KROHNE OPTISWITCH 3x00C 2wire SIL EN - Manual (Page 4)

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
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 toler-
ance
SFF HFT = 0 HFT = 1 HFT = 2
< 60 % not permitted SIL 1 SIL 2
60 % … < 90 % SIL 1 SIL 2 SIL 3
90 % … < 99 % SIL2 SIL 3 (SIL4)
≥ 99 % SIL 3 (SIL4) (SIL4)
1 .2 Planning
The safety function of this measuring system is the identi-
fication and signalling of the condition of the vibrating element.
A difference is made between the two conditions "covered"
and "uncovered".
Relevant standards
Safety requirements
Safety function
4 OPTISWITCH series 3000 • - two-wire
Functional safety
32745
-EN
-080414