4. Conclusion

The PDS method proposed by SINTEF is in line with the principles advocated in the IEC 61508 and 61511 standards. It offers an effective and practical approach to implementing the quantitative aspects introduced in these standards. As such, the proposed quantitative approach to estimating safety-critical unavailability (SUC) can be seen as an alternative to the calculation formulas reported in IEC 61508-6 Annex B. However, these methods differ in a few key respects:

• the approach adopted to take into account the common cause failure factor (ß) ;

• the inclusion of systematic failures in the PDS method;

• the ability to translate periodic test limits into the PDS method.

The PDS method has the advantage of explicitly introducing systematic errors...