VEGA VEGAPULS 56K - Manual (Page 59)

VEGAPULS 56K 59
22915-EN-041227
Supplement
1.4 Determination of safety-related characteristics
The failure limit values for a safety function, depending on the SIL class.
Safety integrity Safety integrity Safety integrity Safety integrity Safety integrity Low demand mode High demand or continuous
level mode
SIL PFD
avg
PFH
4 >10 -5 up to <10 -4 >10 -9 up to <10 -8
3 >10 -4 up to <10 -3 >10 -8 up to <10 -7
2 >10 -3 up to <10 -2 >10 -7 up to <10 -6
1 >10 -2 up to <10 -1 >10 -6 up to <10 -5
(from IEC 61508, part 1/7.6.2)
Safety integrity of the hardware:
Limitations due to the architecture for safety-related subsystems of type B
Safe failure fraction SFF Hardware fault tolerance HFT
0 1 (0) 1) 2
< 60 % not allowed SIL 1 SIL 2
60 % up to < 90 % SIL 1 SIL 2 SIL 3
90 % up to < 99 % SIL 2 SIL 3 (SIL 4)
> 99 % SIL 3 (SIL 4) (SIL 4)
(from IEC 61508, part 2/7.4.3)
1) Acc. to IEC 61511-1, paragraph 11.4.4 the fault tolerance specified acc. to the above chart can be reduced
by one for all subsystems if the following conditions are met:
- instrument has proven during operation
- only process-relevant parameters can be modified on the instrument (e.g. measuring range, …)
- the adjustment of this process-relevant parameters is protected (e.g. password, …)
- the safety function needs less than SIL 4