Testing of Overhead Line Insulators

This article describes insulator testing and the types of tests, such as flashover tests, performance tests, and routine tests carried out to ensure the operational reliability of electrical insulators.

The importance of electricity in the contemporary world stands unquestionable and undeniable. This electricity is generated at the power stations which are located far from our industries and settlements. Hence, transmission lines are used to transmit this bulk amount of power from the generating station to the distribution station efficiently and safely.

There are a lot of techniques that are employed to keep the efficiency of the entire power system high so that the power loss during transmission and distribution (T&D) is as small as possible. Similarly, it is pertinent to transmit and distribute this huge amount of power safely and reliably because the transmission and distribution lines carrying the electrical power run across our cities and villages. If safety and reliability are not emphasized, it may lead to fatal accidents.

Many things are done to ensure the safety and reliability of the transmission line. One among these many things is the proper installation and maintenance of line insulators. These line insulators ensure that the transmission line under operation doesn’t come in contact with other lines or the earth. The rigorous testing methods are put to use to ensure the reliable working of these line insulators. We are going to discuss them in this post.


Types of overhead line insulators

 Various types of line insulators are used in overhead transmission lines. These are listed below.

  • Pin type insulators
  • Post type insulators
  • String insulator unit
  • Suspension insulator string
  • Tension insulator

Read More: Types of Overhead Line Insulators

Testing of insulators

As we discussed above, different types of tests are carried out on the line insulators before they are put to use. The reliability and safe working of the transmission line depends on these line insulators. Hence, there are broadly three types of tests that are carried out on a line insulator.

  • Flashover tests
  • Performance tests
  • Routine tests

Let’s discuss these tests in detail.

Flashover Tests

Under various environmental conditions or due to over-voltage, the air around a transmission line may get ionized, which can cause the formation of an arc. This arc can jump from the transmission line to another line or to the support structure, hence leading to a short-circuit fault. This is known as a flashover.


A line insulator must be able to arrest flashovers, and hence, it is tested for flashover protection. Flashover tests of the line insulators include

  • Power frequency Dry flashover test
  • Power frequency Wet flashover test
  • Impulse frequency flashover test

Power frequency Dry flashover test

In this test, the insulator is connected to the line in the manner it is intended to be, practically. The electrodes of the insulator are connected to a variable voltage source of power frequency. The voltage is gradually increased to a level that is near the flashover voltage, the voltage at which the insulation of the air surrounding the line breaks down and starts conducting. The insulator is subjected to this voltage for a minute to test whether it can withstand the flashover voltage or not. A good insulator should be able to stop flashover from happening, and that’s what is tested here.

Power frequency Wet flashover Test or Rain Test

A transmission line is subjected to various environmental factors, one among which is rain. It increases the moisture in the air, and the flashover voltage of the line reduces. Therefore, the line is more susceptible to faults due to flashover. Therefore, the insulator in use should be able to withstand this situation.

To carry out this test, the insulator is connected to the line as it is intended to be, and a variable voltage source at power frequency supplies its electrodes. In addition to that, the insulator is sprayed with water at an angle of 45° in such a manner that its precipitation should not be more than 5.08 mm/min. The voltage is increased gradually to a level that is near the flashover or breakdown voltage. The insulator is subjected to this voltage for 30 seconds to one minute to test whether it can withstand that voltage or not. This test is also called a one-minute rain test or simply a rain test.

Impulse Frequency Flashover Test

This test is done to analyze the effectiveness of the line insulator during a lightning surge. The insulator under test is subjected to a very high voltage at several hundred kiloHertz of frequency. The spark-over voltage of the insulator, in this case, is noted, and it is known as the impulse spark-over voltage. The ratio of the impulse spark-over voltage to the spark-over voltage at power frequency, which is known as the impulse ratio, is calculated. This ratio should be around 1.4 for pin-type insulators and 1.3 for suspension-type insulators.

Performance Tests of Insulators

  • Temperature Cycle Test: This test is done by putting the insulator at different temperatures in a cycle to note the changes or deterioration in its making, if any. The insulator is subjected to a temperature near 70 deg.C for an hour. Then, it is subjected to 7 deg.C for another hour. This is done three times in a cycle, and then the insulator is thoroughly examined for any damage or deterioration.
  • Puncture Voltage Test: In this test, the puncture voltage of a given insulator is determined. The insulator under test is suspended in insulating oil, and the voltage across the insulator is increased gradually. At a certain voltage level, the puncture of the insulator takes place, which is known as the puncture voltage. It is usually 30% higher than that of the dry flash-over voltage for a suspension-type insulator.
  •  Mechanical Strength Test: For one minute, the insulator is subjected to 250% of its maximum working load. This is done to determine the limit of the mechanical strength of the insulator.
  • Electromechanical Test: This test is done only for suspension-type insulators. The insulator under test is subjected to 250% of the maximum working tensile stress. After this, the insulator is tested for 75% dry spark-over voltage.
  • Porosity Test: The porosity test of an insulator is done to assess the number of pores that the insulator has. For conducting the porosity test, a freshly manufactured insulator is broken into pieces and immersed into a 0.5% to 1% alcohol solution of fuchsine dye under pressure of 150 kg/cm² for several hours. The pieces are then taken out of the solution and examined for absorption of dye. This indicates the degree of porosity of the insulator.

Routine Tests of Insulators

High Voltage Test: This test is usually carried out for a pin-type insulator. The insulator is inverted and is placed under water. The spindle hole is filled with water, and the insulator is subjected to a high voltage for five minutes. A good insulator should be able to sustain this without getting damaged.

Proof Load Test: In this test, each insulator is subjected to 20% excess or 120% of the working mechanical load. The insulator should be able to sustain the load without being damaged.

Corrosion Test: Under this test, the insulator with its metal fitting is suspended in a copper sulfate solution for a minute. The insulator is then taken out of the solution and wiped and cleaned. This procedure is repeated four times. Then, the insulator is examined for any metal deposits on it. A good insulator should have no metal deposit on it after this test.

So, these are the common and important tests that are carried out before an insulator is put to use in a transmission line.

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