Smart grid integrates electrical network, communication systems and information technologies, where increasing architecture interdependency is introducing new challenges in the evaluation of how possible threats could affect security and reliability of power system. While cyber-attacks have been widely studied, consequences of physical failures on real-time applications are starting to receive attention due to implications for power system security. This paper presents a methodology to quantify the impact on observability in state estimation of possible disruptive failures of a common transmission infrastructure. Numerical results are obtained by calculating observability indicators on an IEEE 14-bus test case, considering the simultaneous disconnection of power transmission lines and communication links installed on the same infrastructure.