Rockwell Automation 1715-OF8I Redundant I/O System User Manual - Manual (Page 289)

Rockwell Automation Publication 1715-UM001C-EN-P - March 2014 289
Appendix
E
PFD and PFH Calculations for a SIL 2 System
About PFD and PFH
Calculations
Probability of failure on demand (PFD) is the SIL value for a 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.
Probability of failure per hour (PFH) is typically used to describe safety
performance for high demand applications. Tables in this chapter present PFD
and PFH values for components that are evaluated by TÜV.
Determine Which Values to
Use
Each of the PFD and FPH calculated values provided in this manual is based on
the configuration that the module can be used in, that is 1oo1, 1oo2, 1oo2D, or
2oo2.
Topic Page
About PFD and PFH Calculations 289
Calculations for 1715 I/O Modules 290
IMPORTANT You are responsible for determining which of the values provided are
appropriate for your SIL 2-certified system. Determine which values to use
based on the modules used in your system and the system configuration.