Advances in sensor technology have enabled companies to make significant progress towards achieving condition-based maintenance (CBM). CBM provides the opportunity to perform maintenance actions more effectively. However, scheduling maintenance at the unit level may imply a high maintenance frequency at the asset level, which can be costly and undesirable for safety reasons. In this paper, we consider systems consisting of multiple critical units for which a strict and conservative maintenance strategy is enforced. Although this implies that benefits cannot be obtained by delaying maintenance activities, the clustering of them can be beneficial. We consider two simple, practical systems for condition monitoring that involve either one signal (alarm) or two signals (alert, alarm). Our analysis and results provide general insights into when and how to cluster maintenance operations, with the objective of minimizing the total maintenance costs. Moreover, they show that clustering is essential for a broad range of circumstances, including those at a considered real-life case of equipment maintenance at Europe׳s largest gas field.