This part of IEC 60909 specifies procedures for calculation of the prospective short-circuit currents with an unbalanced short circuit in high-voltage three-phase a.c. systems operating at nominal frequency 50 Hz or 60 Hz, i. e.: currents during two separate simultaneous line-to-earth short circuits in isolated neutral or resonant earthed neutral systems; partial short-circuit currents flowing through earth in case of single line-to-earth short circuit in solidly earthed or low-impedance earthed neutral systems. The currents calculated by these procedures are used when determining induced voltages or touch or step voltages and rise of earth potential at a station (power station or substation) and the towers of overhead lines. Procedures are given for the calculation of reduction factors of overhead lines with one or two earth wires. The standard does not cover: short-circuit currents deliberately created under controlled conditions as in short circuit testing stations, or short-circuit currents in the electrical installations on board ships or aeroplanes, or single line-to-earth fault currents in isolated or resonant earthed systems. The object of this standard is to establish practical and concise procedures for the calculation of line-to-earth short-circuit currents during two separate simultaneous line-to-earth short circuits and partial short-circuit currents through earth, earth wires of overhead lines and sheaths or shields of cables leading to conservative results with sufficient accuracy. For this purpose, the short-circuit currents are determined by considering an equivalent voltage source at the short-circuit location with all other voltage sources set to zero. Resistances of earth grids in stations or footing resistances of overhead line towers are neglected, when calculating the short-circuit currents at the short-circuit location. This standard is an addition to IEC 60909-0. General definitions, symbols and calculation assumptions refer to that publication. Special items only are defined or specified in this standard. The calculation of the short-circuit currents based on the rated data of the electrical equipment and the topological arrangement of the system has the advantage of being possible both for existing systems and for systems at the planning stage. The procedure is suitable for determination by manual methods or digital computation. This does not exclude the use of special methods, for example the super-position method, adjusted to particular circumstances, if they give at least the same precision. As stated in IEC 60909-0, short-circuit currents and their parameters may also be determined by system tests.