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Mechanical force three-phase symmetrical fault

According to IEC 61914, Annex B.3. The Swiss TSO Swissgrid published a modular system guideline (Baukasten) with a different formula.

Symbol
$F_{k,3}$
Unit
kN/m
Formulas
$\alpha_{k,3} f_k {i_{p,3}}^2 \frac{1}{GMD_k} \frac{1}{1000}$IEC 61914
$\alpha_{k,3} \frac{\mu_0}{2\pi} \left(1000i_{p,3}\right)^2 \frac{1}{GMD} \frac{1}{1000}$Swissgrid Baukasten
Related
$f_k$
Factor f (IEC 61914)
$GMD$
Geometric mean distance between phases of the same system [m]
$GMD_k$
Geometric mean distance between phases of the same system, short-circuit [m]
$\mu_0$
Vacuum permeability [H/m]
$\pi$
Archimedes' constant $\pi$
Used in
$F_{k,max}$
Maximal mechanical force [kN/m]