Page 325 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

But now a days, they are frequency used for low voltage distribution lines. Such insulators can be used either
in horizontal position or in a vertical position. They can be directly fix to the pole with a bolt or to the cross arm.
Fig 6 shows a shackle insulator fixed to the pole. The conductor in the groove is fixed with a soft binding wire.
Suspension type insulators
The cost of pin type insulator increases rapidly as the working voltage is increased. Therefore, this type of
insulator is not economical beyond 33 KV. For high voltage (>33KV), it is a usual practice to use suspension type
insulators as in Fig 7. They consist of a number of porcelain discs connected in series by metal links in the form of
a string. The conductor is suspended at the bottom end of this string while the other end of the string is secured
to the cross- arm of the tower. Each unit or disc is designed for low voltage, say 11KV. The number of discs in
series would obviously depend upon the working voltage. For instance, if the working voltage is 66KV, then six
discs in series will be provided on the string.

Fig 7

Advantages
1 Suspension type insulators are cheaper than pin type insulators for voltage beyond 33 KV.

2 Each unit or disc of suspension type insulator is designed for low voltage, usually 11KV. Depending upon the
working voltage, the desired number of discs can be connected in series.
3 If any one disc is damaged, the whole string does not become useless because the damaged disc can be
replaced by the sound one.
4 The suspension arrangement provides greater flexibility to the line. The connection at the cross arm is such
that insulator string is free to swing in any direction and can take up the position where mechanical stresses
are minimum.
5 In case of increased demand on the transmission line it is found more satisfactory to supply the greater
demand by raising the line voltage than to provide another set of conductors. The additional insulation required
for the raised voltage can be easily obtained in the suspension arrangement by adding the desired number
of discs.
6 The suspension type insulators are generally used with steel towers. As the conductors run below the earthed
cross arm of the tower, therefore, this arrangement provides partial protection from lighting.
Strain insulators
When there is a dead end of the line or there is corner or sharp curve, the line is subjected to greater tension. In
order to relieve the line of excessive tension, the strain insulators are used. For low voltage lines (<11KV) shackle
insulators are used as strain insulators. However for high voltage transmission lines, strain insulator consists of an
assembly of suspension insulators. The discs of strain insulators are used in the vertical plane. When the tension
in the lines is excessively high, as at long river spans, two or more strings are used in parallel.
Post insulators
Cap and pin type: Such insulators can be used for mounting of buses, dropout fuses, line conductors, G.O.A.B
(Gang Operated Air Break) switches. These are of outdoor type and are available in 11, 22 and 33KV ranges.
(Fig 8a&b)

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CITS : Power - Electrician & Wireman - Lesson 106-116