What is Insulation Resistance Test.
An insulator is a material that resists the flow of electric current through it. Electric insulation is used to direct electricity along the desired path in a circuit and prevent it from straying down unwanted path causing thereby damage to equipment and injury to personnel.
In theory, all the electric current that flows along a conductive wire reaches its intended destination but in real life, some escapes through the insulation. This is because no insulator is perfect and a certain amount of leakage current would flow through the insulating material (dielectric).
Dielectrics are insulators however in the presence of an electrical field the dielectric material becomes polarized.
Electric Polarization is simply when the positive and negative charges within an insulating material shift in opposite directions in the presence of an electric field. The positive charges within the dielectric are displaced in the direction of the electric field, and the negative charges are displaced in the direction opposite to the electric field
Insulation Resistance Test is a predictive maintenance test. It is basically a measure of an insulating materials resistance to the current that flows across it. Measuring a High resistance value would mean that a little current is escaping and the insulation in good condition while a low resistance value would mean that a significant amount of current may be leaking through and along the insulation.
The insulation resistance is gotten by passing a certain amount of voltage through the conductor and dividing it by the amount of current that escapes
The major reason for Insulation resistance test is to check insulation integrity. IR tests could be carried out after installation to ensure safety and eliminate the possibility of a short-circuit when the system is energized. IR test could also be carried out for maintenance purposes on already installed electrical systems. Electrical systems are exposed to environmental factors like heat, mechanical stress, vibration and corrosive environment etc and this can lead to insulation failure, so IR tests are carried out periodically on cables and motors to get valuable information about the state of insulation deterioration; Then this information is used to predict insulation failure ahead of time.
MINIMUM ACCEPTABLE VALUE. Generally, the rule of thumb for the minimum insulation resistance value acceptable for safe energization is 1 MΩ per 1000 V of applied test voltage plus 1 MΩ.
SHOCK HAZARD FROM TEST VOLTAGE. Large electrical equipment and cables usually have sufficient capacitance to store up a dangerous amount of energy from the test current. Make sure to discharge after the test.
The insulation resistance is gotten by passing a certain amount of voltage through the conductor and dividing it by the amount of current that escapes
ie Resistance (insulation) = 
The major reason for Insulation resistance test is to check insulation integrity. IR tests could be carried out after installation to ensure safety and eliminate the possibility of a short-circuit when the system is energized. IR test could also be carried out for maintenance purposes on already installed electrical systems. Electrical systems are exposed to environmental factors like heat, mechanical stress, vibration and corrosive environment etc and this can lead to insulation failure, so IR tests are carried out periodically on cables and motors to get valuable information about the state of insulation deterioration; Then this information is used to predict insulation failure ahead of time.
Components of the total current that flows during IR Test
Three components of current make up the total current that flows during the insulation resistance test. They are Capacitive charging current, Absorption Current and Leakage Current.
When the test voltage is first applied to the conductor, there is an initial burst of current that starts out high but it decreases as the length of time the voltage is applied increases and flattens out after the insulation becomes charged; this is called capacitive charging current.
Dielectric absorption current also appears at the initial application of the test voltage, just like the capacitive charging current, absorption current starts out high but it depletes at a much slower rate than capacitive charging current. Absorption current drops to nearly zero after some time because the molecules within the insulating material become polarized. This is the current that is required to polarize the insulating material.
After a certain period of time when the influence of absorption current diminishes a small amount of current normally flows through and over the insulation itself. This is the leakage current (IL) that the insulation allows to flow depending on its condition and it is of primary importance as it is used to evaluate the condition of the insulating material under test.
Types Of Insulation Resistance Tests
There are several IR test methods but we would discuss 3 basic types of insulation resistance tests.
1) Short Time or Spot Reading test - in this type of test the voltage is applied for about 30 to 60 seconds. When the test voltage is applied across the equipment under test you will get readings that are constantly rising on your MegOhmeter this is because the equipment under test is charging. At some point when the equipment becomes fully charged the readings will stabilize and you would get the true IR of the equipment under test. This makes it important to note spot reading test is only a true indication of IR when performed on a low capacitance equipment that can become fully charged under 60 seconds.
2) Time Resistance or Dielectric Absorption test - This type of test is not dependent on the size of the equipment under test. It is based on the absorption effect of a good insulation. A good insulation shows continual increase in resistance value over a period of time so the test voltage is applied across the equipment under test for a period of 5 to 10 minutes. The resistance value is taken after 10 seconds, 30 seconds, 60 seconds and every minute thereafter. For an insulation in good condition, the 2-minute reading should be higher than the 60-second reading and the 60-second reading should be higher than the 30-second reading. If the data is collated and plotted on a resistance-time graph the slope should show an uptrend. A flat or downward curve should indicate an insulation in poor condition.
A shortcut to plotting a graph is to take the ratio of two time-resistance readings such as a 60-second reading divided by a 30-second reading known as Dielectric absorption ratio (DAR) or the 10-minute reading divided by a 1-minute reading called Polarization Index (PI). This ratio can give an insight into the condition of the insulation. If the DAR ratio is less than 1 it is safe to say that the insulation failed the test as this would mean that the 30sec resistance value was higher than the 60 sec resistance value. If it is between 1.0 - 1.25 the insulation is in a poor condition and could fail under shock or during starting. If it is above 1.6 then the insulation is in excellent condition. The table below shows general interpretation for DAR and PI values.
