# Dielectric loss in cables

By on

Cable cross section showing
insulation

Dielectrics (insulating materials for example) when subjected to a varying electric field, will have some energy loss.   The varying electric field causes small realignment of weakly bonded molecules, which lead to the production of heat.  The amount of loss increases as the voltage level is increased.  For low voltage cables, the loss is usually insignificant and is generally ignored.  For higher voltage cables, the loss and heat generated can become important and needs to be taken into consideration.

Dielectrics (insulating materials for example) when subjected to a varying electric field, will have some energy loss.   The varying electric field causes small realignment of weakly bonded molecules, which lead to the production of heat.  The amount of loss increases as the voltage level is increased.  For low voltage cables, the loss is usually insignificant and is generally ignored.  For higher voltage cables, the loss and heat generated can become important and needs to be taken into consideration.

Dielectric loss is measured using what is known as the loss tangent or tan delta (tan δ).  In simple terms, tan delta is the tangent of the angle between the alternating field vector and the loss component of the material.  The higher the value of tan δ the greater the dielectric loss will be.  For a list of tan δ values for different insulating material, please see the Cable Insulation Properties note.

Note: in d.c. cables with a static electric field, there is no dielectric loss.  Hence the consideration of dielectric loss only applies to a.c. cables.

## Cable Voltage

Dielectric loss only really become significant and needs to be taken into account at higher voltages.  IEC 60287 "Electric Cables - Calculation of the current rating", suggests that dielectric loss need only be considered for cables above the following voltage levels:

Cable Type   U0, kV
Butyl Rubber 18
EDR 63.5
Impregnated Paper (oil or gas-filled) 63.5
Impregnated Paper (solid) 38
PE (high and low density) 127
PVC 6
XLPE (filled) 63.5
XLPE (unfilled) 127

## Cable Dielectric Loss

Cable Capacitance

Cable capacitance can be obtained from manufacturers or for circular conductors calculated using the following:

$C= ε 18ln( D i d c ) 10 −9 F. m −1$

Given the tan δ and capacitance of the cable, the dielectric loss is easily calculated:

$W d =ω C U 0 2 tan δ$

It is possible to use the above for other conductor shapes if the geometric mean is substituted for Di and dc.

## Symbols

dc - diameter of conductor, mm
Di - external diameter of insulation, mm
C - cable capacitance per unit length, F.m-1
U0 - cable rated voltage to earth,  V
Wd - dielectric loss per unit length, W.m-1
tan δ - loss factor for insulation
ε - insulation relative permitivity
ω - angular frequency (2πf)

More interesting Notes:

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

Star-Delta Motor Starting - Performance

Many questions sent in to the site are in connection with motor starting and in particular star-delta.  For all but the simplest application, there is...

Power Factor

Power factor is the ratio between the real power (P in kW) and apparent power (S in kVA) drawn by an electrical load. The reactive power (Q in kVAr)...

Variable Frequency Drive

Variable frequency drives are widely used to control the speed of ac motors.  This note looks at the mechanisms which enable drive units to control the...

9 power supply issues solved by using a UPS

Installation of a UPS can help in reducing problems due to issues with the power supply.  A lot of people relate this to nine key issues.  Depending on...

Voltage Drop in Installations - Concepts

Problems on achieving maximum voltage drop within an installation come up often. Depending where you live, local regulations will have different limits...

ANSI (IEEE) Protective Device Numbering

The widely used United Sates standard ANSI/IEEE C37.2 'Electrical Power System Device Function Numbers, Acronyms, and Contact Designations' deals with...

Maximum Demand for Buildings

Estimating maximum demand is a topic frequently discussed. Working out how much power to allow for a building can be very subjective . Allowing too much...

DC Component of Asymmetrical Faults

The image (reproduced from IEC 60909) shows a typical fault in an ac system.  From the illustration it can seen that there is an initial dc component ...

Cable Insulation Properties

Cable insulation is used to provide electrical separation between conductors of  a cable.  During the historical development of cables, numerous types...

What does N+1 mean?

The term 'N+1' relates to redundancy and simply means that if you required 'N' items of equipment for something to work, you would have one additional...

## Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note