8
Failure Rates in Accordance to IEC 61508
The following table summarizes the failure rates of the modules, which were calculated from
Failure Mode and Diagnostic Effects in accordance to standard IEC 61508. The information can
be used in calculating probability of dangerous failures using Reliability Block Modeling or Markov
Modeling. Such modeling should consider redundant 3108 analog input modules in a 1oo2, 2oo3
or 3oo4 configuration.
Rates only applicable for 3108 only. 3118 is not safety-rated.
Safe Failure Fraction:
99.31%
Diagnostic Coverage:
98.88%
Failure Rates In Common Circuitry:
Safe Detected 2.5890E-07
Safe Undetected 4.6152E-09
Dangerous Detected 4.2702E-07
Dangerous Undetected 2.4371E-09
Don't Care 2.9158E-07
Failure Rates In Per Channel Circuitry:
Safe Detected 1.4731E-08
Safe Undetected 4.1488E-09
Dangerous Detected 1.9403E-08
Dangerous Undetected 2.5975E-09
Don't Care 1.8720E-08
Average Frequency of a Dangerous Failure per Hour (1oo1D):
5.0346E-09
Average Frequency of a Dangerous Failure per Hour (1oo2D):
1.4557E-10
Average Frequency of a Dangerous Failure per Hour (2oo3D):
6.6370E-11
Mean Time to Restoration for 1oo2D (SIL-3) configurations:
≤ 9 days
Mean Time to Restoration for all other configurations:
No
restriction
In dual redundant configurations, the users must exchange the card within Mean Time to Restore
(MTTR ≤ 9 days) in order to remain within SIL 3 requirements. Otherwise the card will continue
to operate in degraded SIL 2 mode.
