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Insulation Resistance and Polarization Index Test, Step by Step

The insulation resistance test is usually the first electrical test performed on a transformer, and for good reason: it answers the most basic safety question before anything else is attempted. Is the insulation between the windings, and between each winding and earth, still sound? If the answer is no, no other high-voltage test should proceed until the problem is understood, because applying test voltage to damaged insulation can turn a repairable problem into a failure. This guide walks through the insulation resistance and polarization index test the way it is done on site: the equipment, the safety precautions, the connections, the readings, and how to interpret the results.

If you are planning a full commissioning programme, read this guide alongside our power transformer testing and commissioning guide, which shows where insulation testing fits among ratio, winding resistance, vector group and oil tests. For the numbers side of the work, keep our transformer calculators open in another tab. And remember the standing rule of all testing guides on this site: your project specification, the manufacturer's instruction manual and your local safety regulations always override general guidance.

What insulation resistance actually measures

Insulation resistance is the resistance that the insulation system offers to a direct current applied across it. In a healthy transformer, that resistance is very high — hundreds or thousands of megohms — because the insulation is doing its job of keeping current where it belongs. When insulation is wet, dirty, cracked or aged, leakage current finds paths through it and the measured resistance falls. The test therefore gives a single, easy-to-understand number that reflects the overall condition of the insulation, even though it cannot pinpoint exactly where a weakness sits.

It helps to understand what current the test set actually sees. When DC voltage is first applied, several currents flow: a capacitive charging current that decays quickly as the winding capacitance charges, an absorption current that decays slowly as the insulation polarises, and a steady leakage current that continues for as long as voltage is applied. The leakage current is the one that reflects insulation condition, but in the first minute of a test it is masked by the other two. That is exactly why the polarization index exists: by comparing a late reading, when absorption current has largely decayed, with an early reading, the test separates genuine insulation condition from the normal charging behaviour of healthy insulation.

Safety first: isolation, earthing and discharge

No insulation test is ever done on a live transformer. The unit must be isolated from every source of supply, including auxiliary supplies to fans, pumps and tap changer motors, and the isolation must be proved and earthed under the site permit-to-work system. Barriers and warning signs keep other workers clear of the test area, because test leads at several thousand volts are a hazard to anyone who touches them, and the transformer terminals must be treated as live for the whole duration of the test.

The hazard that catches out newcomers is what happens after the test. A winding that has just been tested at high DC voltage stores charge like a capacitor, and it can hold a dangerous voltage long after the test set is switched off. Every test must therefore end with a controlled discharge: the test set's discharge function, followed by applying a portable earth to the winding and leaving it earthed for a period at least as long as the test itself before anyone touches the terminals. Skipping the discharge step is one of the most common causes of electric shock during transformer testing, and experienced engineers treat it as non-negotiable. Personal protective equipment, insulated tools and a second person present during high-voltage testing are part of the same discipline.

Choosing the test voltage and the test set

Insulation testers, commonly called megohmmeters, are available in a range of test voltages, and choosing the right one matters. Small low-voltage equipment is tested at 500 volts or 1 kilovolt. Distribution transformers are commonly tested at 1 or 2.5 kilovolts. Large power transformers are usually tested at 5 kilovolts, and some specifications call for 10 kilovolts on the highest voltage classes. The test voltage should be high enough to stress the insulation meaningfully, but never higher than the manufacturer's recommended test voltage for that winding — overvoltage testing can itself damage insulation, and a test that damages the transformer it was meant to protect has failed completely.

Modern digital insulation testers do much of the work for you: they apply a stable voltage, record readings at set intervals, calculate the polarization index automatically, and store results for the test report. Older analogue sets require the operator to read and time everything manually, which is perfectly workable but demands more attention. Whichever set you use, check its calibration date, inspect the test leads for damage, and confirm the guard terminal is available — the guard is essential for accurate readings on large transformers, as explained below. For help choosing an instrument, see our electrical test tools buying guide, which covers insulation testers alongside multimeters and clamp meters.

Step-by-step: making the connections

A complete insulation resistance test on a transformer measures three combinations, and all three are needed for a full picture. First, high-voltage winding to earth, with the low-voltage winding earthed. Second, low-voltage winding to earth, with the high-voltage winding earthed. Third, high-voltage winding to low-voltage winding, with both windings otherwise isolated from earth. In each case, all terminals of the winding under test are shorted together so the whole winding is at the same potential, and terminals of windings not under test are earthed to prevent induced voltages and to give leakage currents a defined path.

The guard terminal deserves special attention, because it is the difference between a useful reading and a misleading one on a large transformer. Surface leakage current flows over bushings, especially if they are damp or dirty, and this current bypasses the insulation being measured, making the insulation look worse than it is. Connecting the guard terminal to a guard ring or to the bushing skirt intercepts that surface current and diverts it away from the measurement, so the test set measures only the current through the insulation itself. On a wet day or a dirty bushing, the difference between a guarded and an unguarded reading can be dramatic — and the guarded reading is the honest one. Clean the bushings before testing if the specification allows it, and always record whether the guard was used.

