Q and A: Insulation resistance testing

Electrical Tester - 1 June 2019

Insulation resistance testing is one of the most common test techniques applied to electrical equipment of all kinds, so we make no apology for taking this opportunity to look at a few basic questions we often get asked in relation to this important topic.

Q: What is the reason for performing insulation resistance testing? 

A: This type of testing is usually quick and relatively easy to perform, and it requires only commonly available test equipment. It can, however, provide an invaluable early warning of potential weaknesses in insulation that, without attention, might ultimately lead to a costly and disruptive in-service failure. 

Q: How does a polarisation index (PI) test differ from an ordinary insulation test? 

A: In a polarisation index test, the insulation resistance of the test object is measured continuously for a period of 10 minutes. The test set then automatically displays the ratio of the resistance measured after 10 minutes to the resistance measured after one minute. This ratio is the polarisation index. If the insulation is in good condition, the PI will be 2.0 or higher. If the result is lower than this – ratios of less than 1.0 are possible – this is indicative of a problem. This might be nothing worse than surface contamination, but all PI measurements of less than 2.0 suggest the need for further investigation. 

Q: What does the dielectric absorption ratio (DAR) test do? 

A: The DAR test is a timed insulation test like the PI test, but the times involved are shorter. With a DAR test, the insulation is resistance is measured after the test voltage has been applied for 30 seconds, and again after it has been applied for 60 seconds. The DAR is the ratio of the two results. Since the test times are shorter, the DAR measurement is usually lower than the PI measurement. Typically, the DAR will be 1.4 or higher if the insulation being tested is in good condition. 

Q: What is a step voltage (SV) insulation test? 

A: For the SV test, two or more short-time insulation tests are carried out at different test voltages. The first test uses a low test voltage, such as 500 V, and the second test applies an overvoltage, such as 2.5 kV. An ideal insulator will give the same readings irrespective of the test voltage. If, however, the higher voltage measurement shows a lower value of resistance than the low voltage test, this suggests that there may be cracks or voids in the insulation. 

Q: What is a ramp voltage insulation test? 

A: The ramp voltage test can be seen as a “gentler” alternative to the SV test. Instead of increasing the test voltage in steps, this test ramps the test voltage up slowly until the desired overvoltage is reached. The advantage of this procedure is that if, during the steady increase in test voltage, the current in the insulation under test starts to increase rapidly, the test can be terminated before the insulation is permanently damaged. 

Q: Some insulation testers include filtering. What does this do and how does it improve results? 

A: Filtering is provided to reduce the effects of electrical noise on the results produced by the insulation tester. It is particularly useful when carrying out insulation tests in noisy environments such as industrial installations and electrical substations. Insulation testers may include both hardware and software filtering. The performance of the hardware filter is usually specified as a milliampere value for noise rejection. 

Typically, standard instruments intended for general purpose and industrial applications will offer 3 mA of noise rejection. High-end models, which are suitable for use in substations and similarly demanding locations, offer up to 8 mA of noise rejection. Additional software filtering is often provided to smooth out momentary erroneous measurements and to improve the stability of the readings. 

Q: Can general-purpose insulation testers be used for testing transformers? 

A: General-purpose insulation testers can be used to carry out spot tests on transformers but, because of the complex oil-cellulose insulation system used in them, it is not easy to accurately interpret the test results. Insulation resistance tests are favored as both a first test on a transformer that has tripped out of service because it provides a quick determination as to whether the insulation system has failed completely and as the correct test with which to assess a transformer’s core insulation. Otherwise, insulation testing on transformers should be performed using dedicated transformer test equipment.