IEC Standard for Current Transformer Testing: A Complete Guide

Current transformers (CTs) are essential in modern electrical systems. They are used for protection, control, and accurate metering in both industrial and commercial applications. However, ensuring their reliability and performance requires a well-defined testing process. This is where the IEC standard for current transformer testing becomes crucial.

The International Electrotechnical Commission (IEC) has laid down detailed standards for the testing of current transformers. These standards ensure the accuracy, safety, and functionality of CTs under different operating conditions. In this guide, we’ll explore everything about the IEC standard for current transformer testing, including key testing parameters, procedures, and technical considerations.

What is the IEC Standard for Current Transformer Testing?

The IEC standard for current transformer testing refers primarily to the IEC 61869-2 and the earlier IEC 60044-1 standards. These standards define test methods, performance parameters, and classifications for CTs used in low, medium, and high-voltage systems.

IEC 61869-2 is the most recent standard and supersedes IEC 60044-1. It covers the requirements for current transformers used in power systems with voltages above 1 kV.

This standard applies to different types of CTs, including:

• Protection Current Transformers
• Measuring Current Transformers
• Summation Current Transformers
• Core-Balance Current Transformers (CBCTs)

It ensures that every CT performs consistently, regardless of environmental or load conditions.

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Purpose of the IEC Standard for Current Transformer Testing

The primary objectives of the IEC standard for current transformer testing are:

• To verify the electrical and mechanical characteristics of CTs
• To ensure measurement accuracy within defined classes
• To validate insulation, polarity, and burden performance
• To confirm safety and compliance with grid codes
• To standardize global practices for CT manufacturing and field testing

This testing plays a key role in reducing system faults and protecting assets like transformers, switchgear, and circuit breakers.

Key Testing Parameters Under IEC Standard for Current Transformer Testing

The IEC standard defines several performance tests for CTs. Each test focuses on a different aspect of performance. The most critical parameters include:

Each test ensures the CT operates safely and accurately in real-world applications.

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Ratio Error and Accuracy Classes in IEC Standard for Current Transformer Testing

The ratio error is the percentage deviation of the actual transformation ratio from the nominal value. IEC standards define several accuracy classes for CTs based on their intended function:

• For Measuring CTs: Classes 0.1, 0.2, 0.5, 1, 3
• For Protection CTs: Classes 5P, 10P, PX, PR, TPX, TPY

For example, a class 0.5 measuring CT should not have more than 0.5% ratio error at rated current. The phase displacement should also fall within defined tolerances.

Understanding and testing these classes is essential when selecting CTs for sensitive protection schemes like differential protection or distance relays.

Excitation Test: Core Saturation and Knee Point

The excitation test (also called magnetization curve test) is critical for protection CTs. It determines the knee point voltage, which is the point where the CT core begins to saturate.

This is especially important for relays that rely on accurate current replication during faults.

IEC 61869-2 provides a guideline for determining this curve by gradually increasing the voltage on the secondary side and measuring the corresponding current.

• For Class PX CTs, the knee point voltage (Vk) must be ≥ 2 × rated voltage
• The excitation current at 1.1 Vk should be ≤ 30 mA

This test helps ensure the CT will not saturate prematurely during high fault currents.

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Burden and Load Performance in IEC Standard for Current Transformer Testing

CTs are designed to operate accurately at specific burdens (load impedance in ohms or VA). The IEC standard for current transformer testing defines rated burden levels such as 1.25 VA, 2.5 VA, 5 VA, 10 VA, and 15 VA.

A CT tested under 5 VA should maintain its accuracy limits when a load of 5 VA is applied.

Testing ensures the CT will not underperform due to higher-than-expected impedance from meters or relay circuits. This is crucial for remote substations where wiring resistance is high.

Polarity Testing in CTs

Correct polarity is critical for protection systems. A wrong polarity can cause maloperation of differential relays or unintended tripping.

IEC standards require a simple DC method or pulse injection method for polarity testing. One secondary terminal is marked (usually S1 or K), and its polarity should match the primary P1 side.

This test is mandatory during factory acceptance, site commissioning, and after any maintenance.

Short-Time Current Test and Thermal Withstand

IEC 61869-2 outlines the short-time current withstand test. This verifies the CT’s ability to handle large fault currents for a short duration (usually 1 second or 3 seconds).

Example:

• A CT rated for 25 kA for 1 second should withstand 25,000 A RMS without mechanical or thermal damage.
• The insulation and core integrity must remain intact after the test.

This test ensures CT safety during system faults, especially in critical substations.

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Insulation Tests Under IEC Standard for Current Transformer Testing

Insulation testing ensures that no flashover or breakdown occurs under system voltages or transient overvoltages. Common tests include:

• Power Frequency Withstand Voltage Test
• Lightning Impulse Withstand Test
• Insulation Resistance Measurement (in MΩ)

IEC defines these tests based on system voltage level:

CTs must pass these dielectric tests to be certified for grid use.

Temperature Rise Test

The temperature rise test checks whether the CT operates within safe temperature limits under rated load. The standard typically allows a rise of 55–65°C above ambient temperature depending on insulation class (Class A, B, F, or H).

It simulates real-world conditions where CTs are energized for long durations.

IEC requires continuous monitoring of:

• Winding temperature
• Core temperature
• Ambient temperature

After the test, the insulation resistance must still meet acceptable levels.

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IEC Standard for Current Transformer Testing in Field Applications

Field testing of CTs based on IEC standards is also essential. Technicians use advanced test kits like CT analyzers to measure:

• Turns ratio
• Excitation curve
• Burden impedance
• Polarity
• Winding resistance

These portable devices help validate the CT’s health before system energization or during maintenance shutdowns.

Conclusion

The IEC standard for current transformer testing provides a structured, globally accepted framework to ensure CT reliability and performance. Whether it’s verifying accuracy, checking polarity, or testing insulation, each procedure is crucial for safe and efficient power system operation.

Adhering to these IEC standards reduces the risk of faults, ensures compatibility with protection relays, and helps meet regulatory compliance. Engineers, testers, and manufacturers must fully understand and apply these standards at all project stages—design, factory testing, commissioning, and maintenance.

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