16. Insights into the Maintenance Test Effectiveness

The condition of no fault found (NFF) has been accepted by engineers and operators for many years. Whilst there are known general reasons for NFF one of the areas that is overlooked is the importance of the test equipment, whether built-in test equipment or automatic test equipment, and specifically the effectiveness of the test. How much of a system or component is actually being test by the test equipment? This paper explores the system or component fault coverage effectiveness and the impact on NFF. In doing so a module is proposed to establish the relationship between NFF and test coverage effectiveness for different reliabilities. As the component or system reliability improves along with the test effectiveness the percentage of NFF occurrences decreases. Another consideration is the assumed number of random failures that the design engineers include during the development phases. This is usually an underestimate and the combination of this and the ineffective fault coverage have a negative impact on the operational availability.