Introduction to dependability design

Dependability and its components

Equipment failures, unavailability of a power supply, stoppage of automated equipment and accidents are quickly becoming unacceptable events, be it to the ordinary citizen or industrial manufacturers. Dependability and its components: reliability, maintainability, availability and safety, have become a science that no designer can afford to ignore.

This technical report presents the basic concepts and an explanation of its basic computational methods. Some examples and several numerical values are given to complement the formulas and references to the various computer tools usually applied in this field.

Importance of dependability

Prehistoric men had to depend on their arms for survival. Modern man is surrounded by ever more sophisticated tools and systems on which he depends for safety, efficiency and comfort. Ordinary citizen are specially concerned in everyday life by:

• the reliability of the TV set,
• the availability of the mains supply,
• the maintainability of freezers and cars,
• the safety of their boiler valves.

Bankers and, in general, service industries give a lot of weight to:

• computer reliability,
• availability of heating,
• maintainability of elevators,
• fire related safety.

In competitive industries it is not possible to tolerate production losses. This is even more so for complex industrial processes. In these cases one vies to obtain the best:

• reliability of command and control systems,
• availability of machine tools,
• maintainability of production tools,
• personnel and invested capital safety.

These characteristics, known under the general term of DEPENDABILITY, are related to the concept of reliance, (to depend upon something). They are quantified in relation to a goal, they are computed in terms of a probability and are obtained by the choice of an architecture and its components. They can be verified by suitable tests or by experience.

For over 20 years Merlin Gerin has pioneered work in the DEPENDABILITY field: in the past, with its contribution to the design of nuclear power plants or the high availability of power supplies used at the launching site of the ARIANE space program, nowadays, by its design of products and systems used worldwide.

AUTHOR: Schneider Electric expert | Pascal BONNEFOI