A winding resistance test sounds like nothing. Inject DC current, measure voltage drop, divide. You get an ohms reading that you write down and move on.
Then you find out the reading takes ten minutes to settle on a large power transformer. The number changes if the core wasn’t demagnetized first. The phase-to-phase comparison only works at the same temperature. The factory value can’t be compared to your field value without a correction nobody applied. And after you finish, the core is heavily magnetized — which means the next person who runs a TTR or excitation test gets garbage results.
The winding resistance test is one of the most diagnostically powerful tests in transformer work. It catches loose connections, broken conductor strands, shorted turns, OLTC contact problems, and developing winding deterioration. It’s also one of the most procedure-sensitive tests — get the practical details wrong and you get a number that means nothing, or worse, a number that means something different than you think.
This guide covers running and reading the test properly. It assumes you know what a transformer is and have used a micro-ohmmeter. The value is in the practical details that turn an ohms reading into a real diagnostic.
Table of Contents
What the Test Actually Measures
The winding resistance test measures the DC resistance of a transformer winding from terminal to terminal. That sounds straightforward, but several things are bundled into that one number:
The bulk resistance of the conductor itself — copper or aluminum, hundreds or thousands of meters of it wound into the coil. This is the dominant component on a healthy winding.
The contact resistance at every joint along the current path — brazed joints inside the winding, bolted connections at terminals, the moving contacts of any tap changer in the circuit, and the bushing connections.
The cumulative effect of any open or partially-broken conductor strands. Modern transformer windings use stranded conductor (often continuously-transposed cable, CTC), and a few broken strands inside the winding raise the resistance slightly without producing an obvious failure.
What you’re really doing when you measure winding resistance is interrogating that entire current path for any defect that increases its resistance. A loose connection, a degraded braze, a worn OLTC contact, a broken strand — all show up as a higher-than-expected number.
What the test doesn’t measure: insulation, dielectric strength, capacitance, inductance, or any AC behavior. It’s a pure resistance measurement of the conductive path. That’s its limitation and its strength — by focusing on one quantity, it’s sensitive to defects that AC-based tests can miss.
Why DC, and Why the Settling Problem
Two questions people new to the test ask:
Why DC? A transformer winding is mostly inductance from an AC perspective. The inductive reactance at power frequency is far greater than the resistive component — you can’t extract a clean resistance reading from an AC measurement because the impedance is dominated by the reactance. DC eliminates the reactance entirely. At steady-state DC, the inductor is just a piece of wire, and what you measure is its resistance.
Why does it take so long to settle? Because of that same inductance. When you apply DC voltage to a winding, the current doesn’t jump to its final value. It rises according to the winding’s L/R time constant — the larger the inductance and the smaller the resistance, the longer it takes. On a small distribution transformer, current settles in seconds. On a large power transformer with high inductance and low resistance, it can take minutes. The voltage drop you’re measuring is meaningful only after the current has stabilized — before that, there’s still an L(di/dt) component in the voltage that contaminates the resistance calculation.
Modern instruments handle this by injecting current with high compliance voltage (often 40 V or more) to speed up the rise, monitoring the current rate of change, and only reporting a result when di/dt has fallen below a threshold. But the underlying physics still rules: large transformers take time. A test engineer who reads the number too early gets a value that’s higher than the true resistance because the L(di/dt) term is still contributing.
Choosing the Test Current
Test current selection is the first practical decision and it matters more than people realize.
The standards (IEEE C57.152, IEC 60076-1) frame the test current as a percentage of the winding’s rated current. The useful range is roughly 0.1% to 10% of rated, with 1% to 15% being a commonly cited working range. The constraints at each end:
Too low. The voltage drop across the resistance is too small to measure accurately. Noise dominates. Reading is unreliable.
Too high. Two problems. First, the I²R heating of the winding becomes significant — the resistance is changing during the measurement as the winding warms, so the value you measure isn’t the cold value, and trending becomes impossible. Second, sustained high DC current saturates the core, which is bad for the next tests in the sequence and can produce odd readings during the test itself.
Working guidelines that hold up in practice:
- HV windings (high resistance, low rated current): 1–3 A is typical. Minimum 1 A for adequate signal.
- LV windings (low resistance, high rated current): up to 5% of rated, sometimes higher. For very high-current LV windings, 20–50 A is common.
- High-resistance windings (above 100 mΩ): 10 A or less is fine.
- Very low-resistance windings (below 100 mΩ): higher currents (20–50 A) give better accuracy.
Modern instruments often have current ranges from 10 mA to 50 A and pick the appropriate level automatically. If you’re picking manually, err toward the higher end for accuracy but stay well below 10% of rated to avoid heating.
