Dielectric Loss in Cables: Tan Delta and Capacitance

Dielectric loss is the heat generated in cable insulation when the insulation is subjected to a varying electric field. The electric field causes a small realignment of weakly bonded molecules, and part of the electrical energy is converted into heat.

For low voltage cables the loss is usually small and is commonly ignored. At higher voltages, dielectric loss can become significant and should be included in the thermal rating calculation.

Dielectric loss is measured using the loss tangent, or tan delta. In simple terms, tan delta is the tangent of the angle between the alternating electric field vector and the loss component of the insulating material. A higher value of tan δ means a higher dielectric loss.

In d.c. cables with a static electric field, there is no dielectric loss in this sense. The calculation therefore applies to a.c. cables.

When dielectric loss is considered

IEC 60287 indicates that dielectric loss only needs to be considered above certain voltage levels. The threshold depends on the insulation type:

For insulation material context, see Cable Insulation: Thermoplastic and Thermosetting Materials.

Cable dielectric loss calculation

Given the insulation loss factor and cable capacitance, the dielectric loss per unit length is calculated as:

For non-circular conductor shapes, the same form can be used if the appropriate geometric mean dimensions are substituted for D_i and d_c.

Cable capacitance

Cable capacitance can be obtained from manufacturer data. For a circular conductor, it can also be estimated from:

Symbols

Dielectric loss contributes to the heat source term in thermal rating calculations. For the wider method, see IEC 60287 Cable Current Capacity and Cable Thermal Analysis.