Taking the readings: 15 seconds, 60 seconds, 10 minutes

With connections made and the area clear, apply the test voltage and record the resistance at three times: 15 seconds, 60 seconds and 10 minutes. The 60-second reading is the headline insulation resistance value that goes in the report. The 15-second reading, compared with the 60-second reading, gives the dielectric absorption ratio, which is the 60-second value divided by the 15-second value. The 10-minute reading, compared with the 60-second reading, gives the polarization index, which is the 10-minute value divided by the 1-minute value. Both ratios are dimensionless, which is part of their power: because they are ratios of two readings taken on the same winding at the same temperature minutes apart, temperature and winding size largely cancel out, making them far more comparable between transformers and between test dates than absolute resistance values.

During the 10-minute test, watch the reading, not just the final number. On healthy insulation, resistance rises steadily throughout the test as absorption current decays — the curve climbs and keeps climbing. On wet or contaminated insulation, resistance rises a little and then flattens, because leakage current dominates and there is little absorption left to decay. An experienced tester can often judge the result from the shape of the curve before the calculation is even done. Record the full set of readings, the test voltage, the winding and ambient temperatures, the humidity if known, whether the guard was used, and the oil condition if an oil sample was taken at the same time — future engineers comparing results will need all of it.

Interpreting the polarization index

As a general guide for oil-impregnated transformer insulation, a polarization index of 2 or above is considered good: the insulation is dry and healthy, and absorption is behaving as it should. A value between 1 and 2 is questionable and calls for investigation — it may reflect moisture, contamination or simply a cold, damp test day, and the honest response is to check the oil moisture, clean and dry the bushings, and re-test under better conditions before drawing conclusions. A value close to 1 is a warning: the insulation is showing little absorption, which usually means it is wet or contaminated, and the transformer should not be energised or subjected to further high-voltage testing until the cause is found and corrected, typically by drying or oil treatment.

There are important exceptions to keep in mind. Very large transformers can show lower polarization index values simply because their capacitance is so large that absorption takes longer than 10 minutes to decay; on such units the trend over time matters more than any single value. Dry-type transformers and some modern insulation systems have different expected ranges, so the manufacturer's guidance for that specific unit always takes precedence over general rules. And a polarization index is never read alone: it is read alongside the absolute insulation resistance, the oil test results — see our transformer oil testing guide — and the factory test certificate. A transformer whose polarization index has fallen from 3 to 1.5 since its last test has a story to tell even though 1.5 is not yet in the danger zone, and that story is usually about moisture getting in somewhere.

Temperature correction: comparing like with like

Insulation resistance is strongly temperature-dependent: as a rough rule of thumb used throughout the industry, resistance approximately halves for every 10 degrees Celsius rise in winding temperature, and doubles for every 10 degrees fall. That means the same healthy transformer can test at very different resistance values on a cold morning and a hot afternoon, and comparing those two numbers directly would be meaningless. The standard solution is to correct every reading to a common reference temperature — commonly 20 degrees Celsius — using the correction factors in the applicable standard or the manufacturer's guidance, and to always record the winding temperature at the time of test so the correction can be checked later.

Polarization index and dielectric absorption ratio need no temperature correction, because they are ratios of readings taken minutes apart at essentially the same temperature. That is one more reason they are so useful in the field: when the temperature correction factors are uncertain, or when winding temperature cannot be measured reliably, the ratios still give a trustworthy verdict. For the absolute resistance values, though, never compare uncorrected readings from different days and draw conclusions from them — correct first, compare second, and say so in the report.

Common mistakes and a reporting checklist

The mistakes that spoil insulation tests are almost all procedural. Testing without discharging and earthing afterwards. Forgetting the guard on a damp day and condemning healthy insulation. Testing a transformer that has just been oil-processed, when the oil is still full of bubbles, and recording a misleadingly low result. Using a test voltage higher than the manufacturer allows. And the most common of all: recording a single resistance value with no temperature, no guard note and no time stamps, leaving the next engineer unable to compare anything. Each of these is avoidable with a checklist and a little discipline.

A complete test record for each winding combination should include: transformer identity and tap position, test date and time, test voltage, guard used or not, readings at 15 seconds, 60 seconds and 10 minutes, calculated absorption ratio and polarization index, winding and ambient temperature, corrected resistance at the reference temperature, oil sample reference if taken, and the name of the tester. File it with the factory test certificate and every previous site test for that transformer, so the trend is visible at a glance. That file is the transformer's insulation history, and it is what turns a single test from a number into a diagnosis. For the companion topics, read our guide to transformer vector groups and our clamp meter buying guide.

Frequently asked questions

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