Polarization Index Testing or 10-minute Insulation Resistance testing reading is universally accepted as a good way to evaluate motor windings for the build-up of dirt or moisture. We discussed Electrical polarization earlier in this article but failed to mention that the approximate time for complete polarization is usually 10 minutes this is why PI readings can only be gotten after 10 minute readings. Low value of PI indicates that the winding may have been contaminated with oil, dirt or absorbed moisture because PI is basically R10/R1 and the 10-minute reading is expected to be significantly higher than the 1-minute reading but if there is surface contamination a lot of leakage current flows at the same time as absorption current and the megohm readings will not increase with time as fast as an insulation in good condition. This results in a lower polarization index after 10 minutes.
To elaborate
IT = Ic + Ia + IL
By the end of the first-minute Capacitive charging current should have completely decayed so the total current in the system decreases and Resistance increases, Between the second minute to 10 minute as the dielectric get polarized the absorption current decays slowly and resistance rises slowly by the 10th minute whats left in the system should be only Leakage current which is usually very low in a good insulation system leading to very HIGH resistance but a contaminated system would have IL flowing even when Ia is still at a high value not allowing the resistance value to rise.
3) Step Voltage test this type of test simply involves applying an increasing voltage across the equipment under test in steps for the same length of time on each step. It looks at trends in resistance, with respect to varying test voltages. a good insulation should withstand an increase in over-voltage stress and its resistance should remain approximately the same during testing with different voltage levels . If resistance values decrease substantially when tested at higher voltage levels, this should serve as a warning that insulation may have cracked or significantly aged.
1) Short Time or Spot Reading test - in this type of test the voltage is applied for about 30 to 60 seconds. When the test voltage is applied across the equipment under test you will get readings that are constantly rising on your MegOhmeter this is because the equipment under test is charging. At some point when the equipment becomes fully charged the readings will stabilize and you would get the true IR of the equipment under test. This makes it important to note spot reading test is only a true indication of IR when performed on a low capacitance equipment that can become fully charged under 60 seconds.
2) Time Resistance or Dielectric Absorption test - This type of test is not dependent on the size of the equipment under test. It is based on the absorption effect of a good insulation. A good insulation shows continual increase in resistance value over a period of time so the test voltage is applied across the equipment under test for a period of 5 to 10 minutes. The resistance value is taken after 10 seconds, 30 seconds, 60 seconds and every minute thereafter. For an insulation in good condition, the 2-minute reading should be higher than the 60-second reading and the 60-second reading should be higher than the 30-second reading. If the data is collated and plotted on a resistance-time graph the slope should show an uptrend. A flat or downward curve should indicate an insulation in poor condition.
A shortcut to plotting a graph is to take the ratio of two time-resistance readings such as a 60-second reading divided by a 30-second reading known as Dielectric absorption ratio (DAR) or the 10-minute reading divided by a 1-minute reading called Polarization Index (PI). This ratio can give an insight into the condition of the insulation. If the DAR ratio is less than 1 it is safe to say that the insulation failed the test as this would mean that the 30sec resistance value was higher than the 60 sec resistance value. If it is between 1.0 - 1.25 the insulation is in a poor condition and could fail under shock or during starting. If it is above 1.6 then the insulation is in excellent condition. The table below shows general interpretation for DAR and PI values.
Polarization Index Testing or 10-minute Insulation Resistance testing reading is universally accepted as a good way to evaluate motor windings for the build-up of dirt or moisture. We discussed Electrical polarization earlier in this article but failed to mention that the approximate time for complete polarization is usually 10 minutes this is why PI readings can only be gotten after 10 minute readings. Low value of PI indicates that the winding may have been contaminated with oil, dirt or absorbed moisture because PI is basically R10/R1 and the 10-minute reading is expected to be significantly higher than the 1-minute reading but if there is surface contamination a lot of leakage current flows at the same time as absorption current and the megohm readings will not increase with time as fast as an insulation in good condition. This results in a lower polarization index after 10 minutes.
To elaborate
IT = Ic + Ia + IL
Where,
Ic = Capacitive charging current
Ia = Absorption Current
IL = Leakage current
By the end of the first-minute Capacitive charging current should have completely decayed so the total current in the system decreases and Resistance increases, Between the second minute to 10 minute as the dielectric get polarized the absorption current decays slowly and resistance rises slowly by the 10th minute whats left in the system should be only Leakage current which is usually very low in a good insulation system leading to very HIGH resistance but a contaminated system would have IL flowing even when Ia is still at a high value not allowing the resistance value to rise.
3) Step Voltage test this type of test simply involves applying an increasing voltage across the equipment under test in steps for the same length of time on each step. It looks at trends in resistance, with respect to varying test voltages. a good insulation should withstand an increase in over-voltage stress and its resistance should remain approximately the same during testing with different voltage levels . If resistance values decrease substantially when tested at higher voltage levels, this should serve as a warning that insulation may have cracked or significantly aged.
MINIMUM ACCEPTABLE VALUE. Generally, the rule of thumb for the minimum insulation resistance value acceptable for safe energization is 1 MΩ per 1000 V of applied test voltage plus 1 MΩ.
SHOCK HAZARD FROM TEST VOLTAGE. Large electrical equipment and cables usually have sufficient capacitance to store up a dangerous amount of energy from the test current. Make sure to discharge after the test.
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