The Cascaded H-Bridge topology has in recent years become popular for use in medium voltage drives. The reliability of this topology however is of concern due to the large number of semiconductor switches. The natural redundancy possessed by the CHB topology can only be realized if failed devices may be identified and removed from the system. In this paper a fault diagnostic method for open switch fault is proposed. The method is based on the prediction of the active and zero voltage states of each module. With the proposed method, the exact location of the switch under fault in an H-bridge module can be determined quickly. The performance of the proposed technique has been verified through simulation.