Why: The key dependability issue is make equipment and processes work as advertised, which is, without failure. Dependability aims at facilitating co-operation by all parties concerned (supplier, organization, and customer by fostering an understanding of the dependability needs and value to achieve the overall dependability objectives) so it involves harmonizing conflicting issues. Dependability has a better view point from the end user of the equipment or system than from the designer's viewpoint or the maintainer's viewpoint. From a system effectiveness viewpoint, reliability and maintainability provide system availability and dependability.
When: You cannot repair yourself to happiness with a failure prone system as the failure prone system will be viewed lacking dependability to function as required when you need it. Thus dependability is viewed over the longer term and not in convenient snap-shots and dependability also involves life cycle cost issues.
Where: Reliability contributes directly to uptime by avoiding failures whereas maintainability contributes directly to reducing downtime by faster repairs. Thus reliability and maintainability jointly provide impact on dependability of the system. Dependable systems must be ready to function, in an operable state, to produce the desired output, upon demand by the end user, at the specified quantity and quality of output.
These definitions are written by H. Paul Barringer and are also posted on his web site at www.barringer1.com