Glossary · Fault behavior and system states
Safe failure fraction (SFF)
Also known as: SFF
German: Anteil sicherer Ausfälle (SFF)
In functional safety under IEC 61508, the safe failure fraction (SFF) of an element is the ratio of the sum of its safe failure rate and dangerous detected failure rate to its total failure rate. Together with hardware fault tolerance, it is used to determine architectural constraints.
- Functional safety
In one sentence
The safe failure fraction (SFF) is the share of an element's failures that are safe or dangerous but detected, used for architectural constraints.
Example
A transmitter with 300 FIT safe, 250 FIT dangerous detected and 50 FIT dangerous undetected failures has an SFF of about 92 percent.
How it applies
- Architectural constraints: In IEC 61508 Route 1H, the maximum SIL that a subsystem can claim depends on its SFF, its Hardware fault tolerance (HFT) and whether it is a type A or type B element.
- Data: SFF is taken from FMEDA results or the manufacturer's safety manual. Only failure modes relevant to the safety function count.
- Diagnostics: Better diagnostics turn dangerous undetected failures into detected ones and raise the SFF, provided the system reacts to detected faults.
- Documentation: Keep the source of each SFF value and the conditions under which it applies. The documentation team should not copy SFF values into marketing material without these conditions.
SFF vs. diagnostic coverage
Diagnostic coverage (DC) is the fraction of dangerous failures that diagnostics detect. SFF also counts safe failures. An element can have a high SFF even with weak diagnostics if most of its failures are safe.