The paper describes the models of systems and investigates their structural survivability. A threshold estimate of the system survivability is introduced. This estimate depends on the size of the largest connected component of the network model after a destructive impact on it. This estimate is more complex than the canonical structural survivability index, where only the disruption of network connectivity is taken into account.

The state of networks with different topology is investigated when their elements (links) are removed. The introduced indicator depends on the network topology and its size, at the same time, it is well approximated by cubic polynomials.

Keywords: Survivability modeling, Structural reliability, Canonical survivability, Network model, Connectivity component, Survivability index.