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To calculate system faults, we can use the guidance given in IEC 609096 "Short-circuit currents in three-phase a.c. systems. For faults far from the generator faults in three phase systems, each type of fault the symmetrical short-circuit current I"_{k} is given by:
Note:
For several series circuits in the fault loop, the final fault current is given by:
${I}_{k}^{"}=\frac{c{U}_{n}}{\sqrt{3}{\displaystyle \sum _{i}}{Z}_{i}}$ - for a.c. circtuis
${I}_{k}^{"}=\frac{c{U}_{n}}{\sum _{i}{Z}_{i}}$ - for d.c. circuits
Source impedance Z_{Q }, is given by:
${Z}_{Q}=\frac{c{U}_{n}}{\sqrt{3}{I}_{k}^{"}}$ - three phase and single phase systems
${Z}_{Q}=\frac{c{U}_{n}}{{I}_{k}^{"}}$ - d.c. systems
Typically both a maximum fault level (used for rating equipment), and minimum level (used for protections settings) are calculated. When evaluating the maximum and minimum, the following condtions are taken into account:
Note: IEC 60909 recommends resistance calculated at 20 °C for the maximum short circuit, and at the end of short circuit temperature for the minimum short circuit level. Presently, in myCableEngineering we use the conductor operating temperature as the reference for both maximum and minimum fault levels. User specified (input) fault levels are taken as being those with the voltage factor c = 1.
c - voltage factor I"_{k} - initial symmetrical three phase short-circuit current (r.m.s.), A - I"_{k1} line-to-earth - I"_{k2} line-to-line - I"_{kE2E} line-to-line-earth U_{n} - nominal system voltage, line-to-line (r.m.s.), V - positive-negative voltage for d.c. systems Z_{1} - positive sequence short -circuit impedance, Ω Z_{2} - negative sequence short circuit impedance, Ω Z_{0} - zero sequence short circuit impedance, Ω Z_{q} - impedance or any external network, Ω