Measurement of Insulation Resistance

What is Insulation Resistance Test or IR Test?

The Insulation Resistance Test consists in measuring the Insulation resistance of a device under test, while phase and neutral are short-circuited together.

What is Insulation Resistance?

Ideal insulation does not allow an electric current to flow through it. The atoms within an insulating material are tightly bound with one another, hence it is difficult to release electrons from the material, and in normal condition, no current flow through the insulator.

The current through ideal insulation is almost nil for a particular voltage. If the voltage is increased above the dielectric strength or above the breakdown voltage of the insulator the current starts flowing through the insulator. The insulator is said to be in a state of breakdown.

The various insulators have different breakdown voltages and for different system voltages, various types of insulators are used.

IR Test is the oldest and most widely used test for the measurement of insulation resistance. The Insulation Resistance Test measures the Insulation resistance of a device under test. In the IR test, the phase and neutral are short-circuited together. The measured IR value must be higher than the specified IR value as per the standard,

The insulation resistance of the material is not perfect, when voltage is applied some current starts flowing through the specimen. The current is called the leakage current. The IR value must be as high as possible to get a low leakage current. The lower the leakage current, the higher the IR value.

Now let us understand why the IR test is important and what is its significance.

Why Insulation Resistance Test is done?

Measurement of insulation resistance is very important for ensuring the healthiness of the electrical system. The quality of the insulation has prime importance for the reliable operation of electrical equipment.

We carry out the Insulation Resistance Test to ensure the healthiness of the insulation. The higher insulation is an indication of a reliable electrical system. The Insulation Resistance Tester is used to measure the insulation resistance value. The higher insulation resistance is always desirable for electrical systems.

The value of the insulation resistance deteriorates with aging and temperature. The insulation is apt to fail with a rise in temperature above permissible limits and the insulation resistance decrease by half with every rise in 10⁰C temperature. The insulation deterioration has to be monitored for early action before insulation breakdown.

To adjudge the quality of the insulation, we carry out the test like-Insulation Resistance Test(IR Test), Dielectric Absorption Ratio (DAR)Test, and polarization Index(PI) test. In case of variation of insulation resistance from permissible value, we need to take necessary preventive action well in advance.

Factors responsible for insulation quality deterioration

The quality of insulation deteriorates if the surface of the insulator traps moisture on its surface or surface has dirt. The insulation resistance lowers if there are any harsh installation environments, with temperature extremes and/or chemical contamination. All these cause deterioration of insulation resistance.

Electrical stresses: The over-voltage and under-voltage cause stress on the insulation system.
Mechanical stresses: Frequent start-up and shutdown sequences cause high current to flow through the conductor. The high current causes heating of the conductor and lowers the insulation resistance.
Chemical stresses: The proximity of chemicals like oils, corrosive vapors, and dust affect the insulation performance of the materials.
Stresses linked to temperature variations: The frequent start and stops lead to an increase in temperature because of increased current above the rated full load current of the conductor. The temperature variation lowers the insulation resistance.
Environmental contamination causes aging acceleration of insulation.

Regular cleaning is the prerequisite for the reliable operation of the insulator.

Another important fact about insulators is that they have a negative temperature coefficient. The resistance value of the insulator decrease with an increase in temperature. The rise in temperature of an electrical machine, rise in bus bar temperature, or rise in electric panel temperature lead to failure of the insulator.

We need to isolate the equipment if the temperature rise is above the permissible limit. For example, if motor trips with a winding temperature high, we should not start the motor again, and wait for some time till the temperature drops down to the permissible limit.

The conductor has insulation around it.

The failure of the insulation cause a short circuit and it may cause fire hazard and electric shock as well. Therefore, it is a must to ensure the healthiness of the insulation.

Methods of Measuring Insulation Resistance

The insulation tester or Megger is the instrument by which we can measure the insulation resistance. We use the following instruments for insulation resistance measurement.

Direct indicating ohm meter with hand-operated DC generator
With Direct indicating ohm meter with motor operated DC generator
Direct indicating ohm meter with DC battery
Direct indicating ohm meter with a full-wave rectifier
Resistance bridge circuit with galvanometer and battery

We use DC voltage to measure the insulation resistance. We can generate the DC voltage with a hand-driven dc generator or motorized dc generator. Insulation act as a capacitor. The capacitor blocks the DC if the insulation quality is good. If leakage current flows through the insulation, it indicates the poor quality of insulation.

The insulation tester generates DC voltage, and we can select the DC voltage magnitude according to the system voltage of the insulation system. The current flowing through the insulation gives the indication of whether the insulation is good or bad.

test setup of insulation resistance measurement

We connect the positive lead marked as L on the insulation tester to the conductor and connect the negative lead marked as E to the insulator surface. Thus, the DC voltage is applied across the conductive part and earth part of the conductor. Let the voltage be V and the current flowing through the insulator is I.

Insulation Resistance Formula

According to Ohms’s Law;

If the applied voltage is 500 volts and the current through the resistance is 10 Micro-ampere then insulation resistance is;

When we apply constant DC voltage across the conductor and insulator, the current drawn is the sum of the three current elements.

Charging current,
Absorption current and
Leakage current.

Initially, the charging and absorption current is high and it decays very fast. The insulation tester reads insulation resistance low at the start because the charging current is high.

Taking the IR test value just after applying DC voltage does not give the accurate IR Value. Therefore, we should apply the constant DC voltage for at least one minute for the insulation resistance test.

When we apply DC voltage for one minute or less than one minute, the test results may not be so accurate but it fairly gives an indication of the insulation condition. The test is known as a short time or spot reading test.

Safety Precaution After IR Test:

During the IR test, there is charge accumulation between the conductor & insulation. If we touch the conductor after carrying out the IR test, the stored charge finds its path through the human body, and this may cause an electric shock. Therefore, the stored charge energy needs to discharge after carrying the IR test by connecting the conductor to earth potential.

What is the allowable lower limit for Insulation Resistance?

One-mega ohm rule establishes the allowable lower limit of insulation resistance. The one mega-ohm rule states that there will be approximately one megaohm resistance for every 1000 volts of operating voltage.

If the equipment is rated for 6600 Volts, the minimum insulation resistance value should be 6600/1000=6.6 Mega-Ohm.

Limitation of Insulation Resistance (IR) Test

The insulation resistance change with temperature and humidity. Therefore, the spot reading of IR does not give the correct insulation resistance value.

Time insulation resistance test

The spot reading or short time reading of the insulation test does not give a clear picture of the insulation quality. The double reading test or time resistance test like DAR and PI test gives a clearer picture of the insulation condition.

The other factors like temperature and humidity do not affect the results of the insulation resistance measurement in the time resistance test. Therefore, the time resistance test or the double reading test is the best method for checking the insulation resistance.

Dielectric Absorption Ratio & Polarization Index Test:

In the DAR test, we take the insulation resistance reading after 30 seconds and 60 seconds. The 60 seconds IR value reading is divided by 30 seconds IR value reading, and the ratio of 60 seconds IR reading to 30 seconds IR reading is the DAR of insulation. If we divide 10- Minute IR reading by 1-Minute IR reading, the ratio is the polarization Index of insulation.