Many protective devices, as well as spare and standby systems, exhibit what are known as non self-announcing failures; that is, failures can be observed only by inspection. Inspections can usually reveal only whether the device is operational or not, and not the level of deterioration of the device. This paper considers the availability of such systems when degradation is driven by an exogenous random environment. We allow this environment to be quite general; it includes both continuous (graceful) degradation and degradation due to discrete shocks. For periodic inspection schemes, we compute the limiting average availability and discuss opportunities for more effective inspection policies.

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