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

Symbol
$V_{\mathrm{d}}$
Unit
K.m/W
Related
$M_{\mathrm{d}}$
Used in
$T_{\mathrm{4i}}$
Choices
MaterialValue
Metal1.4
Fibre0.83 (air)
0.91 (buried)
Cem1.2 (air)
1.1 (buried)
PVC0.312
PE0.312
PP0.312
Earth0.28
WPE0.003