Getting a reading on your winding resistance meter is only half the job. The real skill is knowing what that number means — whether it indicates a healthy transformer, a developing fault, or something that needs immediate attention.
📖 This guide focuses on reading interpretation. For the full test procedure, instrument selection, and 4-wire Kelvin method, see our Complete Winding Resistance Meter Guide.
Step 1 — Compare Against the Nameplate Value
Every transformer has a nameplate resistance value (in mΩ or Ω) for each winding, measured at a reference temperature (usually 75°C). Your first comparison is always against this number after applying temperature correction.
A reading significantly higher than the nameplate points to increased resistance in the circuit — a loose joint, corroded connection, or deteriorating contact. A reading significantly lower suggests shorted turns. If nameplate data is unavailable, use the first commissioning test record as your baseline.
Step 2 — Check Phase-to-Phase Imbalance
For three-phase transformers, measure all three phases (R, Y, B on HV; r, y, b on LV) and compare them against each other. A healthy transformer shows very similar resistance across all three phases.
| Phase Imbalance | Verdict | Likely Cause |
|---|---|---|
| Below ±2% | ✓ Normal | Healthy winding — no action needed |
| ±2% to ±5% | ⚠ Monitor | Minor asymmetry — record and trend over time |
| Above ±5% | ✗ Investigate | Loose joint, corroded connection, or tap changer issue |
| One phase much lower | ✗ Urgent | Probable inter-turn short — take out of service |
| One phase much higher | ✗ Investigate | Open circuit, high-resistance joint, or broken strand |
Step 3 — Apply Temperature Correction
Winding resistance varies directly with temperature. You cannot meaningfully compare a reading taken at 35°C in the field with a factory test at 75°C without correction. The standard formula for copper windings:
Where Rₑ₅ is the resistance corrected to 75°C, R_measured is your meter reading, and T_winding is the winding temperature at the time of test. For aluminium windings, replace 235 with 225. Always record winding temperature before starting — without it, the reading cannot be reliably compared to any historical or nameplate value.
Step 4 — Read the Tap-by-Tap OLTC Pattern
For transformers with an On-Load Tap Changer (OLTC), measure resistance at every tap position — not just the nominal tap. Resistance should change smoothly and predictably as you step through the taps.
What a healthy OLTC pattern looks like
Each tap step changes the number of active winding turns, so resistance increases or decreases by a consistent increment from tap to tap. The progression from tap 1 to the highest tap should be smooth — no sudden jumps, no reversals.
OLTC fault patterns to watch for
Contact Wear or Pitting at That Tap
A sharp resistance increase at a single tap, with normal values on adjacent taps, indicates worn or pitted contacts at that position. The OLTC may fail under load at that specific tap.
Transition Resistor Failure
An abnormally large step between two adjacent taps — much larger than expected from the winding geometry — points to a failed or degraded transition resistor in the OLTC mechanism.
Intermittent Mechanical Contact
If the reading at one tap position refuses to stabilize, or varies each time you test it, the tap selector contact is making intermittent electrical contact — a mechanical OLTC issue requiring maintenance.
Drive Mechanism or Shared Contact Failure
When all tap positions show higher-than-expected resistance, the fault is likely in a shared path — the OLTC drive mechanism, a common contact, or the winding connections to the tap changer.
Step 5 — Compare with Historical Records (Trending)
A single reading in isolation tells you only part of the story. The real diagnostic power of winding resistance testing comes from trending — comparing today's values against the commissioning reading and every subsequent test.
Normal Ageing
A gradual, very small increase in resistance over many years is normal ageing behaviour. As long as phase balance and absolute values are within limits, no action is needed beyond continued monitoring.
Developing Joint or Contact Degradation
A consistent upward trend across multiple test cycles — even if the absolute value is still within tolerance — signals a joint or contact that is slowly deteriorating. Plan for inspection at the next available outage.
Acute Fault — Investigate Immediately
A resistance that jumps significantly between two successive tests (with no major maintenance in between) is a red flag. Something has changed inside the transformer — a joint has loosened, a strand has broken, or OLTC contacts have degraded sharply. Do not return to service without investigation.
Step 6 — Diagnose Unstable or Drifting Readings
Before diagnosing a fault, rule out measurement errors. Unstable readings are often caused by test setup issues rather than transformer problems.
- Slow stabilization: Normal on large transformers — use a higher test current (10A for power transformers) and allow 60–90 seconds for the inductive winding to settle.
- Gradual drift upward: The winding or leads are heating due to test current — reduce current or shorten test time.
- Random fluctuation: Check all four Kelvin clamp connections. Clean contact surfaces and ensure firm mechanical pressure. Even a slightly loose potential clamp causes erratic readings.
- Very slow settling: May indicate residual magnetism in the core. Allow more time, or demagnetize before testing.
If you have ruled out all test setup issues and the reading is still unstable, the instability itself is a finding — it typically points to an intermittent OLTC contact or a loose internal joint.
Quick Diagnosis Reference
| Reading Pattern | Most Likely Cause | Action |
|---|---|---|
| One phase higher than others | Loose joint / corroded connection / OLTC contact | Investigate at next outage |
| One phase much lower than others | Shorted turns / inter-turn fault | Take out of service immediately |
| All phases high vs nameplate | Thermal deterioration / incorrect temperature correction | Recheck temperature; trend over time |
| Sudden jump at one OLTC tap | Worn or pitted OLTC contact | OLTC inspection and maintenance |
| Fluctuating / won't stabilize | Loose Kelvin clamp or intermittent contact | Check connections; if clean, suspect OLTC |
| Steady increase over successive tests | Developing joint/contact degradation | Plan inspection; increase monitoring frequency |
| Matches nameplate (temp corrected) | Healthy transformer | Record and schedule next test per maintenance plan |
Conclusion
Interpreting winding resistance readings correctly requires four things working together: an accurate meter, proper temperature correction, comparison against a reliable baseline, and systematic tap-by-tap OLTC measurement. A number on its own means little — its value comes from context: what was it before, what are the other phases showing, and does it match the nameplate?
Used this way, winding resistance testing is one of the most powerful diagnostic tools available for keeping transformers reliable and catching problems before they become failures.