# Dielectric loss in cables

Dielectrics (insulating materials for example) when subjected to a varying electric field, will have some energy loss. The varying electric field causes

Dielectrics (insulating materials for example) when subjected to a varying electric field, will have some energy loss. The varying electric field causes

Dielectric loss is measured using what is known as the loss tangent or tan delta (*tan δ*). In simple terms, *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**

Cable Type | U_{0}, 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:

$$C=\frac{\epsilon}{18\mathrm{ln}\left(\frac{{D}_{i}}{{d}_{c}}\right)}{10}^{-9}\text{}F.{m}^{-1}$$

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

$${W}_{d}=\omega \text{}C\text{}{U}_{0}{}^{2}\text{}\mathrm{tan}\text{}\delta $$

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

**Symbols**

*d _{c}* - diameter of

*D*- external diameter of insulation, mm

_{i}C - cable capacitance per unit length, F.m

^{-1 }

*U*- cable rated voltage to earth, V

_{0}*W*- dielectric loss per unit length, W.m

_{d}^{-1 }

*tan δ*- loss factor for insulation

*ε*- insulation relative

*ω*- angular frequency (2πf)

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