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Safety
60
EIO0000001505 12/2014
HFT - Hardware Fault Tolerance and SFF - Safe Failure Fraction
Depending on the SIL for the safety system, the IEC 61508 standard requires a specific HFT in
connection with a specific proportion of safe failures SFF.
The HFT 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 HFT
and the SFF of the system.
IEC 61508 distinguishes 2 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.
Systematic Safety Integrity & Detect 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
reach 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, that is, from design to decommissioning of the
system.
b
≥
3*10
-6
to < 10
-5
a
≥ 10
-5
to < 10
-4
PL Probability of a Dangerous Hardware Failure Per Hour
SFF HTF
Type A subsystem Type B subsystem
0 1 2 0 1 2
< 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