What are the types of Insulators used in Transmission lines? properties of insulating material

What is an Electrical insulator

Electrical insulators have a very high resistive path through which current flow is not possible. It is mostly used in the electrical system to avoid the unwanted flow of the current to the earth from its supporting points such as towers and poles. The poles and towers must be properly grounded, so insulators are used between the towers and poles and in current carrying conductors in order to prevent the flow of current from conductor to earth.


What are the Functions of an insulator

  • It can provide electrical insulation between earthed structure and live conductors in case of operating and overvoltage conditions
  • It acts as a mechanical link between the structure and conductor and the mechanical integrity would be kept under the normal operating and overload conditions

Properties of insulating material

  • It should be strong enough to carry the tension and weight of the conductors
  • High dielectric strength is required to withstand the voltage stress in high voltage system
  • To avoid the leakage current from the conductors to earth it must have a high insulation resistance
  • Impurities must be removed from the insulating material
  • It must not be porous
  • While changing the temperature the physical and electrical property should be affected less

Types of insulators

  • Pin type insulator
  • Post type insulator
  • Suspension type
  • Strain insulator
  • Spool type

Pin type insulators

The insulator which supports the conducting line in case of compression is known as pin and post type. the name pin is derived from its constructional design, it is used for lower voltage transmission which ranges up to 33KV. It is mostly used for straight running transmission lines, depending upon the voltage application pin type can be of one part type two parts type or three parts type.

This type of insulators are applicable for rural and urban distribution it is designed as a pin which is installed on the cross arm of the pole, it is screwed on the pin and the electrical conductor is mounted on the insulator. For high voltage transmission, stronger pin type insulators are used it would be different from the other type in construction they consist of two or three porcelain


Post type insulators

It is similar to the pin type insulator, they are mostly used for high voltage applications, the porcelain pieces cemented together are called as petticoats and the number of petticoats would be greater for the post type insulator because it is used for high voltage applications, it may be mounted horizontally or vertically.



  • It can be used for lower and higher voltages
  • It can be a single stag or multiple stags
  • For higher voltage applications two or more insulators can be fixed together one above the other

Suspension insulators

The pin and post type insulators doesn’t provide satisfactory insulation for very high voltages in transmission lines so in that case we can use suspension insulator which is suitable for high voltage transmissions for voltages beyond 33KV it is not possible to use the pin or post type which won’t be satisfactory because the size and weight of the insulator will be more so in that case we can use suspension insulator.

By adding more insulators to the string distance can be put between the suspension insulator and the string, a string is the full unit of a suspension insulator, and the insulators which are used in the string would be depended on the voltage and the weather conditions


  • If any one of the insulators is damaged in the suspension insulator it can be replaced without changing the entire string
  • Mechanical stresses would be less in this type because the line is on a flexible string
  • Suspension disc can be added or removed according to the voltage level

Strain insulators

In the transmission line if there is any sharp corner or dead end or sharp curve, a great amount of tension will be subjected to the line so in order to remove the line from this tension strain insulators are used, in case of low voltage lines shackle insulators are used as strain insulator and in case of high voltage the strain insulator will have an assembly of suspension insulators. In vertical planes, the disc of strain insulator is used and in case of high tension two or more strings are used in parallel

Spool insulator

It is mostly used in low voltage applications, it can be identified by its shape it is mostly used for secondary mains it may be mounted on a secondary rack or in a service clamp the use of this type of insulator is reduced because of the cabled secondary and service wires


Failures in insulator

  • Electrical breakage of porcelain or puncture
  • Mechanical breakage of porcelain or metal
  • Mechanical separation of cap pin and shell

Causes of failure

  • Cement growth
  • Aging
  • Lightning overvoltages
  • Pollution