Hardware Fault Tolerance (HFT) and Safe Failure Fraction (SFF)
Depending on the SIL * for the safety-related system, the IEC 61508 standard requires a specific hardware fault tolerance (HFT) in connection with a specific proportion of safe failures, shown as Safe Failure Fraction (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.
These types are specified on the basis of criteria which the standard defines for the safety-related elements.
|
SFF |
HFT Type A Subsystem |
HFT Type B Subsystem |
||||
|---|---|---|---|---|---|---|
|
0 |
1 |
2 |
0 |
1 |
2 |
|
|
< 60% |
SIL 1 |
SIL 2 |
SIL 3 |
— |
SIL 1 |
SIL 2 |
|
60% – < 90% |
SIL 2 |
SIL 3 |
SIL 4 |
SIL 1 |
SIL 2 |
SIL 3 |
|
90% – < 99 % |
SIL 3 |
SIL 4 |
SIL 4 |
SIL 2 |
SIL 3 |
SIL 4 |
|
≥ 99% |
SIL 3 |
SIL 4 |
SIL 4 |
SIL 3 |
SIL 4 |
SIL 4 |