Introduction to IEC 909 (BS 7639). Short-circuit current calculation in three-phase AC systems

A calculation of the prospective fault current must usually be made when planning or modifying an electrical system. Such calculations may also be required for operational purposes. The standard and a number of supporting Technical Reports have been produced by Technical Committee 73 of the IEC. This objectives of this standard are to produce a procedure which is: practicable and concise; gives conservative results with sufficient accuracy; and can be used by nonspecialist engineers. To meet these objectives, a procedure based on an equivalent voltage source at the fault point has been used. A number of scaling and correction factors are introduced in the calculations to meet the second objective. The standard, though its main concern may be for maximum short-circuit currents, also addresses minimum fault current conditions and also distinguishes between short-circuits with and without AC component decay. The IEC 909 standard, in its scope, only applied to systems with nominal voltages up to 230 kV, but when it was adopted by CENELEC, this was extended to systems up to 380 kV nominal voltage.