The distribution of capacitance along an insulator, and their size, govern the electric stresses, which excite generation of interference, and the coupling of the generator to the radiating antenna.


A composite insulators like a cylinder of dielectric having a relative permittivity about 6. The field intensity falls away rapidly with increasing distance from the live terminal. The generating discharges occur at or near the live terminal, and the capacitance, which couples the high-frequency currents into the radiating circuit, i.e. the line and tower, is small. Composite insulators are thus significantly quieter as interfering sources than string of discs.
In a string of discs, quite large capacitance–of the order of 30 pF are connected in cascade through the fittings. The voltage distribution is governed purely by these and by the stray capacitance to line and ground, in dry conditions. In such a voltage-dividing circuit the partition is independent of frequency: identical distributions therefore exist for the power-frequency and for the radio-frequency voltage. The units at the line end are more prone to surface corona than the rest.

Because of the high unit capacitance the sources are closely coupled into the line, which presents load impedance equal to one-half of the line’s surge impedance. It is common practice to relieve the line end overvoltage by means of stress-grading fittings.


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