Abstract-The operations of many modern cyber-physical systems, such as smart grids, are based on increasingly interdependent networks. The impact of cascading failures on such networks has recently received significant attention due to the corresponding effect of these failures on the society. In this paper, we conduct an empirical study on the robustness of interdependent systems formed by the coupling of power grids and communication networks by putting real distribution power grids to the test. We focus on the assessment of the robustness of a large set of medium-voltage (MV) distribution grids, currently operating live in the Netherlands, against cascading failures initiated by different types of faults / attacks. We consider both unintentional random failures and malicious targeted attacks which gradually degrade the capability of the entire system and we evaluate their respective consequences. Our study shows that current MV grids are highly vulnerable to such cascades of failures. Furthermore, we discover that a small-world communication network structure lends itself to the robustness of the interdependent system. Also interestingly enough, we discover that the formation of hub hierarchies, which is known to enhance independent network robustness, actually has detrimental effects against cascading failures. Based on real MV grid topologies, our study yields realistic insights which can be employed as a set of practical guidelines for distribution system operators (DSOs) to design effective grid protection schemes.