Heat dissipation coefficient for black surfaces in free air

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.