User manual

20 Publication 1756-RM093F-EN-P - January 2010
Chapter 1 Safety Integrity Level (SIL) Concept
To determine the logic element PFH for each safety loop in the simple
example system shown in the PFH Example, sum the PFH values for
each component in the loop. The PFH Equations by Safety Loop table
provides a simplified example of PFH value calculations for each
safety loop shown in the PFH Example illustration.
When calculating PFH values, you must take into account the specific
requirements of your application, including test intervals.
Safety Integrity Level (SIL)
Compliance Distribution
and Weight
The GuardLogix controller and I/O system may conservatively be
assumed to contribute 10% of the reliability burden. A SIL 3 system
may need to incorporate multiple inputs for critical sensors and input
devices, as well as dual outputs connected in series to dual actuators
dependent on SIL assessments for the safety related system.
Reliability Burden
PFH Equations by Safety Loop
For this loop Sum the PFH values of these components
Total PFH for loop 1 = 1791DS-IB12 + GuardLogix controller + 1791DS-IB4XOX4
Total PFH for loop 2 = 1791DS-IB8XOB8 + GuardLogix controller + 1791DS-IB4XOX4
Actuator
Controller
Output
Module
+V
Sensor
Sensor
40% of
the PFD
10% of the PFD
50% of the PFD
Input
Module
Actuator