Last edit: 06/08/2025
Similarly to the categories of ISO 13849-1, IEC 62061 has four Basic Subsystem Architectures, that allow the use of a simplified approach, like ISO 13849-1. Instead of a graph or a table showing the PFH values, IEC 62061 provides the user with formulas that are, in general, a simplification of Reliability Block Diagrams of the Basic Subsystem Architectures and are intended to provide conservative estimates of PFH.
Those formulas are applicable, provided the following two conditions are satisfied:
- λ·T1 << 1. That means that the MTTF is much greater than T1: the minimum between the Proof Test and the Useful Lifetime of the subsystem.
- During the Useful Lifetime, that is the minimum between the Mission Time and the T10D, the failure rates are constant.
Differences with ISO 13849-1
Some of the differences between the two standards are the following:
- In IEC 62061 the risk of common cause failures is evaluated with a similar table to ISO 13849-1, however there is no minimum value for the scoring.
- In Category 2, ISO 13849-1 requires that the MTTFD of the Test Channel (TE) is not lower than half the MTTFD value of the functional channel. The equivalent of Category 2 in IEC 62061 is the Basic Subsystem Architecture C. In this case, however, there is no minimum reliability level of the Fault Handling Function (λD-FH). In case the value is not according to Table H.3 of the standard, the simplified formula cannot be used for the calculation of the PFH of the subsystem and the general formula for Basic Subsystem Architecture C must be used.
- In ISO 13849-1, the MTTFD of subsystems is limited to 100 years, except for Category 4; in IEC 62061, there is no limitation of the PFH, even when architectural constraint is applied.
How to calculate the PFH of a basic subsystem architecture
The following elements must be taken into consideration, to be able to determine the PFH of a subsystem:
- Each subsystem of the safety function must be associated with one of the four Basic Subsystem Architectures.
- DC and test intervals have to be decided.
- The Common Cause Failure has to be calculated.
- λD or MTTFD of subsystem elements have to be calculated.
- The useful lifetime of components is typically 20 years even if, for components with wear out characteristics, the useful lifetime is limited by T10D.