⚡ Why CT Testing Matters

Current Transformers are the “magnifying glass” of your electrical system. They scale down thousands of amps into readable values for meters and relays. If a CT fails or drifts from accuracy, your protection scheme may trip unnecessarily or fail to operate when needed.

šŸ”§ Step-by-Step CT Testing Procedure

1. Visual & Mechanical Inspection

  • Check nameplate details (ratio, burden, accuracy class).

  • Inspect for cracks, oil leakage, loose terminals, or rust.

  • Verify earthing and mounting.

2. Insulation Resistance (IR) Test

  • Measure insulation between primary–earth, secondary–earth, and primary–secondary using a megger.

  • Acceptable values: typically >1000 MĪ© depending on CT rating.

3. Winding Resistance Test

  • Use a micro-ohmmeter to measure secondary winding resistance.

  • Compare with manufacturer’s data to detect shorted turns.

4. Polarity Test

  • Apply a DC source and check deflection on a galvanometer/voltmeter.

  • Ensures correct polarity marking for proper relay coordination.

5. Ratio Test

  • Apply known current to primary, measure secondary.

  • Compare with rated ratio (e.g., 1000/5 A).

  • Deviation should be within ±0.5% for class 0.5 CTs.

6. Excitation (Saturation) Test

  • Gradually increase voltage on secondary and plot excitation curve.

  • Identifies knee-point voltage and saturation characteristics.

  • Crucial for protection CTs used in differential schemes.

7. Burden Test

  • Connect rated burden (VA load) to secondary.

  • Ensure CT maintains accuracy under load.

8. Phase Angle Error Test

  • Measure phase displacement between primary and secondary currents.

  • Must be within IEC limits for metering/protection accuracy.







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