According to standard IEC 60287-2-1 Ed.2.0

Symbol
$Y_{\mathrm{d}}$
Unit
K.m/W
Related
$M_{\mathrm{d}}$
Used in
$T_{\mathrm{4i}}$
Choices
MaterialValue
Metal0.011
Fibre0.006 (air)
0.01 (buried)
Cem0.006 (air)
0.011 (buried)
PVC0.0037
PE0.0037
PP0.0037
Earth0.0036
WPE0.001