All cables are assumed to be served cables or having a non-metallic surface, and are thus considered to have a black surface. All ducts are also considered to have a black surface, except for metallic ducts, in which case a value of $h_{bs}$ equal to 88 % of the value for a black surface is assumed.

It is also the heat dissipation coefficient for cable surface to tunnel wall.

Symbol
$h_{\mathrm{bs}}$
Unit
W/m²/K5/4
Formulae
$E_{\mathrm{bs}} + D_{\mathrm{o}}^{- g_{\mathrm{bs}}} Z_{\mathrm{bs}}$
Related
$D_{\mathrm{o}}$
$E_{\mathrm{bs}}$
$g_{\mathrm{bs}}$
$Z_{\mathrm{bs}}$
Used in
$K_{\mathrm{A}}$