This paper addresses the impact of the dynamic network reconfiguration (DNR) on the voltage sag observability in smart distribution systems. Optimal placement of voltage sag monitors is generally applied without taking into account the DNR and we show that the voltage sag observability is modified with the network reconfiguration. We also propose two indexes to assess the impact of the dynamic reconfiguration on voltage sag observability. A test case is implemented and the results show the need for new formulations for optimal placement problem, where the DNR should be included to guarantee the full voltage sag observability.