Skin and proximity effect coefficient b for PAC/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.

$0.5356-2.1030734{\cdot}{10}^{-1} z_c+6.495563{\cdot}{10}^{-2} {z_c}^2-1.089373{\cdot}{10}^{-2} {z_c}^3+1.0372874{\cdot}{10}^{-3} {z_c}^4-5.8238557{\cdot}{10}^{-5} {z_c}^5+1.9109965{\cdot}{10}^{-6} {z_c}^6-3.3893677{\cdot}{10}^{-8} {z_c}^7+2.509622{\cdot}{10}^{-10} {z_c}^8$otherwise