Electric power network is a fundamental facility in modern society. The importance to ensure and enhance the security of the power network can never be over emphasized. In this paper, we study how the topology of a power network restricts the security. By focusing on the power balanced condition which is necessary for the security after line outage contingencies, we show that a practical power network cannot avoid collapse. Using a method based on ordered binary decision diagram (OBDD) that fast enumerates the line outages causing network collapses, we obtain an upper bound for power network security, and indicate the transmission lines that are critical to power network security. This method is demonstrated on an IEEE 30-bus network. We hope this work brings insights to the understanding of why power networks cannot avoid collapse and how to enhance the security of an electric power network.