The winding resistance test is one of the most effective diagnostic tools used during transformer maintenance, commissioning, and fault investigation. It reveals internal issues that cannot be detected by visual inspection or insulation testing alone.
By measuring the DC resistance of each winding and tap position, engineers can detect mechanical, electrical, and thermal faults early — preventing costly failures.
📖 For a complete overview of the instrument used to detect these faults, see our Complete Winding Resistance Meter Guide — covering 4-wire Kelvin measurement, specifications, and how to choose the right meter.
1. Loose or High-Resistance Joints
Loose connections or corroded joints increase resistance, leading to overheating and eventual failure.
Symptoms:- One phase shows higher resistance
- Sudden jump in resistance compared to previous tests
- Unstable readings
2. Tap Changer Contact Wear (OLTC/DETC)
The OLTC undergoes thousands of operations and experiences significant wear. Winding resistance testing is the most effective way to detect tap changer issues.
Symptoms:- Irregular resistance pattern across taps
- High resistance in a specific tap
- Large deviation between adjacent taps
Learn more: Why OLTC Condition Assessment Needs Winding Resistance Test
3. Poor Brazing or Defective Lead Connections
Faulty brazing creates localized high-resistance points, which worsen under load and generate excessive heat.
Symptoms:- Inconsistent readings between phases
- Gradual increase over multiple tests
4. Shorted Turns or Inter-Turn Faults
Even a single shorted turn greatly reduces resistance and is extremely dangerous.
Symptoms:- One phase shows significantly lower resistance
- Heating or noise during operation
5. Winding Deformation
Mechanical damage from faults or transportation can alter winding geometry.
Symptoms:- Higher resistance in one phase
- Deviation from nameplate or past data
6. Incorrect Tap Position or Tap Selector Fault
Incorrect tap selection changes effective winding turns, affecting resistance.
Symptoms:- Resistance pattern does not follow expected trend
- Misalignment between tap positions
7. Thermal Deterioration of Windings
Prolonged overheating weakens insulation and increases resistance.
Symptoms:- Gradual increase in resistance over years
- Higher values in heavily loaded phases
8. Incorrect Lead Connections During Testing
Bad Kelvin connections create artificially high or unstable readings.
Symptoms:- Fluctuating values
- Large measurement error
9. Temperature-Related Resistance Variation
Resistance varies with winding temperature. Improper temperature correction leads to false alarms.
Symptoms:- Misinterpretation of readings
- Apparent imbalance when actually normal
10. Incorrect or Inadequate Test Current
Low currents produce unstable readings, while overly high currents heat the winding.
Recommended currents:- 1–2A for distribution transformers
- 5–10A for power transformers
Best Meters for Detecting These Faults
Conclusion
The winding resistance test is one of the most powerful diagnostic methods for detecting transformer faults early. Identifying issues such as loose joints, OLTC deterioration, winding deformation, and turn-to-turn faults ensures the reliability and safety of the power system.
Regular testing, accurate trend analysis, and the use of high-quality instruments are essential for effective transformer maintenance.