12 Rockwell Automation Publication 750-UM002H-EN-P - February 2017
Chapter 1 Safety Concept
PFD and PFH Definitions
Safety-related systems can be classified as operating in either a Low Demand
mode, or in a High Demand/Continuous mode.
• Low Demand mode: where the frequency of demands for operation
made on a safety-related system is no greater than one per year or no
greater than twice the proof-test frequency.
• High Demand/Continuous mode: where the frequency of demands for
operation made on a safety-related system is greater than once per year or
greater than twice the proof test interval.
The SIL value for a low-demand safety-related system is directly related to
order-of-magnitude ranges of its average probability of failure to satisfactorily
perform its safety function on demand or, simply, average probability of failure
on demand (PFD). The SIL value for a High Demand/continuous mode
safety-related system is directly related to the probability of a dangerous failure
occurring per hour (PFH).
PFD and PFH Data
PFD and PFH calculations are based on the equations from Part 6 of
IEC 61508.
These tables provide data for a 20-year proof test interval and demonstrates the
worst-case effect of various configuration changes on the data.
Table 1 - PFD and PFH for PowerFlex 753 and PowerFlex 755 Drives
Table 2 - PFD and PFH for PowerFlex 755T Drive Products
Attribute
PowerFlex 753 and
PowerFlex 755 Drives
Frames 1…7
PowerFlex 755 Drives
Frame 8
PowerFlex 755 Drives
Frame 9
PowerFlex 755 Drives
Frame 10
PFD 3.29E-05 1.73E-04 2.65E-04 3.56E-04
PFH (1/hour) 3.75E-10 1.99E-09 3.04E-09 4.09E-09
SIL CL3333
PLeeee
Category3333
HFT 1 (1oo2) 1 (1oo2) 1 (1oo2) 1 (1oo2)
Mission time 20 years 20 years 20 years 20 years
Attribute Drive Frame 8 Drive Frame 9 Drive Frame 10 Drive Frame 11 Drive Frame 12
PFD 2.48E-04 2.74E-04 3.00E-04 3.26E-04 3.52E-04
PFH (1/hour) 2.86E-09 3.15E-09 3.45E-09 3.75E-09 4.05E-09
SIL CL3 3333
PL e eeee
Category3 3333
HFT 1 (1oo2) 1 (1oo2) 1 (1oo2) 1 (1oo2) 1 (1oo2)
Mission time 20 years 20 years 20 years 20 years 20 years
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