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Safety
EIO0000001505 12/2014
59
The Probability of Failure on Demand (PFD) of low demand operation for different SILs are defined
in IEC 61508 are as follows:
In continuous operation, these changes to the following:
A function is considered as "on demand" if the demand rate is lower than one activation per year.
Otherwise, the function is considered as "High demand or continuous operation".
The hazards of a control system must be identified then analyzed in a risk analysis. These risks
are gradually mitigated until their overall contribution to the hazard is deemed to be acceptable.
The tolerable level of these risks is specified as a safety requirement in the form of a target
probability of a dangerous failure over a given period, stated as a discrete SIL level.
PL - Performance Level
The standard IEC 13849-1 defines 5 Performance Levels (PL) for safety functions.
"a" is the lowest level and "e" is the highest level.
5 levels (a, b, c, d, and e) correspond to different values of average probability of dangerous failure
per hour.
SIL Average Probability of a Dangerous Failure on Demand of the safety function
PFD
SIL4
< 10
-4
SIL3
≥
10
-4
to < 10
-3
SIL2
≥
10
-3
to < 10
-2
SIL1
≥
10
-2
to < 10
-1
SIL
Average Frequency of a Dangerous Failure on Demand of the safety function (h
-1
)
PFH
SIL4
≥ 10
-9
to < 10
-8
SIL3
≥
10
-8
to < 10
-7
SIL2
≥
10
-7
to < 10
-6
SIL1
≥ 10
-6
to < 10
-5
PL Probability of a Dangerous Hardware Failure Per Hour
e
≥
10
-8
to < 10
-7
d
≥
10
-7
to < 10
-6
c
≥ 10
-6
to < 3*10
-6