Why pin insulator is not used above 33 KV line?

Apin insulatoris a ceramic insulator that fitted above the cross-arm of a pole, and which supports the conductor which is secured to the top of the insulator. Asuspension insulatoris a toughened-glass insulator 'dish' which hangs below a cross-arm of a tower, from which a conductor/s is suspended. For increased insulation levels, dishes may be coupled together to form 'strings'.

The manufacturing technique for pin insulators is more expensive than for suspension insulators,and the cost escalates signifcantly with increased voltages, which limits their operating voltage to around 50 kV. In practise, this means limiting their application to 66-kV lines in the United Kingdom (although 33 kV is much more common). Manufacturing costs are approximately proportional to the square (or more!) of the operating voltage.

The advantages of using suspension insulators, on the other hand are:

• less expensive than pin insulators at working voltages above 50 kV
• each insulator is designed for a relatively-low voltage, and the required insulation-level is obtained by connecting a suitable number together, to form strings
• mechanical stresses are reduced, because suspension insulators allow conductors to swing (whereas pin insulators are rigid)
• any failure only requires one insulator to be replaced, rather than the entire chain
• any voltage upgrade can be achieved economically, by adding additional insulators

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