What is a Strain Insulator : Working & Its Applications

It is clear that if the transmission lines are not protected correctly by giving support with towers or poles, then the flow of current will be in the direction of the ground through the towers/poles so it will become dangerous. Thus, transmission lines are always supported on insulators mounted on their support poles/towers. But insulators on transmission lines must have these properties like high electrical, mechanical strength and relative permittivity must be high for insulating material. The insulator material used in transmission lines most frequently is porcelain but sometimes steatite, glass, composite materials, etc. This article discusses an overview of the strain insulator.

What is a Strain Insulator?

Definition: Electrical insulator that works in mechanical strain to resist the balanced electrical cable is known as strain insulator. These insulators give support to the transmission lines as well as radio antennas in electrical wiring. This insulator can be placed between two wires to separate them electrically from each other. The strain type insulator diagram is shown below.

Strain Insulator
Strain Insulator

Working of Strain Insulator

In the transmission line, at a corner line is a great tensile load. To maintain this huge tension, strain insulators are utilized at sharp corners. In high voltage transmission lines, these insulators include a set of suspension insulators. So, the suspension insulator string can be placed horizontal plane whereas the discs of an insulator are arranged in a vertical plane. To maintain high tensions, more than two suspension strings are connected in parallel. For fewer voltage lines like < 11 kV, then shackle insulators are employed like strain insulators.

Strain Insulator Working
Strain Insulator Working

This is designed with porcelain or glass or fiberglass and includes supporting hardware and two cables. This insulator shape will reduce the space between the two cables. Generally, these insulators are in physical tension with radio antennas, overhead power lines, and guy-wires.

Once the line voltage needs more insulation to compare with a single insulator then these insulators are connected in series to provide highly effective insulation. By using hardware, insulators are connected.
If one string is inadequate for the strain, then a weighty steel plate bundles numerous insulator strings mechanically. The single plate is over the hot end & the other one is situated at the support arrangement.

This system is used for long distances like when a transmission line crosses a canal, valley, pond, etc.
This insulator should include significant mechanical strength & the required electrical insulating assets.

  • For 33kV rated system voltage, three-disc insulators are used in insulator string.
  • For 66kV rated system voltage, five-disc insulators are used in insulator string.
  • For 132kV rated system voltage, nine-disc insulators are used in insulator string.
  • For 220kV rated system voltage, fifteen disc insulators are used in insulator string.


To ensure the quality of insulators, this must go through these tests like type tests, performance, and routine tests.

  • The type tests are dry flashover, thirty-second rain, wet flashover & impulse frequency tests.
  • The performance tests are temperature cycle, electromechanical, puncture, mechanical strength, and porosity.
  • The routine tests are high voltage, proof load, and corrosion.
  • For all the insulators, the above tests are appropriate. So these insulators are main electrical insulators used to support and insulate the conductors.

Applications of Strain Insulator

The applications of strain insulators include the following.

  • These are used for electrical wiring in transmission lines to support transmission lines and radio antennas.
  • These are normally used in overhead wiring at outdoors. In this situation, they will expose to rainfall, in urban areas they will expose to pollution. In practice, the shape of the insulator will become significant, because a wet lane from one cable to others can make a low-resistance electrical lane.
  • These are designed for horizontal mounting so the shape of flanges that are used for shed water & strain insulators used for vertical mounting is bell-shaped.
  • These are used as the line is subjected to larger strain like river crossings, dead ends, sharp curves.
  • This insulator reduces extreme tension on the line

Advantages of Strain Insulator

The advantages of strain insulators include the following.

  • These are used for low voltages upto 11kv
  • These are insulated from the ground for low-voltage lines.
  • These are designed with porcelain
  • If the insulator is damaged, the stay or guy wires will not drop to the ground.


1). Where are the strain insulators are placed?

At dead ends on the overhead transmission lines.

2). What is the dielectric potency of porcelain?

The dielectric potency is 60 kV/cm.

3). What is the function of a strain insulator?

These are used in overhead electrical wiring to give support in transmission lines and radio antennas.

4). What are the tests used to ensure the value of an insulator?

They are type, performance &routine tests

5). What are routine tests?

They are high voltage, proof load, and corrosion tests

Thus, this is all about an overview of what is a strain insulator, working, and types of tests, advantages, and applications. Here is a question for you, what are the disadvantages of strain insulators?