Home / Blog / What Is Winding Resistance And Why Do We Measure It
03-10-2025
What is winding resistance and why do we measure it?
Measuring the winding resistance of a transformer is an essential step in inspection and maintenance. Even small deviations in resistance can indicate loose connections, wear on tap changers or incipient short circuits in the windings. Performing this test provides valuable information about the condition of your transformer and reduces the risk of unexpected malfunctions or costly outages.
Measuring winding resistance in a transformer
The winding resistance is the DC resistance of the copper (or aluminum) wire in the transformer windings, including connections, joints, and tap/selector switches (OLTCs) where relevant. In ideal condition, this resistance is very low (milliohms or microohms) for the main windings.
Because resistance is temperature-dependent (for copper, the temperature coefficient is ~0.004/°C), temperature correction must always be considered when comparing measurements.
Why measure this resistance?
The winding resistance test (also referred to as the “DC winding resistance test”) is a form of continuity check. Problems that can be detected include:
- Loose or poorly contacting connections (e.g. clamps, welds)
- Poor contacts in the tap changer (OLTC), or wear/internal issues across different tap positions
- (Partially) short-circuited windings or unintended parallel paths
- Changes in resistance over time, which may indicate degradation
- Unequal resistance values between phases in three-phase transformers
According to common guidelines, an unexpectedly high or low resistance value may indicate defects or abnormalities in the current path (conductors).
Relation to other tests
- The winding resistance test is often performed alongside “turns ratio” and excitation current tests.
- Because the transformer has a magnetic core, the core must be saturated to obtain a stable measurement.
- After the DC test, demagnetization must be carried out to avoid inrush currents when returning the transformer to service.
Measurement principle and challenges
Measurement principle – Ohm’s law
The principle is simple: inject a known DC current (I) into the winding and measure the voltage (U) across it. Resistance = U / I.
In practice, however, the process is more complex due to:
- Inductance of the winding
- Magnetization of the core
- Energy dissipation and time constants
- Sensitivity to disturbances or accidental lead disconnections
Core saturation
Before a stable resistance can be measured, the core must be saturated (so flux variations or magnetic effects no longer influence the result).
- Too low test voltage/current ramp may lead to incomplete saturation ⇒ incorrect values
- The time required depends on inductance, applied voltage/current, and winding resistance
Inductive effects and voltage spikes
A critical safety factor: if a current lead disconnects unexpectedly during the test, the inductance may generate a high voltage spike – posing a risk.
Therefore, test instruments must include a safe discharge path and protection against current interruption.
Temperature and correction
Resistance values are highly dependent on temperature. To compare measurements under different conditions, results are often corrected to a reference temperature (e.g. 20 °C or 25 °C). International standards (e.g. IEEE, IEC) provide correction formulas for copper or aluminum.
Test voltage compliance
The test instrument must provide sufficient voltage compliance (ability to supply voltage) to drive current through the winding despite resistance and inductance. Instruments are typically designed for voltages of 50–60 Vdc.
How HighTest instruments support this
HighTest offers several models in their winding resistance tester range. Key features include:
Feature |
Explanation / benefit |
| Measurement range (from µΩ to Ω) | The WINRES series measures from 0.1 µΩ to 100,000 Ω. |
| Dual measurement channels | Allows simultaneous testing of two windings (e.g. primary and secondary) |
| Automatic demagnetization | Ensures residual magnetism is removed after each test |
| Temperature sensor / automatic correction | Compensates results to reference temperature automatically |
| Intelligent decision criteria | Stops measurement automatically once stability is reached |
| Safety functions | Provides automatic discharge in case of current interruption or fault |
| OLTC / tap changer support | Continuous current during tap transitions and detection of voltage drops |
| Three-phase testing (TRIORES series) | Measure primary and secondary simultaneously without reconnecting leads |
Thanks to these features, HighTest equipment is well-suited to meet the challenges of winding resistance testing in real transformer diagnostics.
Interpretation & diagnostics
Phase-to-phase comparison
For three-phase transformers, resistance balance between the three phases is critical. A deviation > 2–3% may indicate connection imbalance or contact wear.
Comparison with reference values
Compare measurements with factory specifications or historical data. Since temperature strongly affects resistance, always apply correction to the same reference temperature (using the correction formula).
Temperature correction formula (copper):
- = Temperature compensated resistance
- = Measured resistance (uncompensated)
- = Standard temperature (°C)
- = Measured temperature (°C) during test
- = Temperature constant (e.g. 234.5 °C for copper)
Tap resistance profiles (OLTC)
For transformers with on-load tap changers (OLTC), you can measure the resistance in each tap position and generate a resistance vs. tap position graph. Deviations or irregularities in the profile often indicate poor contacts or contact wear.
You can also perform a dynamic measurement (monitor resistance during the switching operation) to detect transition issues.
Diagnostic signals
| Observation | Possible cause |
| Sudden high resistance on one phase | Poor connection, burnt contact, internal damage |
| Low resistance on one phase compared to reference | Possible short circuit or parallel conduction |
| Irregular resistance pattern across taps | Issues in OLTC contacts, poor transition between contacts |
| Changing resistance over time | Deterioration / oxidation / loosening of connections |
| Unstable measurement or drift | Incomplete core saturation, cold connections, measurement errors |
Limits and warnings
- A small deviation in resistance does not automatically indicate a defect – tolerances or standards may apply.
- A large phase-to-phase difference or deviation from historical data are stronger indicators of issues.
- Temperature correction is essential to make measurements comparable.
- After a DC test, demagnetization must be performed to avoid residual magnetization and high inrush current at re-energization.
Practical considerations
Cable quality and connections
Ensure test leads are of low resistance and high quality, with solid, reliable connections at both transformer bushings and instrument side.
Gradual current build-up
Do not inject the maximum test current abruptly – increase slowly to avoid voltage spikes and overshoots.
Full disconnection from voltage sources
Make sure the transformer is de-energized during DC testing, and that other circuits are disconnected.
Stabilization period & monitoring
Wait until the measured value is stable (based on intelligent criteria such as those in HighTest instruments) before recording the result.
Demagnetization after test
Ensure the instrument demagnetizes the core after the test to remove residual flux and minimize inrush currents at re-energization.
Test environment and temperature recording
Record the temperature at the time of measurement (or use sensors) so correction can be applied.
Logging & trending
Store results and compare them with previous measurements as part of condition-based maintenance.
Safety procedures
- Ensure test leads are properly secured and cannot disconnect during testing
- The instrument should provide automatic discharge in case of power loss or fault
- Be aware of inductive voltage spikes when disconnecting
Winding resistance testing is often an integral part of the heat-run test for transformers.
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