This part of IEC 60909 is applicable to the calculation of short-circuit currents: . in low-voltage three-phase a.c. systems; . in high-voltage three-phase a.c. systems; operating at a nominal frequency of 50 Hz or 60 Hz. Systems at highest voltages of 550 kV and above with long transmission lines need special consideration. This part of IEC 60909 establishes a general, practicable and concise procedure leading to results, which are generally of acceptable accuracy. For this calculation method, an equivalent voltage source at the short-circuit location is introduced. This does not exclude the use of special methods, for example the superposition method, adj