Here, the authors study open-circuit faults (OCFs) in the power switches of multilevel converters with a model-based perspective. In specific, the authors address single-phase cascaded H-bridge (CHB) converters with n-levels in its output voltage (CHB-n L), which are designed as shunt active power filters. In this task, the OCFs are modelled by fault profiles with an additive structure in each subsystem of the CHB-n L converter. These additive fault profiles have constant and oscillatory components, where the constant term sign indicates the pair of damaged switches in the H-bridge. Hence, a sliding-mode integral observer is proposed to estimate the constant terms of the fault profiles. The complexity of the observer depends on the number of H-bridges in the CHB-n L converter. As a result, the proposed fault detection and isolation (FDI) scheme relies on the estimated constant terms of the additive fault profiles to achieve a robust and fast diagnosis stage. The proposed model-based FDI scheme is validated experimentally under single and multiple fault scenarios, and model uncertainty. During the evaluation, the derived methodology only requires less than one cycle of the fundamental frequency to isolate the faults, and show robustness to load changes and parametric uncertainty.