How to choose the right insulation tester?

Editor’s note: This article comes from Megger, a designer and manufacturer of portable electrical test equipment. Read the company’s blog postings here.

Megger electical testing report

For more tips on choosing the right insulation tester for your project, check out Megger’s Guidebook here.

Choosing the right insulation tester to get a job done can be a bit overwhelming. With so many models on the market, how do you choose the right one? To figure out which tester is right for your application, it is important to understand why this type of testing is crucial.

First, examine good insulation verses poor insulation. Good insulation has high resistance, while poor insulation has relatively low resistance. Wires are insulated with a resistance sheathing in an effort to contain the conductivity of either a copper or aluminum wire. And, although the wires are insulated, some current leaks out. An imperfection in the insulation will allow for electricity to escape, which can be detrimental to electrical circuits and machinery.

By regularly testing the insulation, problems can be identified and fixed before an accident or equipment failure occurs. Now how do you decide what tester is right for your job?

Logic tells you that you should choose a tester based on application-specific needs. This is not necessarily the case. Choosing the right tester is based more on specifications rather than application. Voltage or voltages needed to perform a test, as well as measuring current, are generally prime considerations when picking a unit that is right for you.

However, power source may be a driving factor depending on experience, personal preference and job related situations like the environment you will be working in. Other criteria to consider include user preference and price.

1. A closer look at voltage requirements. Output voltage applied to equipment should be based on the DC insulation resistance test voltage recommended by the manufacturer. If there is no recommendation, visit the InterNational Electrical Testing Association (NETA) website for recommendations of industry best practices.

2. Measuring current is another factor that needs to be taken into consideration. Are you measuring leakage current or polarization current? Leakage current can be found using an insulation tester and provides you with the information to calculate the amount of contamination that is in the insulation.  You can obtain leakage current by taking a spot reading. Simply apply the test voltage to the component under test, then take a resistance reading after 60 seconds. This is an industry standard to determine leakage current through insulation.

To find polarization current, the insulation resistance tester must run longer (approximately 10 minutes) because this type of current takes longer to build up. Users take a reading at one minute and then again at the 10 minute mark.  This will provide you with a polarization index. This type of test should be done monthly as a part of routine maintenance. The most recent results should always be compared to the previous month’s results.

3. The level of personal experience with insulation testers is also an important factor when choosing a tester that is right for you. If you are experienced and have a firm grasp on interpreting readings, then any tester may be suitable for you depending on your application and environment. For those users who are less experienced, simplicity and limited functions are recommended along with some sort of on-the-job training on insulation resistance testing.

4. Environment is key in determining what type of tester to purchase. Will it be used inside or outside? Will it be used in an industrial plant or in a shop? Will it be used for troubleshooting, preventative maintenance or both?  All of these factors will determine the size and type of tester you will be using.

Once the prime considerations have been determined, field of testers can then be narrowed up or down depending on the price point you need to meet.

After a few principle considerations have been taken into account, specific models can be selected largely based on preference. Some other considerations include analog versus digital, computer capability, and automated testing.

No matter what your testing goals are there is an insulation tester that is right for you.


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