Geometric mean distance between phases of the same system, short-circuit

In case of short-circuit the phases of an AC cable system will repell each other. If the cables are inside a duct it will increase the distance between the phases.

Symbol
$GMD_{k}$
Unit
m
Formulas
$GMD+2\left(\frac{Di_{d}}{2}-\frac{D_{e}}{2}\right) \frac{1}{1000}$separate ducts
$\left(Di_{d}-D_{e}\right) \frac{1}{1000}$common duct, 3 cables
$\left(Di_{d}-D_{e}\right) \sqrt{2} \frac{1}{1000}$common duct, 2 cables