Allen-Bradley 1756-L6 Series, Reference Manual, Page: 107 / 134

Rockwell Automation Publication 1756-RM001I-EN-P - May 2012 107
Appendix
C
PFD Calculations for a SIL 2 System
About Probability of Failure
on Demand (PFD)
Calculations Probability of failure on demand (PFD) is the SIL value for a low demand safety-
related system as related directly to order-of-magnitude ranges of its average
probability of failure to satisfactorily perform its safety function on demand.
IEC 61508 quantifies this classification by stating that the frequency of demands
for operation of the safety system is no greater than once per year in the Low
Demand mode.
PFD calculations are commonly used for process safety applications and
applications where emergency stop devices (ESDs) are used.
Although PFD values are usually associated with each of the three elements
making up a safety-related system (the sensors, the actuators, and the logic
element), they can be associated with each component of the logic element, that
is, each module of a programmable controller.
Tables in this chapter present PFD values for ControlLogix and
ControlLogix-XT components that are evaluated by TÜV.
About the Calculations in
This Manual For the calculations presented in this chapter, these values were used as the two
application-dependent variables:
• Mean Time to Restoration (MTTR) is ten hours.
• Proof Test Interval (T
1
) is listed for each table.
The PFD values in this manual are calculated with formulas explained in
IEC 61508, Part 6, Annex B. Refer to IEC 61508, Part 6, for more information
about calculating PFD values for your system.
Topic Page
About Probability of Failure on Demand (PFD) Calculations 107
About the Calculations in This Manual 107
1-Year PFD Calculations 108
2-Year PFD Calculations 112
5-year PFD Calculations 115
Using Component Values To Calculate System PFD 119
Allen-Bradley Motors