Vega VEGAPULS 61 Safety manual - page 4
λ
dd
Rate for dangerous detected failure
λ
du
Rate for dangerous undetected failure
DC
S
Diagnostic Coverage of safe failures; DC
S
=
λ
sd
/(λ
sd
+
λ
su
)
DC
D
Diagnostic Coverage of dangerous failures; DC
D
=
λ
dd
/(λ
dd
+
λ
du
)
FIT
Failure In Time (1 FIT = 1 failure/10
9
h)
MTBF
Mean Time Between Failure
MTTF
Mean Time To Failure
MTTR
Mean Time To Repair
l
IEC
61508 (also available as DIN EN)
-
Functional safety of electrical/electronic/programmable
electronic safety-related systems
l
IEC
61511-1
-
Functional safety - safety instrumented systems for the
process industry sector - Part 1: Framework, defini-
tions, system, hardware and software requirements
Failure limit values for a safety function, depending on the SIL
class (of IEC 61508-1, 7.6.2)
Safety integrity level
Low demand mode
High demand mode
SIL
PFD
avg
PFH
4
≥
10
-5
… <10
-4
≥
10
-9
… <10
-8
3
≥
10
-4
… <10
-3
≥
10
-8
… <10
-7
2
≥10
-3
… <10
-2
≥10
-7
… <10
-6
1
≥
10
-2
… <10
-1
≥
10
-6
… <10
-5
Safety integrity of hardware for safety-related subsystems of
type B (IEC 61508-2, 7.4.3)
Safe failure fraction
Hardware fault
tolerance
SFF
HFT = 0
HFT = 1 (0)
HFT = 2
<
60 %
not permitted
SIL
1
SIL
2
60 %
… <90 %
SIL
1
SIL2
SIL
3
90 % … <99 %
SIL
2
SIL
3
(SIL4)
≥
99 %
SIL
3
(SIL4)
(SIL4)
Relevant standards
Safety requirements
4
VEGAPULS series 60 - 4 … 20 mA/HART
Functional safety
31338
-EN
-070510