Up to now, preventive or corrective actions to improve power systems' operating states mainly concern power generation rescheduling, load-shedding and tap modifications of in-phase or phase-angle transformers. A method is proposed for relieving overloads by network topology modifications. The switching between busbars and feeders in substations is considered as a control variable. The optimization of the topology is a very difficult task due to the combinatorial nature of the problem and the nonlinearities inherent to networks. The method is based on the Maximal Flow Minimal Cost algorithm. All the possible switching actions are taken into account with, if desired, generation rescheduling and load-shedding. The power system is modelled so as to obtain the same power flow distribution as with a DC load flow. Final checks of security constraints (voltage magnitudes and short-circuit currents) are performed with AC load-flow and fast short-circuit programs. The method is very efficient considering the solution time (important for real time applications).