This part of IEC 60909 is applicable to the calculation of short-circuit currents: ï in low-voltage three-phase a.c. systems ï 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, adjusted to particular circumstances, if they give at least the same precision. The superposition method gives the short-circuit current related to the one load flow presupposed. This method, therefore, does not necessarily lead to the maximum short-circuit current. This part of IEC 60909 deals with the calculation of short-circuit currents in the case of balanced or unbalanced short circuits. In case of an accidental or intentional conductive path between one line conductor and local earth, the following two cases must be clearly distinguished with regard to their different physical properties and effects (resulting in different requirements for their calculation): ï line-to-earth short circuit, occurring in a solidly earthed neutral system or an impedance earthed neutral system; ï a single line-to-earth fault, occurring in an isolated neutral earthed system or a resonance earthed neutral system. This fault is beyond the scope of, and is therefore not dealt with in, this standard. For currents during two separate simultaneous single-phase line-to-earth short circuits in an isolated neutral system or a resonance earthed neutral system, see IEC 60909-3. Short-circuit currents and short-circuit impedances may also be determined by system tests, by measurement on a network analyzer, or with a digital computer. In existing low-voltage systems it is possible to determine the short-circuit impedance on the basis of measurements at the location of the prospective short circuit considered.The calculation of the short-circuit impedance is in general based on the rated data of the electrical equipment and the topological arrangement of the system and has the advantage of being possible both for existing systems and for systems at the planning stage. In general, two short-circuit currents, which differ in their magnitude, are to be calculated: ï the maximum short-circuit current which determines the capacity or rating of electrical equipment; and ï the minimum short-circuit current which can be a basis, for example, for the selection of fuses, for the setting of protective devices, and for checking the run-up of motors. NOTE The current in a three-phase short circuit is assumed to be made simultaneously in all poles. Investigations of non-simultaneous short circuits, which may lead to higher aperiodic components of short-circuit current, are beyond the scope of this standard. This standard does not cover short-circuit currents deliberately created under controlled conditions (short-circuit testing stations). This part of IEC 60909 does not deal with the calculation of short-circuit currents in installations on board ships and aeroplanes.