This lecture covers the calculation of short-circuit currents in electrical networks, focusing on symmetrical three-phase systems and isolated two-phase systems. It explains the determination of nodal impedances, equivalent diagrams, and the application of Thévenin's theorem. The instructor discusses the calculation of DC currents at specific nodes, the reduction of networks, and the matrices required for impedance calculations. Various examples are provided to illustrate the concepts, including the calculation of current in different network configurations. The lecture emphasizes the importance of considering only relevant components in the calculations and neglecting nonlinear phenomena.