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What This Appendix
Contains
Calculated Values
113
Probability of Failure on Demand (PFD),
Probability of Failure per Hour (PFH), and
Mean Time Between (MTBF) Data
This appendix lists calculated values for probability of failure on
demand, probability of failure per hour, and mean time between
failure.
See the table that shows the values.
Calculated values of probability of failure on demand and probability
of failure per hour appear in the table and must be calculated for the
overall devices within the system to comply with the SIL level
required for application.
The proof test interval of the 1791DS-IB4XOW4 module must
IMPORTANT
not exceed 0.5 years because the maintenance interval for the
relay contacts must not exceed a period of 6 months to satisfy
safety category 4 in accordance with EN954-1.
Within the proof test interval, every I/O module must be functionally
tested by individually toggling each input point and verifying that it is
detected by the controller. Additionally, each output point must be
individually toggled by the controller and user-verified that the output
point changes state.
Calculated Values for Probability of Failure on Demand (PFD) - 1791DS-IB12,
1791DS-IB8XOB8, 1791DS-IB4XOW4
Proof Test Interval
1791DS-IB12
Years
Hours
0.25
2190
2.202E-07
0.5
4380
4.384E-07
1
8760
8.757E-07
2
17,520
1.754E-06
5
43,800
4.419E-06
10
87,600
8.962E-06
Appendix
1791DS-IB8XOB8
1791DS-IB4XOW4
2.213E-07
4.697E-06
4.395E-07
9.532E-06
8.768E-07
1.963E-05
1.755E-06
4.151E-05
4.421E-06
1.207E-04
8.963E-06
2.978E-04
Publication 1791DS-UM001H-EN-P - April 2009
B
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