JUMO dTRANS T06 - Determining the Safety Integrity Level (SIL)

6 Safety Manual
41
6.6 Determining the safety integrity level (SIL)
The achievable safety integrity level is determined by the following safety-related parameters:
- Average probability of dangerous failures of a safety function on demand (PFD
avg
),
- Hardware fault tolerance (HFT) and
- Safe failure fraction (SFF).
The specific safety-related parameters for the 707071 measuring system can be found in the table in the "Safety-related pa-
rameters" chapter.
The following table shows how the "safety integrity level" (SIL) depends on the "average probability of dangerous failures of a
safety function of the entire safety-related system" (PFD
avg
) according to DIN EN 61508. The "low demand mode" is consid-
ered here, i.e. the demand rate for the safety-related system averages once a year.
Table for high demand PFH and low demand PFD
Safety integrity level
(SIL)
Operating mode with high demand rate
PFH (high demand mode)
Operating mode with low demand rate
PFD
avg
(low demand mode)
4 ≥ 10
-9
to <10
-8
≥ 10
-5
to <10
-4
3 ≥ 10
-8
to <10
-7
≥ 10
-4
to <10
-3
2 ≥ 10
-7
to <10
-6
≥ 10
-3
to <10
-2
1 ≥ 10
-6
to <10
-5
≥ 10
-2
to <10
-1