CT Ratio Test: Formula, Calculation, and Testing Procedure
Learn the CT ratio test formula, calculate current transformer ratios, compare expected and measured readings, and understand common testing errors.
What Is a CT Ratio Test?
A current transformer (CT) ratio test checks the relationship between primary current and secondary current. It helps engineers determine whether the measured current relationship matches the CT’s nominal ratio under the test conditions.
CT ratio testing is useful during commissioning, maintenance, troubleshooting, and checks after CT wiring changes. The test method depends on the CT design, available equipment, and required accuracy.
What Is the Formula for Calculating a CT Ratio?
The nominal CT current ratio is calculated using the rated primary and secondary currents.
Formula:
Where:
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\(I_p\) = rated primary current in amperes (A)
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\(I_s\) = rated secondary current in amperes (A)
For example, a CT rated 800/5 A has a nominal ratio of:
The nominal ratio is therefore 160:1. A primary current of 400 A corresponds to an ideal secondary current of 2.5 A, assuming the nominal ratio applies under the stated operating conditions.
How Do You Calculate the Expected CT Secondary Current?
Use the primary current and nominal CT ratio to calculate the expected secondary current.
Formula:
Where \(K\) is the nominal CT ratio expressed as a numerical value.
| CT nameplate ratio | Primary current | Expected secondary current |
|---|---|---|
| 100/5 A | 100 A | 5 A |
| 200/5 A | 100 A | 2.5 A |
| 400/5 A | 200 A | 2.5 A |
| 800/5 A | 400 A | 2.5 A |
| 1000/1 A | 500 A | 0.5 A |
These values are theoretical calculations. Actual readings may differ because of the CT’s characteristics, test conditions, measurement uncertainty, and secondary-circuit burden.
How Do You Perform a CT Ratio Test?
Follow the CT manufacturer’s instructions and the approved test procedure. Confirm the nameplate ratio, terminal markings, test current, instrument ranges, and secondary-circuit arrangement before applying current.
A typical test workflow includes these steps:
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Identify the CT rating. Record the primary and secondary current ratings from the nameplate.
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Prepare the test equipment. Select a suitable current injection source and instruments with appropriate current ranges and accuracy.
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Verify the connections. Follow the approved wiring diagram and confirm the intended primary current path.
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Apply the specified test current. Use the test method and current level required for the CT and the equipment manufacturer’s instructions.
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Record the readings. Measure the applied primary current and corresponding secondary current.
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Compare the results. Calculate the measured ratio and compare it with the nominal ratio, considering the test method’s uncertainty and acceptance criteria.
Never open the secondary circuit of an energized CT. Isolate the equipment and follow the approved safety procedure before changing connections.
How Do You Calculate the Measured CT Ratio?
Divide the measured primary current by the measured secondary current.
Example:
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Measured primary current: 400 A
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Measured secondary current: 2.48 A
The measured ratio is approximately 161.29:1, compared with the nominal ratio of 160:1.
This difference does not automatically mean that the CT has failed. Engineers must consider the test method, instrument uncertainty, CT specifications, and applicable acceptance criteria before making a decision.
What Causes CT Ratio Test Errors?
Several factors can affect the measured ratio.
| Possible cause | What to check |
|---|---|
| Incorrect nameplate ratio | Confirm the rated primary and secondary currents |
| Incorrect test connections | Check the primary current path and terminal references |
| Measurement uncertainty | Verify instrument accuracy and current range |
| Secondary-circuit burden | Confirm the test arrangement and connected load |
| CT operating characteristics | Check the manufacturer’s specifications and applicable test conditions |
| Unstable test current | Verify that the source and readings remain within the required test conditions |
A ratio discrepancy should be investigated systematically rather than judged from one reading alone.
What Equipment Is Used for CT Ratio Testing?
The required equipment depends on the CT design and the selected test method. Common options include a CT ratio tester, a primary current injection tester, and suitable instruments for measuring primary and secondary currents.
For primary injection applications, the Wrindu RDSL-82-6000D Primary Current Injection Tester provides up to 6000 A output current and a rated capacity of 36 kVA. Its two ammeters support monitoring across large and small current ranges.
Suitable for high-current injection applications where the required current falls within the equipment’s specified output range. Confirm that the proposed setup is suitable for the CT and test procedure.
What Is the Difference Between a CT Ratio Calculation and a Ratio Test?
A ratio calculation uses rated or assumed values to determine the expected current relationship. A ratio test uses measured values to assess the current relationship under actual test conditions.
Neither calculation alone nor a basic primary injection reading proves the CT’s complete accuracy performance. Additional tests may be required for accuracy class, phase displacement, excitation characteristics, or other specified properties.
Conclusion
The CT ratio formula provides a straightforward way to calculate the expected secondary current and evaluate measured readings. A reliable assessment requires correct connections, suitable instruments, documented test conditions, and acceptance criteria appropriate to the CT.
For the broader procedure, see the CT Primary Injection Testing Guide.
