Vega VEGAPULS 61 - 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
- Function
al 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 SIL1 SIL2
60 % … <90 % SIL1 SIL2 SIL3
90 % … <99 % SIL2 SIL3 (SIL4)
≥99 % SIL3 (SIL4) (SIL4)
Relevant standards
Safety requirements
4 VEGAPULS series 60 - 4 … 20 mA/HART
Functional safety
31338-EN-070510