How to Test a Transformer: Winding Resistance, Insulation, and Turns Ratio

A transformer appears to be one of the simplest electrical devices — two coils of wire on a shared core — but it fails in at least five distinct ways, and each failure mode requires a different test to detect it. This guide covers every test from a basic multimeter winding resistance check to insulation resistance testing, polarity testing, and turns ratio measurement — the complete electrical commissioning and maintenance test sequence for transformers from small signal types up to large distribution units.

How Transformers Fail

Understanding the failure modes shapes the test sequence. The five main failure mechanisms are:

• Winding open circuit: A conductor break inside the winding — usually from a surge current or mechanical failure. The affected winding shows infinite resistance. Symptoms: no output voltage from the affected winding.
• Winding short circuit (inter-turn fault): Two adjacent turns of wire make contact, creating a shorted loop. This reduces the effective turns count, increases current, and causes localised overheating. Difficult to detect without an LCR meter or turns ratio tester.
• Winding-to-core insulation breakdown: The insulation between the winding and the iron core fails — creating a ground fault. Detected by insulation resistance (megger) test.
• Winding-to-winding insulation breakdown: Insulation between primary and secondary fails — creates a direct electrical path between them. Also detected by megger test. Particularly dangerous in isolating transformers where this destroys the safety isolation.
• Core saturation / wrong turns ratio: Core damage, rewind errors, or connection errors cause abnormal turns ratio and may result in saturation under load. Detected by turns ratio test.

Test 1: DC Winding Resistance

Winding resistance is the DC resistance of each winding's copper conductor. It is used to detect open circuits, poor connections, and inter-turn shorts (an inter-turn short reduces the effective length of the conductor and therefore reduces measured resistance).

Use a multimeter on the resistance range, or a dedicated low-resistance ohmmeter (micro-ohmmeter) for large transformers where winding resistance may be in the milliohm range.

1. Disconnect all external connections from the transformer.
2. Measure the resistance between each pair of winding terminals. For a centre-tapped secondary, measure each half separately and the full winding.
3. Compare readings between phases on three-phase transformers — they should be within ±2% of each other. Larger differences indicate a problem.
4. Compare to the nameplate or design specification if available.

Test 2: Insulation Resistance (Megger Test)

This test checks the insulation quality between windings and between each winding and the core/earth. Use a 500 V DC megger for transformers rated up to 1 kV; use 1000 V DC for HV transformers; use 2500 V DC or higher for power distribution transformers above 11 kV.

Perform three separate measurements:

1. Primary winding to earth: Connect megger Line terminal to the primary winding, Earth terminal to the core/tank. Short all primary terminals together and short all secondary terminals together and connect them to earth. Test for 1 minute.
2. Secondary winding to earth: Connect megger Line terminal to the secondary winding (all terminals shorted together), Earth terminal to core/tank. Primary terminals all shorted to earth. Test for 1 minute.
3. Primary to secondary: Connect megger Line terminal to primary (all shorted), Earth terminal to secondary (all shorted). Core/tank must be floating (disconnected from earth) for this test.

Minimum acceptable insulation resistance values per IS 2026 (Power Transformers):

• LV winding (up to 1.1 kV): minimum 200 MΩ (new), 100 MΩ (maintenance)
• HV windings: see manufacturer specification; typically ≥ 1000 MΩ for HV equipment in good condition
• Primary-to-secondary: must meet or exceed the lower of the two individual values

Test 3: Turns Ratio Test

The turns ratio test verifies that the voltage ratio between windings matches the nameplate specification and detects inter-turn short circuits that may not be visible in winding resistance measurements.

Simple AC method (suitable for small transformers):

1. Apply a known, safe AC voltage to the primary (e.g., 230 V from the mains, or a lower voltage from a variac for safety).
2. Measure the open-circuit secondary voltage with a multimeter in AC voltage mode.
3. Calculate the ratio: Turns Ratio = V_primary / V_secondary. This should match the nameplate ratio within ±0.5% for a good transformer.

Test 4: Polarity Test

Transformer polarity defines the phase relationship between primary and secondary voltages. Additive polarity means the voltages add when windings are connected in a specific way; subtractive polarity means they oppose. Polarity matters critically when:

• Paralleling transformers (wrong polarity causes a short circuit)
• Connecting current transformers for metering and protection
• Building centre-tapped supplies (wrong polarity on one winding means half the output is wrong)

Simple polarity test procedure:

1. Connect a short between one primary terminal (H1) and one secondary terminal (X1).
2. Apply a known AC voltage to the primary (H1–H2).
3. Measure the voltage between H2 and X2 (the remaining terminals).
4. If the reading is V_primary + V_secondary: additive polarity. If V_primary − V_secondary: subtractive polarity.

Test 5: No-Load (Open-Circuit) Test

The no-load test measures the core losses (iron losses) and the magnetising current. It is performed by applying rated voltage at rated frequency to the primary with the secondary open-circuited:

• Measure input power (using a wattmeter or power analyser) — this equals the core loss in watts
• Measure input current — this is the magnetising current, typically 0.5–3% of full-load current
• An abnormally high no-load current indicates core damage, incorrect core material, or core saturation

Test 6: Load Test (Short-Circuit Test)

The short-circuit test determines winding impedance (copper losses) by applying a reduced voltage to the primary while the secondary is short-circuited through a current-limiting impedance. This test is typically done at a manufacturing facility with proper equipment, not in the field — it requires precise control to avoid dangerous currents. It determines:

• Percentage impedance voltage (% Z) — important for fault current calculation
• Copper losses at full load
• Leakage inductance

For field verification, compare the measured percentage impedance voltage to the nameplate value (typically 4–6% for distribution transformers). Values significantly outside specification indicate winding problems.

Oil-Filled Transformer Tests

Large oil-filled power transformers require additional testing beyond the electrical tests above:

• BDV (Breakdown Voltage) test: Tests the dielectric strength of the transformer oil — the oil must withstand a high AC voltage across a standard gap before arcing. See our dedicated guide on the transformer oil BDV test.
• Dissolved Gas Analysis (DGA): Laboratory analysis of gases dissolved in the oil — specific gas signatures indicate specific fault types (overheating, arcing, partial discharge) before they become visible failures.
• Moisture content: Water in transformer oil dramatically reduces its dielectric strength. Karl Fischer titration or capacitive moisture sensors measure oil water content in ppm.