Skin and proximity effect coefficient b for GIL conductor

The equation for $z_c<5$ is identical in Cigré Guide 2018 (2003), Elektra 100 (1985), and Elektra 125 (1989).

The equation for $z_c>30$ in Cigré Guide 2018 (2003) is missing a factor 2 in the square-root and the equation in Elektra 125 (1989) has the factor 5 falsely inside the square-root.

Coefficients of the polynomial interpolation are taken from Table II of Elektra 125 (1989) whereas the first value used in CIGRE Guide 218 (2003) is wrong.

Symbol
$b_{c}$
Formulae
$\frac{56}{z_{c}^{2} + 211}$$z_{c} < 5$
$\frac{2}{4 \sqrt{2} \sqrt{z_{c}} - 5}$$z_{c} > 30$
$2.509622 \cdot 10^{-10} z_{c}^{8} - 3.3893677 \cdot 10^{-8} z_{c}^{7} + 1.9109965 \cdot 10^{-6} z_{c}^{6} - 5.8238557 \cdot 10^{-5} z_{c}^{5} + 0.0010372874 z_{c}^{4} - 0.01089373 z_{c}^{3} + 0.06495563 z_{c}^{2} - 0.21030734 z_{c} + 0.5356$otherwise