Insulation Resistance Testing Quick Tips

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Insulation resistance testing refers to the process of measuring the resistance of electrical insulation materials which are used to isolate conductive materials from each other. It is commonly performed on electrical equipment like cables, transformers, generators, and motors to detect any potential defect and hence, to ensure all the insulation materials are working properly. 

During the insulation resistance test, a voltage is applied to the insulated material, and the resulting current is detected. Ohm’s law is used to calculate the resistance. It is performed using a megohmmeter which can generate a high voltage while measuring low currents. 

Insulation Resistance Testing Made Easy: Tips You Need to Know

To stay safe and to get the best outcomes for insulation testing, it is significant that the testing is conducted correctly. Following are some good working practices:

  1. Use the right test leads. 

Always use a lead set that is intended for the instrument, suitable for the test voltage you’re planning to use, and appropriate for the test object you are working on. If the connections are not made cautiously, the test lead will become accidentally disconnected. This leaves the test object charged to an alarmingly high voltage. 

Avoid using any test lead with indicates any sign of damage. Also, never try to repair damaged or worn leads- replacing them is the safest option. 

  1. Select the best test voltage. 

Test sets today allow testing to be performed at voltages up to 15KV. Higher voltage testing can provide additional and more useful information regarding the situation of the test object’s insulation. Although using a voltage that is too high for a specific test object to withstand can seriously damage it. 

Always check for supplier information for the object under test and follow the guidelines it states about testing. If not, then seek assistance from the producer of your insulation tester. 

  1. Select the most appropriate test. 

A quick one-off insulation test often offers useful data, but modern insulation resistance measurement sets have so much more to offer. This includes features like step voltage, dielectric displacement, ramp tests, dielectric absorption ratio, and polarisation index. 

Full information regarding these features and how to perform them is always mentioned in your instrument handbook. If not, you can also consult the manufacturer. 

  1. Use an instrument with a high measuring range. 

If your device considers all measurements above 1 TΩ as infinity, you may not be able to detect a significant drop in the insulation resistance of your test object, from 30 TΩ to 2 TΩ, since your last test. While the latest result may still be within an acceptable range, such a drastic reduction in resistance could be an early indication of a developing problem. Having an instrument with a wider measuring range would help detect such situations.

  1. Finish the test before disconnecting the test set. 

Test objects hold a lot of charge, and specifically, the stored charge is lethal, when they are being tested at high voltages. Modern testers provide solutions to this issue. This is done by safely discharging the test object when the test has run to termination or when it is completed by the consumer. 

When test leads are disconnected prematurely, the discharge operation cannot be performed, and the test object will remain dangerous and charged. 

  1. Use the guard terminal.

Surface leakage over testing materials such as bushings can highly decrease their apparent insulation resistance, and consequently, there have been several cases of insulators being scrapped when all that was truly significant to clean them. Through a test set guard terminal, you can greatly reduce the effect of surface leakage on the test outcomes. This is done when connected to a bare wire wrapped around the surface of the object under test. 

Moreover, do remember that making two measurements, one with and one without the guard terminal connected, can offer a very great indication of whether or not the insulation requires cleaning. 

  1. Record and trend your outcomes. 

A single insulation resistance measurement offers an intelligent indication of insulation status. However, a series of quantification over time, with the outcomes trending and recorded, will indicate a lot more. If the insulation resistance of your test material is reducing over time then it’s a good opportunity to find out why, appropriately before it reduces to the point of failure. 

Precise records also rapidly indicate any prompt deviation from the usual insulation resistance values, which is always a strong sign that more investigation is required. 


Insulation resistance test is a significant aspect of safety programs and electrical maintenance as they can help identify potential defects before they cause equipment failure or safety hazards. It should be conducted periodically as part of routine maintenance or after any significant modifications or repairs to electrical equipment. 

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