Fluctuating insulation testing readings: What is the actual measurement?

If you aren’t experienced with insulation testing, you may become extremely frustrated when you are performing a test and the readings keep changing.  From time to time, customers will call us asking why their readings keep changing and what is it that they are doing wrong.  The answer is nothing.  You are not doing a thing wrong.  In fact, you are more than likely doing everything right!

Experienced operators barely notice the changes in measurements as they are performing their testing, but if you are someone who generally works with multimeters, which tend to give stable readings immediately, this can be confusing.

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With an analog insulation tester, the pointer will peg sharply towards the low end as the test begins, then slowly moves towards infinity, which is its rest position.  Digital testers do the same thing, however, it is more difficult to observe a time trend on a digital tester since it only produces ‘dancing’ numbers, where on an analog the movement of the pointer is observable.  This is why nearly all experienced users prefer using analog.

So why do the readings keep changing no matter what type of tester you use? Well, that is easy to answer: you are seeing the effects of charging currents.  Basically, the test voltage pulls not only the resistive (leakage) current – which is really what the operator is interested in, but also the capacitive and absorptive current.  These currents that are being detected are not a problem, per se, only in that they interfere with taking the proper test measurement.

There are other circumstances that interfere with the measurement.  A large capacitance is found in cables with long runs of parallel conductors, wire harnesses, items with large winding, like motors, generators and transformers.  Ideally, the designer would like the current to flow neatly in the direction of the circuit; however, if another wire or turn of the same winding, the current will cut straight across and short circuit if it is not prevented from doing so by the insulation around the wires.

Absorption can also occur, playing with the results.  This takes place in the insulation rather than the circuitry.  Absorption is also a charging current but it is caused by the molecular realignment of the insulating material itself under the influence of the applied voltage field. 

Since insulation is a poor conductor and circuitry is a good conductor, absorption is slow and capacitance is quick and the tester does not distinguish between any of this.  It simply measures the total current flow. 

So with all of that said, what is the correct reading? They all are at some point.  But what the operator really wants to know is, what is the leakage current?  This is the measurement that shows a piece of equipment’s relative condition.  In the easy scenario, when the test item is small, the correct measurement is the one where the pointer stabilizes. With larger pieces of equipment, the measurement is trickier because although it may look like the pointer has stopped, it continues to move for a long time, even after the naked eye can’t recognize the movement.  So in this case, if an operator is performing something like an installation test where all he needs is to ‘meet spec’ just wait for the pointer to hit an acceptable reading and terminate the test.  No need to wait and wait and wait if it passes an inspection point you need.

Look at it this way, in order to make an intelligent judgement about the electrical condition of a test item, it is not necessary to measure to the extreme value, taking more and more time for smaller and smaller increases. Standardize on the time interval.  This will assure that you are always working from the same point on the time-resistance curve.

Knowing these factors and taking them under consideration when performing a test should provide a clear answer to your measurement reading. If you still have concerns, try consulting with another expert or consider updating your equipment. While the most common answer is explained here, sometimes it can be beneficial to upgrade to the latest technology as well.