Presentation: Session 6 Methoden & Tools II

The argument of operational reliability enjoys steadily increasing popularity in justification of the safety of products. Furthermore, this argument can be expediently applied for the proof of safety in plant engineering. In particular, the argument of operation reliability can reduce the expense of safety verification for planned systems which are already in operation in the same or similar situation. Evidence of operational reliability is therefore often considered as a cost-effective alternative to a more comprehensive safety verification.

However, various requirements in different standards (IEC 61508, IEC 61511, ISO 13849, EN 50129) are provided for the proof of operational reliability. The requirements are different, especially in regards to the observation period. There is a consensus on all listed standards in terms of equality of the operating and environmental conditions. Moreover, most of standards do not address following important questions:

– Does the operational reliability refer to the component, to individual function of the component, to a principle considered in the component, or to the protection concept?
– How equal must the components be, so that the operational reliability may still be used for the newer component?
– To which extent can a component be proven in operation if it demonstrably falls outside the thresholds set in current standards (e.g. tolerated failure rate, EMC characteristic curve, heat resistance) or contains systematic errors that have never led to a hazard during operation?

The present article shows how operational reliability of the electronics in systems can be proven. To this end, it first compares the requirements for operational reliability from the known standards. Subsequently, the article addresses the important issues left out in the standards. It presents four types of equalities (building, function, shape and architectural equality), based on which it describes the legal situation of the operational reliability within the framework of the question of the equality of components. Finally, an overview of the safety aspects which are not considered in the context of the argument of operational reliability is given as well.