Imperfect contact thermal factor

Factor to account for imperfect thermal contact between conductor or screen, sheath or armour and surrounding or adjacent non-metallic materials.

0.7 is recommended for most cases.
1.0 is used for conductor and screen of oil-filled cables.
0.9 can be used when the metallic component is completely bonded on one side to the adjacent medium (e.g. aluminium foil to oversheath).
0.5 is used for isolated screen wires, not fully embedded, situated under an extruded tube and air spaces are present between the wires.

Screen wires are considered isolated if the wires cover < 70% of the circumference. Isolated screen wires are considered fully embedded if they are separated by at east one wire diameter and fully surrounded by non-metallic materials, then a value of 0.7 can be used, otherwise 0.5.

Symbol

$F_k$

Used in

$\epsilon_k$

Heat loss allowance factor

$M_k$

Thermal contact factor [s$^{1/2}$]

Choices

Value	Case
1.0	for conductor and screen of oil-filled cables
0.9	if completely bonded on one side
0.7	recommended value
0.5	for isolated screen wires, not fuly embedded