USA: +1-585-535-1023

UK: +44-208-133-5697

AUS: +61-280-07-5697

Line Insulators

3.5. Line Insulators

The insulators of a transmission line are its most important item, since the operation of a line cannot be any better than the insulators that support the conductors. Transmission line insulators must possess good mechanical strength and good insulating qualities under all conditions of weather and temperature and must not deteriorate fast. Insulators are made of glass, porcelain, and patented compounds. Glass is the cheapest material and when properly made will produce satisfactory insulators for low-voltage work, such as telephone and telegraph, and under favourable conditions may be used upto 25 kV. Though there are a number of patented compounds on the market, these do not seem to offer much competition with porcelain, since porcelain has very good electrical characteristics as well as high mechanical strength.

Transmission line insulators may be classified as follows:

  1. Pin type
  2. Suspension type
  3. Strain type. 6

    Fig. 14. Various types of insulators. (b) Suspension type insulators

    • For low voltages, pin-type insulators made of glass are generally used. Pin-type insulators made of porcelain are designed for voltages upto about 90 kV but are seldom used on lines above 66 kV.
      • For voltages above 66 kV it is generally desirable to use suspension insulators.
      • Strain insulators may be of pin or suspension type. Upto about 30 kV pin-type insulators are satisfactory, but for higher voltages the suspension type is generally used. Strain insulators are used on dead-end towers at bends or corners of transmission lilies, or when mailing long spans. Extra heavy suspension units are made for such service, but often standard units may be used. On ordinary straight line dead-end towers a single string is often sufficient but for severe service two more strings may be connected in parallel.