A general dependability analysis procedure consists of the following task (as applicable):
a) System definition
Define the system to be analysed, its modes of operation, the functional relationships to its environment including interfaces or processes. Generally the system definition is an input from the system engineering process.
b) Dependability requirements / goals definition
List all system reliability and availability requirements or goals, characteristics and features, together with environmental and operating conditions, as well as maintenance requirements. Define system failure, failure criteria and conditions based on system functional specification, expected duration of operation and operating environment (mission profile and mission time). IEC 60300-3-4 should be used as guidance.
c) Allocation of dependability requirements
Allocate system dependability requirements or goals to the various sub-systems in the early design phase when necessary.
d) Dependability analysis
Analyse the system usually on the basis of the dependability techniques and relevant performance data.
1) Qualitative analysis
- Analyse the functional system structure.
- Determine system and component fault modes, failure mechanisms, causes, effects and consequences of failure.
- Determine degradation mechanism that may cause failures.
- Analyse failure / fault paths.
- Analyse maintainability with respect to time, problem isolation method, and repair method.
- Determine the adequacy of the diagnostics provided to detect faults.
- Analyse possibility for fault avoidance.
- Determine possible maintenance and repair strategies, etc.
2) Quantitative analysis
- Develop reliability and/or availability models.
- Define numerical reference data to be used.
- Perform numerical dependability evaluation.
- Perform component criticality and sensitivity analyses as required.
e) Review and recommendations
Analyse whether the dependability requirements/ goals are met and if alternative designs may cost effectively enhance dependability. Activities may include the following tasks (as appropriate):
- Evaluate improvement of system dependability as a result of design and manufacture improvement (e.g., redundancy, stress reduction, improvement of maintenance strategies, test systems, technological processes and quality control system).
NOTE 1. The inherent dependability performance measures can be improved only by design. When poor measured values are observed due to bad manufacturing processing, from the operating point of view, observed dependability performance measures can be enhanced by improving the manufacturing process.
- Review system design, determine weakness and critical fault modes and components.
- Consider system interface problems, fail-safe features and mechanisms, etc.
- Develop alternative ways for improving dependability, e.g., redundancy, performance monitoring, fault detection, system reconfiguration techniques, maintenance procedures, component replaceability, repair procedures.
- Perform trade-off studies evaluating the cost and complexity of alternative designs.
- Evaluate the effect of manufacturing process capability.
- Evaluate the results and compare with requirements.
NOTE 2. The general procedure summarizes, from an engineering point of view, the specific dependability programme elements from IEC 60300-2, which are applicable for dependability analysis: dependability specifications, analysis of use environment, reliability engineering, maintainability engineering, human factors, reliability modeling and simulation, design analysis and product evaluation, cause-effect impact and risk analysis, prediction and trade-off analysis.