This paper aims at showing the effectiveness of multi-levelmaintenance decision-making (i.e., system level and unit level) in the framework of condition-based maintenance (CBM) strategies applied to multi-unit systems. Specifically, based on the degradation/failure model of a k-out-of-n:F system, two opportunistic predictive maintenance strategies with different maintenance decision levels are proposed. In the first one, the decisions are relied only on the remaining lifetime (RUL) of the units, while in the second one, they are based on both RULs of the system and its units. The RUL is associated to a conditional reliability level calculated when an opportunity occurs (i.e., at inspection times), and maintenance actions are assumed perfect (i.e., replacement). The cost models of these strategies are developed on the basis of semi-regenerative theory, and are compared with each other. The comparison allows us to give some conclusions on the advantage of combining the maintenance decisions on both system level and unit level.

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