# Electrical field strength

With the exception of low-voltage cables, the electric field in power cables is limited to the volume of the dielectric by an inner and an outer conductive layer. The field distribution or voltage gradient is equivalent to a homogenous cylinder and is therefore represented by a homogenious radial field.

The value of the voltage gradient at point x within the insulation can be calculated using a logarithmic relationship. The electrical field strength is highest above the conductor screen, below the insulation and lowest below the insulation screen, above the insulation.

In high-voltage DC cables, a temperature gradient can change the conductivity of the insulator which leads to significant distortion of the electric field.

The values for dielectric strength of insulating materials is taken from chemistry.mdma.ch .

Symbol
$E_i$
Unit
kV/mm
Formulae
 $\frac{U_e}{r_x \ln\left(\frac{r_{osc}}{r_{isc}}\right)}$ homogenous radial field $\frac{\delta_i U_e \left(\frac{r_x}{r_{osc}}\right)^{\delta-1}}{r_{osc} \left(1-\left(\frac{r_{isc}}{r_{osc}}\right)^{\delta_i}\right)}$ function of the radial position $r_x$
Related
$\delta_i$
$r_{isc}$
$r_{osc}$
$r_x$
$U_e$
Used in
$\kappa_i$
$w_d$
Choices
MaterialDielectric strengthReference
PE21.7
HDPE19.7
XLPE19.7same as HDPE
XLPEf19.7same as HDPE
PVC11.8range between 11.8 and 15.7
EPR23.6like Polypropylene/polyethlyene copolymer
IIR23.6
PPLP28.7like Aramid papers, calendered
Mass12.2like Aramid papers, uncalendered
OilP12.2like Aramid papers, uncalendered
PP23.6
SiR26.0range between 26 and 36
EVA19.3like Ethylene-chlortrifluoroethylene copolymer