The Core Saturation Problem on Large Units
A real headache on large power transformers: the inductance is so high that current settling takes painfully long. The traditional fix is to drive enough current that the core saturates — once the core is saturated, the inductance drops to a fraction of its unsaturated value, and the L/R time constant collapses. Current settles quickly.
The catch: saturating the core can require 100 A or more on large units. Most field instruments can’t deliver that.
The practical workaround is the dual-winding method: connect the test current through both the secondary and primary windings in series. The current in the primary side assists in saturating the core, even though you’re measuring the secondary. The total ampere-turns are higher, the core saturates, and the secondary current settles much faster than it would alone.
This is built into many modern transformer winding resistance kits as a standard mode. If you’re working on a transformer where the reading just won’t settle, ask whether your kit supports dual-winding excitation, or whether you can manually configure it. It can cut a 15-minute measurement to under a minute.
Demagnetization: Before and After
The DC current you injected during the test leaves the core heavily magnetized. This matters for two reasons.
It affects subsequent tests. Excitation current measurements (in TTR, in no-load loss tests, in dedicated excitation testing) are sensitive to residual core magnetism. Running winding resistance and then immediately running TTR or excitation gives unreliable results on the second test. The core needs to be demagnetized between the two.
It affects energization. If a transformer is energized with significant residual core magnetism, the inrush current on the first cycle can be much higher than the design value. Protective relays may trip on inrush. Sustained DC offset in the inrush current can cause unexpected protection operations or even damage. A transformer that’s just had a winding resistance test should not be put straight into service without demagnetizing the core.
The fix is the demagnetization function built into modern test sets — it applies a decaying alternating current that progressively reduces the residual flux to near zero. Run it after the resistance test, every time. The few minutes it takes are far cheaper than the consequences of skipping it.
A test plan implication: sequence the tests so winding resistance comes after TTR and excitation, not before. If both happen and resistance comes first, demagnetize before moving on.
Temperature Correction: Where Most People Get Tripped
The resistance of a winding changes significantly with temperature. Copper has a temperature coefficient of roughly 0.4% per degree Celsius near room temperature; aluminum is similar. A winding measured at 20°C reads about 20% lower than the same winding at 75°C.
That means comparing a field measurement at 25°C to a factory reading at 75°C, without correction, gives a phantom 20% “increase” that’s just temperature. Most acceptance and trending criteria assume the values being compared are at the same temperature.
The IEC and IEEE standards specify a reference temperature for the correction: 75°C for most oil-immersed transformers (IEC also uses 85°C for some classes; IEEE generally uses 75°C). All measured values get corrected to the reference before comparison.
The correction formula for copper windings:
R₇₅ = Rₘ × (234.5 + 75) / (234.5 + T)
Where Rₘ is the measured resistance, T is the actual winding temperature in °C, and 234.5 is the inferred zero-resistance temperature for copper (sometimes rounded to 235 in older references). For aluminum windings, the constant becomes 225 instead of 234.5.
The practical question is what to use for the winding temperature T. You can’t put a thermometer inside the winding. The conventions:
- If the transformer has been de-energized long enough that top and bottom oil temperatures have equalized (within about 5°C of each other), use the average oil temperature as the winding temperature. This typically requires 3 to 8 hours of de-energized rest, depending on transformer size.
- If you’re testing without oil (factory tests, dry units), use the surrounding air temperature.
- If a winding temperature indicator (WTI) is present and reading meaningfully, use that.
The shortcut of “use the oil temperature” assumes thermal equilibrium has been reached. A transformer just taken out of service has hot windings inside cooler oil — using the oil temperature in that state gives a corrected resistance that’s wrong. The wait time matters.
For accurate trending, record the actual temperature with every reading. A measurement without a recorded temperature is a measurement that can only be compared to other measurements at the same unknown temperature — which is to say, not useful for trending.
Acceptance Criteria
Three comparisons matter, and they apply differently depending on whether you’re testing a new or in-service transformer.
Phase-to-phase agreement. All three phases should read within about 1% of each other at the same tap position and the same temperature. A larger variation points to a fault on the affected phase — a loose connection, a broken strand, an OLTC contact problem. This is the most useful diagnostic in field testing because it doesn’t depend on any historical record; you have all three phases right there.
Comparison to factory test value. After temperature correction to the same reference (typically 75°C), field measurements should be within about 5% of the original factory value. The 5% tolerance accounts for differences between lab and field conditions: instrument differences, lead resistance, exact temperature measurement, and so on. A 2% deviation is unremarkable. A 5% deviation warrants investigation. A 10% deviation is a clear flag.
