Vega VEGABAR 54 Operating Instructions Manual page 55

Relevant standards
Safety requirements
Proven in use
VEGABAR 54 - 4 ... 20 mA/HART
IEC 61508 (also available as DIN EN)
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- Functional safety of electrical/electronic/programmable
electronic safety-related systems
IEC 61511-1
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- Functional safety – safety instrumented systems for the
process industry sector – Part 1: Framework, defini-
tions, system, hardware and software requirements
The failure limit values for a safety function, depending on the
SIL class (of IEC 61508-1, 7.6.2)
Safety integrity level
SIL
4
3
2
1
Safety integrity of the hardware for safety-relating subsystems
of type B (IEC 61508-2, 7.4.3)
Safe failure fraction
SFF
<60 %
60 % up to <90 %
90 % up to <99 %
≥99 %
Acc. to IEC 61511-1, paragraph 11.4.4 the failure tolerance
HFT can be reduced by one for part systems proven in use if
the following conditions are met:
The instrument is proven in use
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Only process-relevant parameters can be modified on the
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instrument (e.g. measuring range, current output in case of
failure ...)
The modification of these process-relevant parameters is
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protected (e.g. password, ...)
The safety function requires less than SIL4
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Functional safety acc. to IEC 61508-4 (SIL)
Low demand mode
PFD
avg
-5 -4
≥10 up to <10
-4 -3
≥10 up to <10
-3 -2
≥10 up to <10
-2 -1
≥10 up to <10
Hardware fault
tolerance
HFT = 0 HFT = 1 (0)
not permitted SIL1
SIL1 SIL2
SIL2 SIL3
SIL3 (SIL4)
High demand mode
PFH
-9 -8
≥10 up to <10
-8 -7
≥10 up to <10
-7 -6
≥10 up to <10
-6 -5
≥10 up to <10
HFT = 2
SIL2
SIL3
(SIL4)
(SIL4)
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