Last edit: 06/08/2025
The Mission Time represents the period within which the component failure rate is considered constant and it is a value defined by the component manufacturer; here its definition.
[ISO 13849-1] 3.1 Terms and definitions
3.1.37 Mission Time TM. Period of time covering the intended use of an SRP/CS
Both IEC 62061 and ISO 13849-1 assume a component constant failure rate period of 20 years, but its manufacturer may decide a longer time. However, based upon the usage frequency in the specific machinery, the period within which the failure rate is considered constant can be less than the 20 years indicated by the component manufacturer: for components with wear out characteristics, that period is limited by the T10D.
That is the reason why, in the new edition of IEC 62061, it was decided to use the term Useful Lifetime; hereafter its definition:
[IEC 62061] 3.2 Terms and definitions
3.2.42 Useful Lifetime. Minimum elapsed time between the installation of the SCS or subsystem or subsystem element and the point in time when component failure rates of the SCS or subsystem or subsystem element can no longer be predicted, with any accuracy.
The Useful Lifetime can be defined as the minimum value between the Mission Time, indicated by the component manufacturer, and the T10D calculated by the machinery manufacturer.
Please bear in mind that the Mission Time TM is a characteristic of the component not of the Safety Control System and it can only be specified by the component manufacturer. That is different from the Proof Test interval Ti or the T10D, which are determined by the machinery manufacturer.
Each component within the SCS can have a different Mission Time. Normally, at the end of the Mission Time, the component has to be replaced, if the Safety Control System needs to continue its operations. In the simplified approach of ISO 13849-1, the Mission Time is assumed to be 20 years [ISO 13849-1; §6.1.8]. IEC 62061 does not give an indication, since it focuses on the Useful Lifetime.
In high demand mode, if the useful lifetime of the subsystem is larger than or equal to the useful lifetime of the SCS, a Proof Test is not necessary and in PFH estimations T1 is equal to the useful lifetime. Where it is smaller, the subsystem or subsystem element should be replaced during the useful lifetime of the SCS if a Proof Test is not possible. In IEC 62061, the parameter T1 is defined as “the proof test interval of the perfect proof test or useful lifetime whichever is the smaller”.
The concept is also present in ISO 13849-1 and it is called “operating life time”.