Impedance of armour inner surface

Definition according to Cigré guide 531 chapter 4.2.1, analogue to parameter $z_3$ for screen.

Symbol
$z_{ar,i}$
Unit
$\Omega$/m
Formulae
$\frac{m_{e67} \rho_{ar}}{2\pi r_{e6}} \operatorname{coth}\left(m_{e67} \left(r_{e7}-r_{e6}\right)\right)-\frac{\rho_{ar}}{2\pi r_{e6} \left(r_{e6}+r_{e7}\right)}$