Relationship between λD and MTTFD

Last edit: 06/08/2025

We remind that both IEC 62061 and ISO 13849-1 are assuming a constant failure rate of subsystem elements. With that assumption:

MTTF and MTTF_D are normally indicated in years. λ values are normally given in FIT (Failure in Time), whereby 1 FIT means one failure in 10⁹ hours.

One year is approximately 8 760 hours. Therefore, a MTTF value can be converted into a λ value with the following formula:

Considering that for electromechanical and pneumatic components, the B_10D value is given and knowing that:

The failure rate λ_D can be calculated from the B_10D with the following formula:

Also in this case, the useful lifetime of the component is limited by its T_10D.

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Index

PT 12: Functional Safety in High Demand: Introduction to IEC 62061 The Architectural Constraints The simplified approach Proof Test in High Demand mode Mission Time and Useful Lifetime The avoidance of Systematic Failures Relationship between λD and MTTFD

Core Competencies

Machinery Directive Machinery Safety Process Safety Functional Safety UL and CSA Conformity Electrical Safety and Arc Flash Mitigation ATEX Risk Gas Furnaces - CE Marking

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