TABLE OF CONTENTS As indicated in Section 5.1, it is usual to employ 'pin-type' insulators for distribution voltages up to 30 or 40 kV. Above this voltage and up to about 132 kV system voltage 'line post' insulators may be used or, more usually, from 40 kV to the highest transmission system voltages the 'cap and pin' insulator is used.
In Germany and in parts of the world influenced by German design practice, the 'long rod' insulator is sometimes used. In recent years, other designs of insulator have been developed, notably rigid insulators utilising a central load-bearing core formed from glass fibre bonded to form a rod, and having plastic sheds. This type of unit is often referred to as a 'composite insulator'.
The pin-type insulator is so called because in use it is screwed onto a galvanised forged steel 'pin' which is mounted vertically on a metal or wooden crossarm.
For low voltage systems, 6.6 to 11 kV, it is usual to have a one-piece insulator shed in which the porcelain is loaded largely in compression. A typical pin-type insulator is shown in Figure 5.1 a. The sketches show that the top of the porcelain body is formed into a groove into which the conductor is bound by means of wire or fixed with the aid of special clips. Toughened glass pin-type insulators require a metal cap; this holds together the 'diced' pieces of glass which result if the glass becomes shattered.
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