KROHNE OPTISWITCH 5xx0C NAMUR SIL EN - Manual (Page 4)

l
IEC 61508
-
Functional safety of electrical/electronic/programmable elec-
tronic safety-related systems
l
IEC 61511-1
-
Functional safety - safety instrumented systems for the
process industry sector - Part 1: Framework, definitions,
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 the hardware for safety-related subsystems of type A
(IEC 61508-2, 7.4.3)
Safe failure fraction Hardware
fault toler-
ance
SFF HFT = 0 HFT = 1 HFT = 2
< 60 % SIL 1 SIL 2 SIL 3
60 % … < 90 % SIL 2 SIL 3 (SIL4)
90 % … < 99 % SIL 3 (SIL4) (SIL4)
≥ 99 % SIL 3 (SIL4) (SIL4)
According to IEC 61511-1, paragraph 11.4.4, the failure tolerance HFT
can be reduced by one for service-proven subsystems if the following
conditions are met:
l
The instrument is service proven
l
Only process-relevant parameters can be modified on the instru-
ment (e. g. measuring range, current output in case of failure …)
l The modification of these process-relevant parameters is pro-
tected (e. g. password, …)
l
The safety function requires less than SIL4
The assessment by Change Management was a part of the "service
proven" verification.
Relevant standards
Safety requirements
Service proven
4 OPTISWITCH series 5000 • - NAMUR
1 Functional safety
32747
-EN
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