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Skin and proximity effect coefficient a for PAC/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}{315+3{z_{encl}}^2}$$z_{encl}<5$
$\sqrt{\frac{z_{encl}}{2}}-1$$z_{encl}>30$
$0.19701-1.56295{\cdot}{10}^{-1} z_{encl}+7.3796{\cdot}{10}^{-2} {z_{encl}}^2-9.02854{\cdot}{10}^{-3} {z_{encl}}^3+6.27032{\cdot}{10}^{-4} {z_{encl}}^4-2.69028{\cdot}{10}^{-5} {z_{encl}}^5+7.0674{\cdot}{10}^{-7} {z_{encl}}^6-1.04301{\cdot}{10}^{-8} {z_{encl}}^7+6.62315{\cdot}{10}^{-11} {z_{encl}}^8$otherwise
Related
$z_{encl}$
Factor z to calculate skin effect coefficients for enclosure
Used in
$y_{encl}$
Skin and proximity effect factor for PAC/GIL enclosure