Vega VEGACAL 60 Series, Safety Manual

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4
1 Functional safety
VEGACAL series 60 • Two-wire 4 … 20 mA/HART
35593-EN-181129
– Functional safety - safety instrumented systems for the process
industry sector - Part 1: Framework, definitions, system, hard-
ware 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 permit-
ted
SIL1 SIL2
60 % … < 90 % SIL1 SIL2 SIL3
90 % … < 99 % SIL2 SIL3 (SIL4)
≥ 99 % SIL3 (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 fol-
lowing conditions are met:
•
The instrument is service proven
•
Only process-relevant parameters can be modified on the instru-
ment (e. g. measuring range, current output in case of failure …)
•
These process-relevant parameters are protected (e.g. password,
…)
•
The safety function requires less than SIL4
The assessment by change management staff was a part of the
"service proven" verification.
1.2 Planning
The measuring system generates on the current output a signal be-
tween 3.8 mA and 20.5 mA corresponding to the level.
This analogue signal is transmitted to a connected processing unit to
monitor the following conditions:
•
Exceeding a preset level
•
Falling below a preset level
When the switching point set on the processing unit is reached, a
signal is output.
Safety requirements
Service proven
Safety function