Home Home Improvement Learn How Insulation Resistance Test is Done

Learn How Insulation Resistance Test is Done

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The Insulation Resistance (IR) test is the most widely applied test for estimating the accuracy of windings or cables within motors, switchgear, transformers, and electrical installations. It is most frequently carried out on different kinds of electrical wires and cables. It is commonly used as a consumer acceptance test such as a production test along with minimum IR for each unit length recurrently defined by the consumer.

Several factors impact the insulation resistance testing such as the type of usage, the device installation, and the operating circumstances. Know more about IR testing and its functionality in the subsequent sections.

Defining Insulation Resistance Testing

It is described as the test which is used to measure resistance offered between any two points that carry electrical current. IR testing is also known as the megger test. This test is applied to know how the insulation or dielectric is efficient whilst constraining the flow of electric current. Hence, this test is not only favourable in assessing the insulation quality during the product manufacturing but also in how the product is applied over time.

Why is Insulation Resistance Testing Performed?

Due to certain circumstances, there is some loss in electric flow along the way when it is being supplied through a conducting wire. Hence, you need to carry out insulation testing to check the possibilities of current leakage as it could be greatly unsafe. The insulation resistance test is significant as it helps to avoid injury and damage. You can use IR testing as:

  • A troubleshooting tool
  • An installation needs to make sure specifications are fulfilled
  • A periodic preventive maintenance task
  • To check the reliability and condition of electrical equipment
  • To determine the service life of insulation

Understanding the Insulation Resistance Test Principle

The IR testing principle is to use the voltage to estimate the steady-state leakage current and post that divide the voltage by the current (R= V/I). The IR test is successful when the IR beats the significant value otherwise meets.

The dimensional target IR can be measured by estimating the electric current flow to the targeted once the voltage “V” is implied and post that, the implied voltage “V” can be divided by the resulting current “I”.

How is Insulation Resistance Test Measured?

Ohms Law is used to measure insulation resistance. The flow of the current can be estimated using the formula R= V/I when you provide a voltage supply around the resistor. It is the most significant method as well as you must be aware of the varying form of current such as leakage, capacitive charge, conduction, and polarization current.

An insulation resistance tester can be used to measure insulation resistance. It comprises three terminals wherein two leads are directly linked with the circuit to execute the test and the third lead is linked with a guard terminal. The latter enables the user to choose a particular electrical element within large plant machinery.

What are the Different Types of Insulation Resistance Tests?

Short-time or Spot-Reading Test: This test can be easily done by connecting a Megger unit over the insulation that is required to be tested. Following this, it is managed for a short span, commonly for less than 30 sec or more than 60 sec. Moreover, it is significant to remember that factors like temperature, humidity, and insulation circumstances have an impact on the reading.

Time-resistance method: This technique is reasonably independent of temperature and sometimes provides you with decisive data without considering past test records. It is dependent upon the absorption impact of good insulation when compared to contaminated or moist insulation. This test is also known as the absorption test.

It is a significant test as it is independent of machinery size. The boost in resistance for dry and clean insulation takes place in the same way irrespective of whether a motor is small or large. Hence, you can compare various motors and design standards for new machinery irrespective of their horsepower ratings.

Dielectric absorption ratio: It is used in recording insulation data and is defined as the ratio of two time-resistance readings. For example, a three-minute time resistance reading can be divided by a 60sec, such that the Megger tool will provide the best results and make the process easier.

What Are the Factors That Would Affect the Values of the Insulation Resistance Test?

  • Capacitance charging current: This begins at a high value and lowers down once the insulation has been electrified to full voltage.
  • Conduction or leakage current: A short significantly stead current both over and through the insulation.
  • Absorption current: It is initially high which drops afterward. Also known as the time-resistance method. It is said that IR testing must be performed to prevent risks like short-circuits and electric shock that occurs when the insulation in electrical equipment, devices, and parts utilized in buildings, industrial units, and other areas deteriorates over a long-time of application.


Insulation resistance is the oldest and most popular test used for assessing the quality of the insulation. Also, it is found that it is a quality of an electrical unit reduces with time, and environmental circumstances such as moisture, dust particles, temperature, and humidity. Hence, it is significant to check the machinery insulation resistance at regular times to sustain the electrical shock. Lastly, the insulation resistance test must be measured, monitored, and tested depending on the machinery or the system’s lifecycle.

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