When estimating the MTTFD (mean time to dangerous failure) of a component for an ISO 13849-1 performance level calculation, the standard defines a strict order of priorities in section 6.1.4:
- Use manufacturer’s data. Note: When MTTFD data is provided by the manufacturer (e.g. for electromechanical components), the number of operations in the real application must not be higher than the number of operations indicated by the manufacturer.
- Use the methods in Annex C of ISO 13849-1 (see below).
- Use failure rate field data from identical component applications in similar environments, collected over a significant period of time — provided the collection and analysis method results in a reasonable level of confidence in the data. See IEC 61508-7:2010, B.5.4 for further information on field data.
- Choose 10 years. If none of the above options are available, ISO 13849-1 allows you to simply assume an MTTFD of 10 years as the final fallback.
So if you don’t have manufacturer’s MTTFD data and Annex C methods or trustworthy field data are not applicable, the standard-sanctioned fallback value is MTTFD = 10 years.
Source: ISO 13849-1:2023, section 6.1.4 Mean time to dangerous failure (MTTF_D)
Annex C methods
Annex C of ISO 13849-1 gives several methods for calculating or evaluating MTTFD values for single components. Any of these methods can be used if it is applicable to the component in question:
- C.2 — Good engineering practices method: Estimating MTTFD or B10D according to Table C.1, based on the application of good engineering practices for different kinds of components. See ISO 13849-1 Annex C.2: Good engineering practices method for MTTFd estimation.
- C.3 — Hydraulic components: Estimating MTTFD for single hydraulic components such as valves. See ISO 13849-1 Annex C.3: Estimating MTTFd of hydraulic components.
- C.4: A means of calculating the MTTFD of pneumatic, mechanical and electromechanical components from B10 (see C.4.1).
- C.5 — Electronic components: Lists typical MTTFD values for electronic components from the SN 29500 database. See ISO 13849-1 Annex C.5: MTTFd data of electronic components (SN 29500).
Source: ISO 13849-1:2023, Annex C.1 General
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