Skin and proximity effect coefficient a for GIL enclosure

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

The equation for $z_c>30$ is acc. Elektra 125 (1989) whereas in Cigré Guide 2018 (2003) and Elektra 100 (1985) the factor 1 is falsely outside the square-root.

Coefficients of the polynomial interpolation are taken from Table II of Elektra 125 (1989). The values are identical in CIGRE Guide 218 (2003).

Symbol
$a_{encl}$
Formulae
$\frac{7 z_{encl}^{2}}{3 z_{encl}^{2} + 315}$$z_{encl} < 5$
$\frac{\sqrt{2} \sqrt{z_{encl}}}{2} - 1$$z_{encl} > 30$
$6.62315 \cdot 10^{-11} z_{encl}^{8} - 1.04301 \cdot 10^{-8} z_{encl}^{7} + 7.0674 \cdot 10^{-7} z_{encl}^{6} - 2.69028 \cdot 10^{-5} z_{encl}^{5} + 0.000627032 z_{encl}^{4} - 0.00902854 z_{encl}^{3} + 0.073796 z_{encl}^{2} - 0.156295 z_{encl} + 0.19701$otherwise