What is Suspension Insulator : Construction, Working & Its Types

An electrical insulator works on the principle of resistivity to oppose the flow of electric current and protect electrical equipment from short circuits (by isolating electrical conductors from making accidental contacts). Some of the examples of the insulator are polymer, wood, plastic, etc. The main application of an insulator is an over headed transmission line, which is supported by poles or towers to prevent any current leakage. Transmission Line Insulators are classified into several types like pin type, suspension type, post type, strain type, spool-type, ceramic type, non-ceramic type, etc. This article describes the suspension insulator and its types.

What is Suspension Insulator?

Definition: A suspension type insulator protects an over headed transmission line like a conductor. Generally, it is made up of porcelain material that includes single or a string of insulating discs hanged over a tower. It operates at above 33KV and overcomes the limitation of pin type insulator like the following.

  • Its size and weight increases above 33KV
  • It is difficult to handle and replace a unit insulator
  • The replacement of a damaged insulator is costly.

Properties of Insulator Material

The following are the properties of any insulating material they are,

  • They should be mechanically strong
  • The dielectric strength of the material should withstand high voltage stress
  • The electrical insulating resistance should be high
  • The material should be free from impurities, no cracks and non-porous
  • The physical properties and electrical properties of an insulator should not be affected due to change in environment
  • The safety factor must be taken.

Suspension Insulator Construction and Working

It consists of two main parts they are the cross-arms and the insulators (also called disc insulator) with the number of metallic links. A suspension insulator or suspension string is developed by connecting a number of insulators in series with the help of metallic links, where the conductor is suspended by the bottom-most insulator and the top end of the insulator are secured by cross-arms. These kinds of insulators are mainly used in an over-headed line.


String Efficiency Derivation

The string efficiency of suspension insulators can be derived using the following diagram. It consists of 3-disc string suspension insulators with a metallic link between them to provide a capacitive effect between them. The effect can be either self-capacitive or mutual capacitive. Let us assume shunt capacitance = k * self-capacitance. Due to the presence of shunt capacitance, the current in each disc varies.


On applying Kirchoff’s law at node ‘A’

where I1, I3, I2 and i1, i2, i3 = current flow in the conductor;

V1,V2,V3 = Voltage

K = constant

ω = 2πf

I2 = I1 + i1


V2 =V1 +V1k

V2 = (1 + k) V1   ………………..1

Applying Kirchoff’s at node ‘B’

I3 = I2 + i2

V3ωC = V2ωC + (V2 + V1)ωkC

V3 = V2 + (V1 + V2)k

V3 = kV1 + (1 + k) V2

V3 = kV1 + (1 + k)2 V1       (from 1)

V3 = V1 [k + (1 + k)2]

V3 = V1 [k + 1 + 2k + k2]

V3 = V1 (1 + 3k + k2) ………(3)

The voltage between the conductor and the earth tower is,

V = V1 + V2 + V3

V = V1 + (1 + k)V1 + V1 (1 + 3k + k2)

V = V1 (3 + 4k + k2) ……….(4)

From the above equations, we can say that at the topmost disc, the voltage is minimum, whereas at the bottom-most disc the voltage is maximum. Hence the unit nearest to the conductor experiences maximum electrical stress which may also lead to a puncture. It is represented as the ratio of string efficiency.

String Efficiency = String Voltage / (Number of Discs x The Conductor Voltage)

Where efficiency is directly proportional to the uniform distribution of the voltage. In an ideal condition, the efficiency is equal to 100% if the voltage across each disc is equally distributed and in the practical world it is not possible. Practically it is better to use shorter strings in the insulator than a larger string to get 100% efficiency.

Types of Suspension Insulator

They are further classified into two types, they are

Cap-and-Pin Type

 It consists of forged steel cap and galvanized forged-steel pin which are connected to porcelain. These units are either joined by socket and ball or by pin-clevis connections.


Interlink Type

It is also called a Hewlett type insulator. The porcelain presented here consists of two curved channels at 90 degrees to each other, with a U-shaped steel link passing through these channels connecting the unit.


On comparison, the interlink type is more mechanically stronger than the cap-and-pin type. The main advantage of both of them is that the metallic link which is present continues to support even if porcelain breaks. The disadvantage experiences high electric stress.


The advantages of suspension type insulator are

  • Low cost
  • Low voltage (about 11KV)
  • Highly flexible


The disadvantages of suspension type insulator are

  • Costlier than pin-type and post-type insulator
  • Increases the spacing between conductor
  • Increases the height of the tower.


The applications of suspension type insulator are


1). Why do we need insulators?

We require insulators in-order to prevent any electric leakage in the system or circuit.

2). Is water an insulator?

No, water is not an insulator.

3). What is the best insulator?

The best insulator is a Vacuum.

4). What are 7 insulators?

The 7 insulators are

  • Glass fiber
  • Wood
  • The paper which has Dry property
  • Air which has the dry property
  • Wood which has the dry property
  • Porcelain
  • Crystals like Quartz.

5). Can you charge an insulator?

Yes, one can charge an insulator.

6). What is the principle of Suspension motor?

A suspension motor works on the principle of insulation, which prevents the current leakage in electric equipment.

7). What are the different types of insulators?

The different types of insulators are Pin type, suspension type, post type, suspension type, strain type, spool-type, ceramic type, non-ceramic type, etc

Thus, This is an overview of an insulator, it is a material used to oppose the flow of current. It plays an important role in an electric system by preventing the current leakage. There are different types of insulators but this article summarises suspension type insulator, which operates above 33KV. The main advantage of suspension insulator is that it uses low voltage and is highly flexible. These kinds of insulators can be mainly seen in railway lines, over headed poles, etc.