A hybrid maintenance policy for a protection system under internal degradation and shocks: a case study in a steel industry

Protection systems are designed so that they can prevent disasters when critical events (demands) occur. These systems are usually inactive and their functionality is only noticeable during a demand or through inspections. Furthermore, the level of deterioration of the system can worsen due to the actions of external events (shocks), which can accelerate the process of its natural wear. Thus, in order to ensure the readiness of such systems, we modeled a hybrid inspection and replacement policy for safety brakes subject to shocks. The policy considers additional inspections beyond the scheduled inspections subject to misclassification. The system is replaced when any inspection indicates that it has failed, either immediately after the Mth inspection, or immediately after the Kth met demand, or immediately after an unmet demand - whichever comes first. Our model is motivated by a real case of a safety brake in a steel mill. We compared this policy with two other classical policies and saw that it has significant cost-saving potential in relation to both. The results indicate how important inspections are for systems with hidden failures and show when the observation of shocks should be prioritized over inspection actions.