An insulation resistance test measures the resistance of the insulation system between a winding and ground, or between windings, by applying a DC voltage and measuring the resulting leakage current as a resistance value. The procedure produces the raw data that IR, PI and DAR interpretation use.
Performing it correctly means preparing a safe test circuit, selecting a test voltage appropriate to the winding insulation, connecting so that surface leakage is controlled, applying the voltage for a defined time, discharging the winding afterward, and recording the conditions that make the result comparable with earlier tests.
Prepare and Verify the Test Circuit Is Safe
The transformer must be isolated and locked out from every energy source, and zero energy must be verified at the terminals before the tester is connected. The winding may hold charge from previous testing or from capacitive coupling, so the preparation includes discharging and verifying before the connection is made. The test area is controlled while the high-voltage tester is in use, and the operator confirms that no one is touching the circuit before the voltage is applied.
Record the transformer identity, the winding to be tested, the tap position, the winding temperature and the ambient conditions before starting. The temperature is essential for interpretation, because insulation resistance changes strongly with temperature, and a reading without its temperature context cannot be compared with earlier results.
Select Test Voltage by Winding Rating Context
The test voltage is chosen in relation to the rated voltage of the winding and the guidance that governs the test. The purpose is to stress the insulation enough to produce a meaningful measurement without exceeding what the insulation system is designed to withstand in service. Different standards and manufacturer instructions give different voltage guidance for the same winding class, so the correct choice comes from the document that governs your test, not from a universal table.
Record the voltage used with every reading. A change of test voltage between tests changes the measured resistance, so results are only comparable when the same voltage, the same duration and the same connection arrangement were used. If the procedure allows a range of voltages, choose the value defined by the governing document and keep it constant for the unit and its history.
Connect for Winding-to-Ground and Inter-Winding Checks
Insulation resistance is measured between defined points: winding to ground, between windings, and sometimes between phases within a winding group. The connection arrangement determines what the reading represents, and it must match the record. For a winding-to-ground test, connect the high-voltage lead to the winding terminals and the return to the grounded tank or frame, with the other windings handled according to the procedure, usually grounded or floating as the test defines.
Use the guard terminal to control surface leakage where the measurement demands it. Surface leakage along bushing porcelain, terminal boards or damp surfaces can dominate the reading and make healthy insulation look poor. The guard redirects that surface current away from the measuring circuit so that the reading represents the insulation volume rather than the surface condition. Guarding is not always required, and the procedure should state when it is used.
Apply Voltage and Record at Defined Intervals
Apply the test voltage and hold it for the defined duration while the charging and absorption currents decay. The standard record is the resistance at one minute, and where PI and DAR are required, the readings at thirty or sixty seconds and at ten minutes are recorded as well. The timed readings describe the polarization behaviour of the insulation, which is what the ratios interpret.
Watch the reading during the test. A resistance that rises steadily and then stabilizes is the normal behaviour of healthy insulation; a reading that falls during the hold period, or that fluctuates, is a signal that deserves attention even before the final value is interpreted. Note any instability in the record, because it may indicate partial discharge, surface effects or a connection problem rather than the insulation volume.
Discharge and Guard Against Recharge
At the end of the test, the winding holds the charge applied by the DC voltage, and it must be discharged before the connections are changed or the transformer is left. Use the tester’s discharge function, allow the discharge to complete, and verify with a voltmeter that no voltage remains. On a large winding, the charge can reappear after the first discharge as the insulation relaxes, so verify again immediately before touching the terminals.
Discharge is also the protection against surprises between tests. If several measurements are planned on the same transformer, discharge and verify between every connection change, and ground the winding if it will be left while other work continues. The discharge step is part of the safety chain, not an optional end-of-test courtesy.
What to Log for Meaningful Comparison
| Log field | Purpose |
|---|---|
| Winding, terminals and guard use | Defines what the reading represents. |
| Test voltage and duration | Makes the result comparable with earlier tests. |
| Timed readings (30 s, 60 s, 10 min) | Provides the data for DAR and PI. |
| Winding and ambient temperature | Enables temperature-aware interpretation. |
| Instrument and calibration | Keeps the record traceable. |
| Observations during hold | Captures instability that raw values miss. |
The logged data is what the interpretation article in this series uses to separate a real change from seasonal or setup variation. A complete log converts a single reading into evidence that can be compared next year.
Weather and surface condition influence the measurement in ways the log must capture. A damp morning, condensation on the bushings or recent rain can depress an insulation resistance reading by creating surface leakage that has nothing to do with the insulation volume, and the guard connection is the tool that separates the two. When a low reading appears on a damp day, clean and dry the accessible surfaces, repeat the measurement with guarding, and compare the two results before concluding that the insulation has deteriorated. The same discipline applies between windings: measure winding to ground and winding to winding as separate records, because a low inter-winding reading and a low winding-to-ground reading point to different insulation paths and different possible causes. On a three-phase transformer, also compare the phases under identical conditions, because a single low phase is a stronger signal than a uniform reduction across all three, which may again point to a common surface or weather effect rather than to the winding insulation itself.
The order of measurements can protect the evidence. Perform the insulation resistance test before tests that inject current into the windings, or after demagnetisation following such tests, so that the insulation readings are not distorted by the electrical state of the core. Keep the tester and leads dry, and check the leads themselves before use, because a damaged lead with a tracking path can add a parallel leakage route that no guard arrangement can fully remove. When several windings are tested in sequence, discharge and verify between each measurement, and record the time of day and the weather as part of the log; those details are what allow a later engineer to separate a real insulation change from a change in the conditions under which the test was performed.
Frequently Asked Questions
What test voltage should I use for a transformer insulation test?
The voltage is chosen in relation to the rated voltage of the winding and the guidance that governs the test. Different standards and manufacturer instructions give different values for the same class, so use the voltage defined by the governing document and keep it constant for the unit’s history.
When is the 60-second reading taken?
The one-minute reading is the standard insulation resistance record, taken sixty seconds after the test voltage reaches its set value. Where DAR or PI are required, additional readings are taken at thirty or sixty seconds and at ten minutes so the ratios can be calculated.
Why must the winding be discharged after the test?
The DC test voltage charges the winding capacitance, and the charge remains after the tester is disconnected. The winding must be discharged and the voltage verified before connections are touched, and the discharge should be verified again because charge can reappear as the insulation relaxes.
For the instruments used in this test, see the transformer maintenance test equipment range, and for interpretation of the results read the insulation resistance, PI and DAR interpretation guide in this series.