CT Ratio Test vs. CT Accuracy Test: What Is the Difference?

What Is the Difference Between a CT Ratio Test and a CT Accuracy Test?

A CT ratio test checks the relationship between primary and secondary current. A CT accuracy test evaluates whether the current transformer meets specified measurement performance requirements under defined test conditions.

A ratio check is one part of CT assessment, but it does not automatically prove compliance with an accuracy class. Accuracy testing may require additional measurements, controlled burdens, specified current levels, and phase-displacement evaluation.

What Does a CT Ratio Test Measure?

A CT ratio test compares the expected current relationship with the measured relationship.

For a CT rated 400/5 A, the nominal ratio is:

At a primary current of 200 A, the ideal secondary current is:

If the measured secondary current is 2.48 A, the measured ratio is approximately 80.65:1. The discrepancy should be evaluated against the test method’s measurement uncertainty and the CT’s applicable requirements.

A basic current ratio measurement does not necessarily measure phase displacement or establish the CT’s accuracy class.

What Does a CT Accuracy Test Measure?

CT accuracy testing evaluates how closely a CT reproduces the primary current as a secondary current according to its specified performance requirements.

Depending on the CT type and applicable standard, the assessment may include:

• Ratio error: The difference between the actual transformation relationship and the nominal ratio.

• Phase displacement: The phase-angle difference between the primary and secondary current references, interpreted according to the applicable convention.

• Specified operating conditions: Test current, frequency, burden, and other conditions required by the relevant standard.

• Accuracy class compliance: Whether the measured performance satisfies the limits for the declared class.

The exact test points and limits depend on the CT’s design, rated burden, accuracy class, and applicable standard. Engineers should not apply a single universal acceptance limit to every CT.

How Is CT Ratio Error Calculated?

Ratio error is calculated from the measured transformation ratio and the nominal ratio, using a clearly defined convention.

For a conventional ratio-error calculation:

Where:

• \(K_n\) = nominal transformation ratio

• \(I_p\) = measured primary current

• \(I_s\) = measured secondary current

For a nominal 400/5 A CT, \(K_n=80\). If the primary current is 200 A and the secondary current is 2.48 A:

The calculated ratio error is −0.8% under this convention. The sign and acceptance limit must be interpreted using the convention and requirements specified by the applicable test method or standard.

This calculation alone does not establish accuracy-class compliance because phase displacement and the required test conditions may also matter.

Which Standards Apply to CT Accuracy Testing?

The applicable standard depends on the CT type, intended use, and market requirements.

These standards should not be treated as interchangeable. Before testing, confirm the standard named in the CT documentation, contract, or commissioning specification.

The exact test points, burden conditions, and acceptance limits must come from the applicable standard and CT specification. Do not infer compliance from a generic ratio calculation.

What Equipment Is Needed for CT Ratio and Accuracy Testing?

The required equipment depends on the measurement objective.

A primary current injection tester can supply current to a suitable primary circuit. It should not be assumed to provide every measurement needed for a standards-based CT accuracy test.

Can a 6000 A Primary Injection Tester Perform a Complete CT Accuracy Test?

Not by output current rating alone. A tester’s maximum current describes its output capability; it does not establish its ability to measure ratio error, phase displacement, or accuracy-class compliance.

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.

For a complete accuracy assessment, select a test system that can measure all required parameters at the specified test points and conditions.

When Is a CT Ratio Test Enough?

A basic ratio test may be suitable when the scope is limited to checking the nominal current relationship under a defined test arrangement. The test report should state the applied current, measured secondary current, calculated ratio, equipment used, and relevant test conditions.

A ratio test alone is not enough when the task requires formal accuracy-class verification, phase-displacement measurement, or confirmation of additional protection-CT performance requirements.

Engineers should follow the commissioning plan, maintenance specification, and CT manufacturer’s documentation to determine the required test scope.

How Should Engineers Choose Between CT Ratio and Accuracy Testing?

Choose the test based on the question that needs to be answered.

The required test scope should be established before equipment is selected. This helps avoid treating a single measurement as proof of properties that the test did not assess.

Conclusion

A CT ratio test checks the primary-to-secondary current relationship, while a CT accuracy test evaluates performance against specified requirements. The tests overlap in their use of current measurements, but they answer different engineering questions.

For reliable results, document the CT rating, test method, equipment, measurement conditions, and applicable acceptance criteria. Use a test system with the required capabilities for the specific assessment.

For a broader explanation of current injection methods, see the CT Primary Injection Testing Guide.