A transformer turns ratio (TTR) test verifies the turns relationship between windings, while a winding resistance test verifies the integrity of the conductive path through windings, joints and tap changer contacts. They answer different diagnostic questions: ratio testing detects changes in effective turns, and resistance testing detects changes in the resistive path. Use them together to separate winding, connection and tap changer problems.
The Two Tests Answer Different Diagnostic Questions
The TTR test applies a low-voltage excitation to one winding and measures the induced voltage in the other, expressing the result as a ratio that should match the nameplate within the applicable reference. It is sensitive to anything that changes the effective turn count or the magnetic coupling between windings. The winding resistance test applies a DC current and measures the voltage drop, producing a resistance value that is sensitive to the physical condition of the conductive path.
The distinction matters for fault localisation. A defect that changes the effective turns, such as an inter-turn short in a portion of the winding, affects the ratio. A defect that increases resistance, such as a loose joint or a degraded tap changer contact, affects the resistance reading. Because the two tests probe different properties, a normal result on one does not clear the other.
Consider a practical example: after a through-fault, the protection operated and the transformer is isolated. The ratio test is normal on all phases, which rules out a major change in effective turns. The winding resistance test shows one phase slightly higher at a specific tap. The combination tells the investigation where to look: not at the winding turns, but at the connection and tap changer contact path on that phase. Neither test alone would have localised the evidence this well.
What a TTR Test Can and Cannot Reveal
A TTR test can reveal a change in turns ratio, an incorrect tap position, a phase-relationship error and, in some patterns, evidence of inter-turn or winding issues that alter the effective turns. It is fast, non-destructive and one of the first tests performed after transport, winding work or a fault, and it is routinely used to verify the vector group and tap changer alignment.
A TTR test cannot reveal the condition of joints, contacts and connectors, because those lie in the resistive path rather than the turns path. It also cannot detect core or mechanical deformation directly. A normal TTR result therefore does not prove that the winding connections are sound or that the transformer survived mechanical stress, which is why resistance and frequency-response tests exist alongside it.
The TTR test is also the tool that verifies tap changer alignment in the turns sense: if the selector has moved to the wrong tap, the ratio at that position deviates from the nameplate profile. This makes the test valuable after tap changer maintenance and after transport, where mechanical linkage settings can shift. The result, however, is a turns verdict, not a contact-quality verdict; the contact condition is covered by the resistance test.
What Winding Resistance Testing Can and Cannot Reveal
Winding resistance testing can reveal high-resistance joints, loose or burned connections, degraded tap changer contacts and changes in the winding conductor path. Because it measures the resistive path phase by phase and tap by tap, it localises the evidence to a phase, a tap position or a common connection, which makes it one of the most useful localisation tools in transformer diagnostics.
It cannot reveal a turns-ratio error that does not change the resistive path, and it is insensitive to mechanical deformation of the core and windings unless the deformation also changes the conductive path. It also requires careful temperature correction and stable measurement conditions, so a single raw value without normalisation cannot be compared with history.
Two measurement disciplines make resistance testing trustworthy: four-wire (Kelvin) connections, which remove lead and clip resistance from the result, and temperature correction to a common reference. Without them, a healthy connection can look bad, or a deteriorating joint can be hidden by a temperature shift. The resistance value is only meaningful when the measurement conditions are recorded with it.
Fault Patterns That Require Both Tests
Some fault classes show up in only one test, and others require both to be understood. An inter-turn short can change the ratio on the affected phase while leaving resistance nearly unchanged at the terminals. A loose bushing connection raises resistance on the affected phase without changing the ratio. Tap changer contact wear raises resistance at specific taps while ratio remains correct. A combination of ratio and resistance changes may indicate both winding and connection damage, which is common after a severe through-fault.
| Suspected defect | TTR response | Winding resistance response | Suggested combination |
|---|---|---|---|
| Inter-turn short | Possible ratio change on affected phase | Usually limited change at terminals | TTR first, then excitation current |
| Loose or burned joint | Normal | Higher resistance on affected phase | Resistance test, thermal verification |
| OLTC contact degradation | Normal | Higher resistance at specific taps | Tap-by-tap resistance trend |
| Post-fault winding damage | Possible ratio change | Possible resistance change | Both tests plus SFRA where indicated |
Use the matrix as a reasoning aid, not as a rulebook. The actual response depends on the fault location, winding configuration and measurement sensitivity, so record the raw evidence and let the pattern, not the expectation, drive the conclusion.
For example, a severe inter-turn fault in a winding section may change both the ratio and the resistance if the shorted turns alter the measurable current path, while a fault close to the neutral may be less visible at the terminals. The lesson is that a single normal reading never proves the absence of damage; the tests are strongest when their results are consistent and weakest when one result contradicts the expected pattern without explanation.
Choosing the Test Sequence After an Abnormal Result
When either test is abnormal, confirm repeatability first with corrected setup, then run the complementary test while the transformer is isolated. A practical sequence is ratio first because it is quick and covers the turns relationship, then winding resistance to localise resistive-path issues, then excitation current when the pattern suggests a core or parallel-circuit condition. Add SFRA when mechanical deformation is suspected after transport or through-fault current.
Keep the sequence documented: the order matters for interpretation, because each test changes the information state and some tests affect the magnetisation of the core. Record the reason for each additional test so the final report explains why the evidence was collected.
After a ratio anomaly, the immediate question is whether the change is in the turns or in the measurement path. The resistance test answers the path question; if resistance is normal, the evidence moves toward the turns or magnetic circuit, and an excitation current test is the natural next step. If resistance is abnormal at the same location, the investigation focuses on the connection and contact path. Sequencing the tests in this way keeps every step purposeful.
Evidence to Preserve in the Final Report
Preserve the raw data and the conditions for both tests: instrument and calibration date, settings, excitation voltage and frequency for the TTR test, test current and stabilisation time for the resistance test, winding temperature, tap positions, lead arrangement and the reference values used. Include the computed deviations, the phase spread and the tap profile, plus the baseline results being compared against.
Close with a factual conclusion that distinguishes what was measured from what is inferred. State which patterns were confirmed, which follow-up tests are recommended and which hold point or review is required before the transformer returns to service. A report that separates evidence from interpretation is the strongest support for the next maintenance decision.
Structure the report so a second engineer can reproduce the reasoning: the transformer and event context, the baseline used, the test conditions, the raw and normalised values, the comparison and deviation, the interpretation and its confidence, and the recommended action. Include instrument serial numbers and calibration dates in the header of each test section so the evidence chain is complete.
Frequently Asked Questions
Which test should be performed first?
Ratio testing is usually performed first because it is fast, non-destructive and covers the turns relationship. If the ratio result is abnormal or the pattern suggests a connection or tap changer issue, add winding resistance testing while the transformer is isolated. The responsible engineer should confirm the sequence for the specific situation.
Can these tests replace each other?
No. They probe different properties: the turns relationship versus the resistive path. A normal result on one does not clear the other, and a complete post-fault or acceptance investigation normally includes both, together with insulation and, where indicated, frequency-response measurements.
Why do both tests need baseline data?
Because deviation from the transformer’s own history is usually more meaningful than an isolated absolute value. Baseline data from the factory or an earlier test turns a single reading into a trend and allows the pattern to be judged against the unit’s known behaviour.
For the lifecycle framework that places these tests in context, see the complete power transformer testing checklist. To compare ratio and resistance test systems for your needs, review transformer testing equipment and request a technical proposal.