What is Insulation Resistance Test : Working & Its Applications

The insulation resistance test is not the latest test but it is used most frequently to find out the reliability of cables or windings within transformers, switchgear, motors, & electrical installations. The IR measurement test is a common test performed on different kinds of cables & electrical wires. This test is frequently used as a customer acceptance test like a production test including minimum insulation resistance for each unit length frequently specified by the customer. There are some factors that affect the IR test like the device installation, the type of usage & the operating conditions. This article discusses an overview of an Insulation resistance test and its working advantages.

What is Insulation Resistance Testing?

The insulation resistance test definition is the test that is used to measure the whole resistance between any two points which are separated through electrical insulation. This test is also known as the megger test. Thus, this test is used to determine how the dielectric or insulation is effective while restricting the electric current flow. So the IR tests are very helpful not only in verifying the insulation quality when the product is manufactured but the product is used over time also.

Insulation Resistance Test
Insulation Resistance Test

Why Insulation Resistance Test is Required?

Once an electric current is supplied throughout a conducting wire to reach the destination but due to some reasons, there is a loss in electric current along the way. Thus we need to perform the insulation testing to verify if there is any current leakage because this leakage could be very harmful.
This testing is very essential in avoiding injury and damage.

So this test should be conducted to make sure the electrical equipment’s reliability. By doing this test one can find out if the electrical equipment is in good condition & we can identify the service life of insulation almost. The insulation testing should be performed while fixing new electrical equipment, thus while testing electrical equipment, we can verify that the insulation is in good condition for the operation.

Insulation Resistance Test Principle

The insulation resistance test principle is to apply the voltage to measure the steady-state leakage current and after that divide the voltage by the current (R = V/I). If the IR meets otherwise beats the necessary value, then the IR test is successful.

The measurement’s target insulation resistance can be calculated by measuring the flow of current to the target once the voltage ‘V’ is applied and after that, the applied voltage ‘V’ can be divided through the resulting current ‘I’.


Insulation Resistance Test Circuit Diagram

Insulation resistance can be measured by using an instrument namely a megger. This instrument can be power-driven through an inbuilt DC generator or higher voltage battery. Megger is also known as Megohmmeter. This device is a high resistance ohmmeter including a generator. This device includes three connections like Line terminal (L), earth terminal (E), and guard terminal (G). These three terminals are specified with L, E & G in the following figure.

Insulation Resistance Testing Circuit
Insulation Resistance Testing Circuit

The resistance can be simply measured in between two terminals like L&E whereas the guard terminal is provided for other testing purposes where once resistance must be isolated from another.

The generator in the instrument is line operated or hand-cracked which develops a high DC voltage to cause a little current throughout & on surfaces of insulation being tested. Once the insulation is measured then the resistance value can be displayed over an indicator scale.

The insulation can be tested by using the following procedure;

Before using the megger instrument, we need to check whether this instrument provides infinity value once it is not connected and once the two terminals are connected together then it should provide zero value & the handle is turned.

To perform an insulation resistance test, you have to verify the cable is detached from any devices and ensure that no eddy current within the device through earthing it. The instrument must be in normal working temperature as resistance is related to temperature.

Make sure that both ends of the cable are separated from each other. Now connect the two terminals of megger to conductors which have to be tested. If the reading displays infinity then the insulation of the conductor is good.

The importance of this test is to test the reliability of the insulation between conductors. This test helps in finding the short circuit issues within the circuit.

Types of Insulation Resistance Test

There are three types of insulation resistance tests that can be performed by a megger or mega-ohm meter like the following.

Short-Time or Spot-Reading Test

This type of insulation resistance test can be simply performed by connecting a Megger device across the insulation which needs to be tested. After that, it is operated for a short time period, generally 60 seconds. Throughout this test, it is most significant to keep in mind that humidity, temperature & insulation conditions have an effect on the reading.

Time-Resistance Method

This type of testing method frequently provides conclusive data with no records from earlier tests. This test mainly depends on a good insulation absorption effect as compared to contaminated insulation. Throughout this insulation test, a device like Megger will take straight readings at exact times & note downs the differences. This kind of time resistance test must show a common increase within resistance over a certain period of time if the insulation is fine.

Dielectric Absorption Ratio

This is the ratio of two time-resistances readings and it is very useful in recording insulation information. For instance, a one-minute reading can be divided by a 30sec reading; so a Megger device will make this test easier and gives the best results. So this device can run for one minute; but, it allows you to note down readings at the 30-second gap & at the minute gap.

How to Measure Insulation Resistance Test?

The insulation resistance can be measured by using Ohms Law. If we provide a voltage supply across a resistor then the current flow can be measured with the formula R=V/I. This is a fundamental technique in fact; we should be aware of different types of current like capacitive charge, polarization, leakage & conduction current.

Insulation resistance can be measured through an insulation resistance tester. This tester includes three terminals where two leads are simply connected to a circuit to perform the test and the third terminal is connected to a guard terminal. This terminal allows you to select specific electrical components within large plant machinery.


The advantages of the insulation resistance test include the following.

  • The insulation resistance test is used to determine how effective the insulation is while restricting the current flow.
  • These tests are very useful for verifying the insulation quality.
  • This test is recommended to avoid electrical shocks.
  • It is a quality control measure used at the time electrical equipment is created.


The insulation resistance test applications include the following.

  • This test is performed to avoid hazards like short-circuits & electric shocks caused once the insulation within electrical devices, equipment & parts utilized in buildings, and industrial plants corrupt over long times of use.
  • The IR test is used to measure the whole resistance between any two points divided through electrical insulation.

Thus, this is all about an overview of insulation resistance and it is a quality of an electrical system that decreases with time, environmental conditions like humidity, temperature, dust particles & moisture. So it is essential to verify the equipment’s insulation resistance at regular intervals to keep away from electrical shock. Insulation resistance test needs to be monitored, measured, and tested based on equipment or the lifecycle of the system. Here is a question for you, what is a megger?