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What is insulation testing and why it should be done?

What is insulation-resistance?

The insulation resistance can be described as the resistance offered by the insulation which covers a conducting wire that carries electrical current. The resistance that is offered by the insulation of the cable in the leakage current path is called the insulation resistance of the cable. Certain factors would affect the insulation resistance test such as the installation of the device, operating conditions, and the type of usage.

Why should we do insulation testing?

When an electric current is sent through a conducting wire it would reach the destination but due to certain reasons some of the electric current would be lost along the way. So we need to do the insulation testing to check if there is any leakage of current and this current leakage could be harmful. Insulation resistance testing is really important to prevent damage and injury. This test must be conducted to ensure the reliability of the electrical systems and also the equipment. The electrical resistance of insulating material is to direct a voltage, and it can be detected by measuring the leakage of current which flows through the insulation. By doing an insulation resistance test we can determine if the equipment is in proper condition, and we can almost detect the service life of the insulation by this test.

The insulation testing must be done when installing a new electrical machinery, so while testing this equipment we can make sure that the insulation is in proper condition for the operation and it would also be useful as a reference for the future testing.  

What are the problems that could happen if there is an insulation failure?          

  • Electric shocks could be a problem
  • Burn injuries or fire accidents could happen
  • Plant operation could be interrupted

What is a good insulation?

Most of the electric wires in an industry will be covered by electrical insulation, so mostly all the electrical equipment’s power wires will be insulted. The wire could be copper or aluminum which will be a good conductor and the insulation will resist the current and will keep the current in its path along the conductor. So an important matter which should be considered is that no insulation is perfect and because of that some of the electric currents would flow through the insulation to the ground. This small current will be one microampere so it is very low and this is the major part of the insulation testing. If the voltage is high then a high current will flow through the insulation, this won’t be a problem if the insulation is good but if the insulation deteriorates then it is a big problem.

What is the reasons for the insulation to go bad?

  • It could be because of vibration
  • Excessive heat or cold
  • Vibration
  • Dirt
  • Oil
  • Corrosive vapor
  • Moisture from the processes
  • Humidity can also be a reason for the insulation failure

Insulation could go bad if the conductors are subjected to mechanical stress, the mechanical stress is caused by the short circuit current, thermal expansion, contraction of the conductors and it can also happen because of vibrations. So we need to check the insulation frequently and the insulation tests must be conducted.

What is a total current in insulation testing?

The testing of insulation resistance in a cable is done to determine the resistance value to the current flow across it. If there is a high resistance then it means a very low current is escaping and if the measured value of the resistance is low then it means the leakage is high. We can do the insulation test by pressurizing the conductor with a given voltage and dividing it by the current that escapes the insulation. Certain factors would affect the current flow through insulation and they are capacitive current, absorption current, and leakage current.

Capacitive current

This current is the initial surge of the current which occurs when the voltage is applied to the conductor. So the initial surge will be high and it would reduce when the conductor is fully charged.


The absorption current is similar to the capacitive current it also starts at high and then drops. So when the voltage builds up then the absorption level in the insulation would change from high to low and this change would affect the storage of the potential energy in the insulation. The absorption current is required while doing the time resistance method of insulation testing.

Leakage current

The leakage current is the small steady current that is present through and over the insulation and it is also called as conduction current. If there is an increase in leakage current then it would be because of the deteriorating insulation.

What are the procedures that must be done before doing insulation testing?

We should check certain conditions before starting the insulation testing, we must do a visual inspection to make sure that the surface dust and moisture have been removed from the component which is going to be tested. We must isolate the component from other connected systems. We must also check that if the component which is going to be tested have been isolated from the system ground, and equipment like lightning-arrester, capacitor, and control transformer.

What are the factors that would affect the insulation test?

The insulation resistance value is affected by various factors like temperature, humidity, moisture. We must note the ambient temperature value during the test, the relation between the temperature and insulation resistance is inversely proportional. 

How insulation test is done?

The insulation tester leads must be connected between one phase conductor and earth. We must select the proper test voltage and duration for the testing. Before disconnecting the test leads the test object must be discharged through the ground. The actual insulation resistance could be high or low according to the factors such as temperature and also the moisture content of the insulation

What are the types of insulation resistance test?

Proof testing

This type of insulation testing would be very useful while the installation of the new electrical equipment a proof testing can determine the errors in the system and also the incorrect installation. This test is done by applying a DC voltage through the de-energized circuit using an insulation tester. In order to do a successful test, there shouldn’t be any failure during the duration of the measurement. The proof testing voltages will be very high when compared to the routine maintenance test method. We can determine the test voltage by checking the nameplate of the device which is going to be tested. The proof test can be done in any equipment without considering its capacitance, mostly it is done with a single voltage and this voltage will be between 500 and 5000v and the test duration will be one minute. The test voltage will be higher than the manufactures rated voltage.

Time resistance method

In this type, the test voltage will be applied for a certain period of time it could be ten minutes. So after applying the voltage the readings are taken up to three or four times in a minute and this will be done for the whole process. So readings will be taken, for almost ten minutes. So this testing is done by taking successive readings at the fixed time intervals and this testing will be really effective if there is a presence of moisture or contaminants. So as we discussed above the absorption current will be initially high and gradually it will be reduced. So if the insulation is good then the resistance level will increase but if the insulation is not good then resistance will be flattened out.

Step voltage test

In this type of test, the insulation will be tested by using two or more voltages, and then their result would be compared. If the insulation is good then it will show a consistent resistance regardless of the applied voltage. So if the resistance level decreases if there is an increase in voltage then the insulation is aging and it won’t last long. The test voltage is started at low voltages and then the voltage is increased and the duration of the test could be one minute.

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