What is the cable Capacitance ?

Capacitance (C), is the property of a circuit or a circuit element, which permits the storage of electricity when potential difference exists between the conductors. Its value is expressed as the ratio of a quantity of electricity to a potential difference, and is usually measured in picofarads per metre (pF/m). A capacitance value is always positive. The alternating-current phenomena associated with electrical capacitance is important in wire and cable. At most commercial voltages and frequencies used in power distribution, the capacitance effects are negligible, although the total capacitance of a length of cable will affect the charging current and reactive current flow. At relatively high voltages, the current due to capacitance may reach sufficient value to affect the circuit, and insulations for such applications are designed using materials with a moderately low dielectric constant.

For a single core screen cable.

C = e r / [18 * loge( D / d )] ( m F/km)

where

er = relative permittivity of insulation = dielectric constant

PE 2.3 PP 2.5

XLPE 2.5

EPR 3

Paper 3.3-3.9

Butyl Rubber 4

PVC 5-8

D = diameter over the insulation (mm) d = diameter of the conductor (mm)

NB: Capacitance calculations for multicore and multipair cables are not as easily calculated, and several different formulae can be found in various texts as approximations.

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