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6 Safety-related applications
6.15
Excerpt from FMEDA report
Normally open configurations C1-C4
Failure category
Safe detected (λ
Safe undetected (λ
Dangerous detected (λ
Dangerous undetected (λ
No effect
Total failure rate (safety function)
a
SFF
DC
b
SIL AC
a
The complete subsystem will need to be evaluated to determine the overall safe failure fraction. The
number listed is for reference only.
b
SIL AC (architectural constraints) means that the calculated values are within the range for hardware
architectural constraints for the corresponding SIL but does not imply all related IEC 61508 require-
ments are fulfilled. In addition it must be shown that the device has a suitable systematic capability for
the required SIL and that the entire safety function can fulfill the required PFD / PFH values.
Normally closed configurations C5-C8
Failure category
Safe detected (λ
Safe undetected (λ
Dangerous detected (λ
Dangerous undetected (λ
No effect
Total failure rate (safety function)
a
SFF
DC
b
SIL AC
a
The complete subsystem will need to be evaluated to determine the overall safe failure fraction. The
number listed is for reference only.
b
SIL AC (architectural constraints) means that the calculated values are within the range for hardware
architectural constraints for the corresponding SIL but does not imply all related IEC 61508 require-
ments are fulfilled. In addition it must be shown that the device has a suitable systematic capability for
the required SIL and that the entire safety function can fulfill the required PFD / PFH values.
28
)
SD
)
SU
)
DD
)
DU
)
SD
)
SU
)
DD
)
DU
IEC 61508:2010
Failure rates (in FIT)
C1
C2
Normal
Normal
0
187
252
6
48
83
104
104
241
341
80 %
75 %
9 %
7 %
SIL 2
SIL 2
IEC 61508:2010
Failure rates (in FIT)
C5
C6
Normal
Normal
0
177
232
6
58
103
104
104
241
341
75 %
69 %
9 %
5 %
SIL 2
SIL 2
C3
C4
Inverted
Inverted
0
0
189
6
6
51
106
106
246
79 %
75 %
9 %
SIL 2
SIL 2
C7
C8
Inverted
Inverted
0
0
179
6
6
61
106
106
246
75 %
69 %
8 %
SIL 2
SIL 2
0
254
6
86
346
6 %
0
234
6
106
346
5 %
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