Trending across successive field tests. Change exceeding about 2-3% between successive tests at the same tap and corrected to the same temperature is significant, even if all values are individually within the 5% of factory. The trend is often the more sensitive diagnostic on an in-service unit — a winding that’s slowly increasing in resistance over years of testing is showing a developing problem, regardless of the absolute value.
Note that all three of these checks need temperature correction to be meaningful. The phase-to-phase comparison is the partial exception — if all three phases are measured within minutes of each other, they’re at the same temperature, and the comparison works even without correction. The other two checks fail completely without correction.
Testing Every Tap
If the transformer has a tap changer, run the resistance test on every tap position. Both no-load tap changers (off-circuit) and on-load tap changers should be tested through their full range.
On a tap changer, the resistance change per tap step should be smooth and roughly uniform. The progression across taps tells you whether the tap selector is making proper contact on every position. A tap with markedly higher resistance than its neighbors is a tap with a contact problem. A tap that reads open (no current flow possible) is a tap where the contact isn’t making at all.
OLTC condition assessment is a major application of winding resistance testing. The traditional static test (DC injection, wait for settling, read) catches steady-state contact problems but misses transient behavior — a tap that makes contact in the static test might still have a momentary break during the actual tap operation. Modern dynamic resistance measurement, where the resistance is monitored continuously during a tap change operation, catches these transients. If your kit supports dynamic OLTC testing, it’s worth running on transformers where the OLTC condition is in question.
Fault Signatures: What the Numbers Tell You
Pulling the diagnostic patterns together:
Single phase reads high. Loose connection or developing fault on that phase. Check terminal connections first, then suspect internal joint or strand damage. The same fault often shows up in TTR (slight ratio error and elevated excitation current on the same phase).
All three phases read uniformly high vs factory. Usually a temperature correction error, not a transformer problem. Recheck the temperature assumption. If correction is right and the deviation persists, suspect a global problem like incorrect calibration on your instrument or systemic test setup error.
One tap reads markedly different. Tap changer contact problem on that position. If the resistance is high but stable, contact is worn or oxidized. If the reading is unstable, the contact is intermittent.
Resistance trending up over successive tests. Developing problem — likely a deteriorating connection (corrosion, loosening) or progressive strand damage. Worth correlating with DGA results; the localized heating from a degrading connection often shows up as carbon monoxide and carbon dioxide trends in dissolved gases.
Resistance settles much slower than usual. May indicate changed inductance — but more often points to a measurement setup problem. Check connections, leads, and grounding before suspecting the transformer.
Resistance won’t settle at all. Almost always a connection problem in the test setup, not a transformer fault. Check the Kelvin clamps, lead integrity, and grounding.
Practical Setup Points
A few things that affect the numbers:
Kelvin 4-wire connection. Use it. The voltage-sense leads measure the voltage drop without lead resistance contaminating the reading. On low-resistance windings, lead resistance can easily exceed winding resistance, so a 2-wire measurement is hopelessly inaccurate.
Clean contacts. Bushing terminals oxidize. Test clamp jaws get dirty. Clean both before connecting. A poor instrument-to-bushing contact looks like a high winding resistance.
Stable current. The reading is only valid once current has fully settled. Wait for the instrument’s stability indicator before reading. Don’t force a reading at a fixed time — the time it takes depends on the transformer.
Comparable conditions. When trending across multiple tests, hold as much constant as possible: same instrument, same test current, same temperature (corrected), same tap position. Differences in any of these contaminate the trend.
Safety. Even though the voltage is modest, the energy stored in a large inductor when you disconnect can be substantial. Modern test sets discharge the winding before disconnecting, but the discipline of waiting for the discharge indicator before removing leads is non-negotiable.
The Takeaway
The winding resistance test is fundamentally simple and operationally subtle. Getting it right means accepting that it takes time on large transformers, choosing a sensible test current, measuring and recording the winding temperature, applying the right correction formula, running every tap, demagnetizing afterward, and reading the result with the right comparison — phase-to-phase, factory baseline, or historical trend — depending on what question you’re asking.
Done that way, it catches a remarkably wide range of faults: loose connections, broken strands, developing winding deterioration, OLTC contact problems, brazed joint degradation. Combined with TTR — which catches turns problems and connection errors — it covers most of the diagnostic territory accessible from a transformer’s terminals without applying high voltage. The two tests together, properly run, are the foundation of low-voltage transformer diagnostics.
Done as a quick injection and an ohms readout without the temperature correction or the settling discipline, it’s a measurement that occupies time without producing trustworthy information. The test is the same; the discipline around it is what makes it diagnostic.