User Manual - Series B Instruction Manual

8 Rockwell Automation Publication PFLEX-UM003B-EN-P - July 2012
General Description
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
These PFD and PFH calculations are based on the equations from Part 6 of
IEC 61508 and show worst-case values.
This table provides data for a 20-year proof test interval and demonstrates the
worst-case effect of various configuration changes on the data.
PowerFlex 40P
PFD and PFH for 20-year Proof Test Interval
PowerFlex 70
PFD and PFH for 20-year Proof Test Interval
Terminology
Attribute Test Result
PFD 1.74E-05
PFH 2.0E-10
SFF 81%
Attribute Test Result
PFD 3.4E-05
PFH 3.9E-10
SFF 81%
Abbreviation Full Term Definition
PFD Probability of
Failure on Demand
The average probability of a system to fail to perform its
design function on demand.
PFH Probability of
Failure per Hour
The probability of a system to have a dangerous failure occur
per hour.
SFF Safe Failure Fraction The sum of safe failures plus the sum of dangerous detected
failures divided by the sum of all failures.
PFLEX-UM003.fm Page 8 Wednesday, July 18, 2012 8:26 AM