mode Safety Loop involving a pressure transmitter, an analogue barrier, two electronic safety modules and the STO (Safe Torque off) of a Variable Speed Drive.

Nous devons connaître les données de sécurité de la fiche technique de chaque composant ; par exemple, nous devons connaître λDU, SFF et le type de composant. Il est ensuite nécessaire d’établir la structure de chaque sous-système et la période de test d’épreuve Ti.

Ci-après, des exemples de données :

The PFDavg of the 1oo1 structure can be calculated with the following formula, resulted from the integral in par. 2.2 Terms and definitions from 61508-4

The SIL architecture of each subsystem has been fixed using table 2 and table 3 of the so called Route 1H.

PFDavg_TOT of the loop is the sum of the PFDavg of each component.

The system reaches SIL 2.

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