A winding resistance test measures the DC resistance of a transformer winding by injecting a stabilized direct current and measuring the voltage drop. The procedure produces the raw data that temperature correction and result evaluation turn into a defensible comparison with the factory baseline.
Performing it correctly means preparing and isolating the transformer, connecting the test leads so that lead resistance does not enter the measurement, choosing a current and waiting until the reading settles, measuring every phase and tap position that the procedure requires, and discharging the winding before any connection is changed.
Prepare the Transformer and Verify Isolation
The transformer must be isolated, locked out and verified at zero energy before the test leads are connected. Confirm that the windings are disconnected from the system on every side, that no auxiliary supply can back-feed the circuit, and that the test area is controlled. The isolation and verification discipline is described in the transformer testing safety article in this series and applies fully here, because the winding can hold charge and the test itself stores magnetic energy that must be discharged afterward.
Record the transformer identity, the tap position, the winding temperature and the ambient conditions before the measurement starts. The winding temperature is essential for the later correction to the reference temperature, and it should be measured or estimated at the winding rather than taken from the air temperature alone.
Connect Test Leads with Kelvin Principles in Mind
The measured resistance of a transformer winding is small, often well under one ohm, so the resistance of the test leads and connections is not negligible. Four-terminal or Kelvin-style connections separate the current path from the voltage-sensing path: the current leads inject the test current, and the sense leads measure the voltage drop across the winding itself, excluding the lead resistance. Connect the sense leads inside the current leads at the winding terminals so that the voltage measurement spans the winding rather than the leads.
Clean and tighten every connection point. A corroded or loose terminal adds resistance that the four-terminal arrangement cannot fully exclude if it lies inside the sense loop. Confirm the connection to the correct phase and terminal according to the test plan, and record which terminals were used so that the measurement can be repeated exactly on the next test.
Choose the Current and Wait for Settling
The test current should be high enough to produce a stable voltage drop but low enough to avoid unnecessary heating of the winding. The right current depends on the winding resistance and the transformer size, and the selection logic is covered in the dedicated test current guide in this series. Whatever current is chosen, the reading must not be taken until the current has stabilized and the inductive transient of the winding has decayed, because a reading taken during the transient is not the true DC resistance.
Settling time varies with the winding and the test current. Large power transformer windings can take a considerable time to reach a stable reading, and the practical discipline is to watch the displayed value until it stops changing rather than to assume a fixed settling period. Some instruments indicate stability automatically; when they do not, allow the reading to settle and confirm it by repeating the display check before recording the value.
Measure Each Phase and Tap Position
The test plan defines which phases and tap positions are measured. On a three-phase transformer, measure each phase and compare the readings with the factory baseline phase by phase, because a single-phase anomaly is more significant than a uniform shift. Where a tap changer is fitted, measure at every tap position required by the procedure and record the tap with each reading; the resistance changes with tap position, and the comparison is only valid between the same positions.
Between measurements, the connections are changed and the safety discipline repeats: discharge and verify before disconnecting, then connect the next phase or tap. The rhythm of the work is deliberately slow because the errors that destroy the value of the test are connection errors and premature readings, not a lack of measurement speed.
Discharge and Verify Before Disconnecting
When the test current is interrupted, the winding is an inductive circuit storing magnetic energy, and it will produce a voltage as the energy decays. Use the tester’s discharge function and wait for the instrument to indicate that the winding is safe, then verify with a voltmeter that no voltage remains before the connections are touched. On large windings the discharge can take time, and the verification is the step that protects the operator from the stored energy.
Do not disconnect a live test circuit to save time. The few seconds saved by breaking a connection under current can produce an arc and a hazardous voltage, and it can also corrupt the measurement by interrupting the current before the reading is valid. Discharge, verify, then disconnect, in that order, every time.
Record Conditions and Results for Comparison
| Record field | Purpose |
|---|---|
| Transformer and winding identity | Ties each reading to one unit and one winding. |
| Tap position and terminals | Makes the measurement repeatable on the next test. |
| Test current | Documents the condition of the measurement. |
| Winding temperature | Enables correction to the reference temperature. |
| Raw and corrected resistance | Preserves the data and the calculation for review. |
| Instrument and calibration | Makes the record traceable to a calibrated source. |
Record the raw value together with the corrected value, the temperature, the current and the settling behaviour. The evaluation of the results, including the acceptable difference from the factory baseline, is covered in the winding resistance result evaluation guide in this series; the procedure’s job is to produce data that evaluation can trust.
Settling behaviour deserves its own attention on large units. The winding behaves as a series resistance with a large inductance, and the time constant of the circuit can be significant; the displayed current rises toward its set value and the voltage measurement only becomes meaningful once the current is stable. Some testers apply the current, wait for the inductance transient to decay, and only then begin the resistance calculation, which is the correct sequence. If the tester displays a reading during the transient, that reading is not the DC resistance and should not be recorded. A practical check is to repeat the measurement on the same phase without changing anything: two identical measurements that agree confirm the reading was stable, while disagreement indicates that the current or connections were not settled. This repeat check is inexpensive compared with the cost of evaluating a false result later, and it is especially valuable on the largest winding, where settling is slowest and the temptation to record an early reading is strongest.
The timing of the test relative to de-energization also affects the result. A winding that has recently carried load is warmer than the rest of the transformer, and measuring before the temperature stabilizes produces a reading that cannot be corrected reliably. Where the test plan allows, let the unit cool to a stable temperature, or measure at a consistent point in the cooling curve and record it, so that the temperature input to the correction is meaningful.
Once the measurements are complete, compare the raw pattern before applying any correction logic. The phase-to-phase spread at the same tap position, and the progression of values across tap positions, should follow the transformer’s expected pattern; a value that jumps out of sequence is worth re-measuring before the test leaves the site. Compare the corrected results with the factory baseline and with the previous site test, and report both comparisons, because a change since the last site test may indicate a developing condition even when the factory comparison is still within tolerance. The record should close with the conclusion: values compared, basis used, result within or outside the governing criterion, and any recommended follow-up. A procedure that ends with a signed, complete record has done its job; a procedure that ends with a display reading that was never written down has not.
Frequently Asked Questions
What test current should I use?
The current should be high enough for a stable reading but low enough to avoid unnecessary heating, and the right value depends on the winding resistance and transformer size. The selection logic is explained in the test current guide; in general, use the lowest current that gives a stable, repeatable reading within an acceptable settling time.
How long should I wait for the reading to settle?
Wait until the displayed value stops changing rather than assuming a fixed time. Large power transformer windings can take a long time to settle because of the inductive transient, and a reading taken before stabilization is not the true DC resistance. Use the instrument’s stability indication where available.
Which windings and tap positions should I measure?
Measure every phase and every tap position required by the test plan, and compare each reading with the factory baseline at the same phase and tap. The comparison is only valid between the same positions, because winding resistance changes with tap position.
For the instruments used in this test, see the winding test equipment range, and for the wider programme read What Tests Are Required for a Power Transformer.