IEC 61703 Ed. 2.0 b:2016

Mathematical expressions for reliability, availability, maintainability, and maintenance support terms provide quantitative measures used in engineering and system management.

Reliability (R(t)) is the probability that a system or component performs its required function under stated conditions for a specified period of time. It is often expressed as:
R(t) = e^(-λt)

where λ is the failure rate and t is the time.

Availability (A) represents the proportion of time a system is in a functioning condition. Mathematical expression:
A = MTBF / (MTBF + MTTR)

where MTBF is Mean Time Between Failures and MTTR is Mean Time To Repair.

Maintainability (M(t)) is the probability that a failed system will be restored to operational effectiveness within time t, often defined as:
M(t) = 1 - e^(-μt)

where μ is the repair rate.

Maintenance Support Terms include various metrics such as:

• Mean Time To Failure (MTTF)
• Mean Time To Repair (MTTR)
• Mean Down Time (MDT)

IEC 61703:2016 provides mathematical expressions for selected reliability, availability, maintainability, and maintenance support measures defined in IEC 60050­192:2015. In addition, it introduces some terms not covered in IEC 60050-192:2015. They are related to aspects of the system of item classes (see hereafter).

According to IEC 60050-192:2015, dependability [192-01-22] is the ability of an item to perform as and when required, and an item [192-01-01] can be an individual part, component, device, functional unit, equipment, subsystem, or system.

To account for mathematical constraints, this standard splits the items between the individual items considered as a whole (e.g., individual components) and the systems made of several individual items. It provides general considerations for the mathematical expressions for systems as well as individual items, but the individual items, which are easier to model, are analysed in more detail with regard to their repair aspects.

This standard is mainly applicable to hardware dependability, but many terms and their definitions may be applied to items containing software.

This second edition cancels and replaces the first edition published in 2001. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition:

• Standard made as self containing as possible;
• Item split between individual items and systems;
• Generalization of the dependability concepts for systems made of several components [introduction of the conditional failure intensity (Vesely failure rate)];
• Introduction of the state-transition and the Markovian models;
• Generalization of the availability to production availability;
• Introduction of curves to illustrate the various concepts.

Keywords: mathematical expressions for dependability