16 EAV64334 06/2017
PL - Performance Level
The standard IEC 13849-1 defines 5 Performance levels (PL) for safety functions.
Level a is the lowest level and e is the highest level.
Five levels (a, b, c, d, and e) correspond to different values of average probability of dangerous failure per
hour.
HFT - Hardware Fault Tolerance and SFF - Safe Failure Fraction
Depending on the SIL for the safety system, the IEC 61508 standard requires a specific hardware fault
tolerance HFT in connection with a specific proportion of safe failures SFF (Safe Failure Fraction).
The hardware fault tolerance is the ability of a system to execute the required safety function in spite of the
presence of one or more hardware faults.
The SFF of a system is defined as the ratio of the rate of safe failures to the total failure rate of the system.
According to IEC 61508, the maximum achievable SIL of a system is partly determined by the hardware
fault tolerance HFT and the safe failure fraction SFF of the system.
IEC 61508 distinguishes two types of subsystem (type A subsystem, type B subsystem).
These types are specified on the basis of criteria which the standard defines for the safety-relevant
components.
Fault Avoidance Measures
Systematic errors in the specifications, in the hardware and the software, usage faults and maintenance
faults in the safety system must be avoided to the maximum degree possible. To meet these requirements,
IEC 61508 specifies a number of measures for fault avoidance that must be implemented depending on
the required SIL. These measures for fault avoidance must cover the entire life cycle of the safety system,
i.e. from design to decommissioning of the system.
Performance
level
Probability of a dangerous Hardware Failure per Hour
e
10
-8
≤...< 10
-7
d
10
-7
≤...< 10
-6
c
10
-6
≤...< 3 x 10
-6
b
3 x 10
-6
≤...< 10
-5
a
10
-5
≤...< 10
-4
SFF HFT type A subsystem HFT type B subsystem
012012
< 60% SIL1 SIL2 SIL3 — SIL1 SIL2
60% <... < 90% SIL2 SIL3 SIL4 SIL1 SIL2 SIL3
90% <... < 99 % SIL3 SIL4 SIL4 SIL2 SIL3 SIL4
> 99% SIL3 SIL4 SIL4 SIL3 SIL4 SIL4
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