Current Transformer (CT) Selection Criteria

Aug 20, 2025

Technical Articles

Introduction:

Digital Panel Meters (DPMs) are widely used in industrial and commercial electrical systems for accurate monitoring of electrical parameters. The accuracy of these meters is highly dependent on the Current Transformer (CT) used. Selecting the right CT is crucial, especially when working with meters of high accuracy classes such as 0.2s, 0.5s, and 1.0.

This article outlines the ideal approach to CT selection to ensure optimal performance and measurement accuracy.

Protection vs. Measuring CTs:

Measuring CTs -

Designed for accurate measurement of current.
Used with meters s analysers.
Prioritize accuracy class, burden, and linearity.
Typically have accuracy classes like 0.2s, 0.5s, 1.0.
Measuring CTs saturates at high currents, which is acceptable since it’s not meant to operate during faults
Handles lower burden – to load on the secondary circuit of measuring device.

Protection vs. Measuring CTs:

Protection CTs -

Designed to operate during fault conditions.
Used with protective relays to detect overcurrent, short circuits, etc.
Focus on saturation characteristics and knee-point voltage.
Accuracy classes are different (e.g., 5P10, 10P10) indicating performance under fault conditions.
P stands for Protection. The number (e.g., 5 or 10) indicates the percentage error at the accuracy limit factor (ALF)
This CTs Designed to avoid saturation even at 10 to 20 times the rated current, ensuring reliable relay operation.
Handles higher burden to drive protection relays.

Important Note: Measuring CTs should not be used for protection purposes, and vice versa. Always select CTs based on the intended application.

Understanding Accuracy Classes:

Accuracy classes are typically 0.2, 0.2s 0.5, 0.5s, or 1.0, indicating very precise measurement at rated current.

Applicable standards: Class 1 as per IEC 62053-21 C Class 0.5, 0.5S, 0.2, 0.2S as per IEC 62053-22.

The “s” in 0.2s and 0.5s denotes special accuracy, especially at low current levels. Refer the below table for better understanding.

Key Parameters for CT Selection:

1. CT Accuracy Class

Choose a CT with an accuracy class equal to or better than the DPM’s class.

For 0.2s meters → CT class should be 0.2s or better.
For 0.5s meters → CT class should be 0.5s or better.
For 1.0 meters → CT class can be 1.0 or better.

2. Burden (VA Rating)

Burden is the total impedance (in ohms) of the connected devices (meters, cables) on the CT secondary. Expressed in VA at rated secondary current (usually 1 A or 5 A).
Ensure the CT burden matches the meter’s input burden.
Avoid overloading or underloading the CT, as it affects accuracy.

3. Rated Primary Current

Select based on the maximum expected load current.
Avoid oversizing, which reduces accuracy at low currents.
For variable loads, consider CTs with extended range or multi-ratio CTs.

4. Rated Secondary Current

Common values: 1A or 5A.
1A should be used for long length of the cable between meter C CT. this will reduce the Voltage drop / Power Drop. Lower burden on CT improves precision in measurements.
5A to be used where cable length between Meter C CT is low. Hence Voltage drop is negligible which Result into precision improvement.

Advantage of Site selectable CT / PT in DPM: Lauritz Knudsen Electrical C Automation Digital Panel Meters have feature of Site-selectable CT and PT. This feature offers exceptional flexibility and convenience during installation and commissioning of digital meters. It allows users to configure the meter for different CT and PT ratios directly at the site without replacing hardware or recalibrating the system. This reduces installation time, eliminates the need for multiple meter variants, and minimizes inventory costs. Additionally, it ensures accurate scaling of measured values for diverse applications, from low-voltage panels to high-voltage systems, improving adaptability and reducing errors caused by incorrect ratio settings. Overall, this feature enhances operational efficiency, simplifies maintenance, and supports quick system upgrades.

Conclusion:

Accuracy classes in measuring CTs play a vital role in ensuring precise current measurement for metering and monitoring applications. Classes like 0.2 and 0.5 are suited for high-accuracy needs, while 0.5s is ideal for revenue metering due to its superior performance at low loads. Understanding these classifications helps in selecting the right CT for reliable and efficient power system operation.