Factors to Consider When Choosing Current Transformers

Current transformers (CTs) are essential components in electrical systems, used to measure and
monitor electrical currents for various purposes, including protection, control, and metering.
Choosing the right CT for your application ensures accurate readings and proper system
functioning. In this blog, we’ll explore the key factors to consider when selecting current
transformers.

Factors to Consider When Choosing Current Transformers

1. Application Type

The first consideration is the intended use of the current transformer. CTs are typically used for:

Metering:

Accurately measuring current for billing and monitoring purposes.

Protection:

Detecting faults or abnormal current flow to trigger safety mechanisms in case of overcurrent or short circuits

Different applications require different accuracy and performance levels from a CT. For example, protection CTs are designed to handle high fault currents, while metering CTs prioritize precision for accurate billing.

2. Primary Current Rating

The primary current rating refers to the current that flows through the primary winding of the
CT. It’s critical to choose a CT with a primary current rating that matches the current in the circuit
where it will be installed.

Application	Typical Primary Current Rating
Residential Buildings	50-200 A
Industrial Plants	500-2000 A
Power Grids	Above 2000 A

Selecting the correct primary rating ensures that the CT accurately transforms the current without
being overloaded.

3. Accuracy Class

The accuracy class defines the level of accuracy the CT provides. Depending on your application,
the accuracy required might vary

Metering CTs:

Often have higher accuracy (e.g., 0.5 or 1.0 class) to ensure precise measurements

Protection CTs:

Can tolerate a lower accuracy class (e.g., 5P or 10P) as they need to detect faults and overcurrent but do not need extreme precision for normal operations.

CT Type	Typical Accuracy Class
Metering	0.2, 0.5, 1.0
Protection	5P, 10P

4. Burden Rating

The burden rating refers to the load the CT can support while maintaining its accuracy. The
burden is usually expressed in VA (Volt-Ampere). If the burden is too high, the CT may not provide
accurate readings.

Burden (VA)	Application
2.5 VA	Short distance between CT and meter
5-10 VA	Longer distances or higher load

5. Insulation Level and Safety

Since CTs are used in high-voltage environments, the insulation level is a critical factor. It
determines the CT’s ability to handle the voltage without breaking down. Choose CTs with
appropriate insulation for the voltage levels in your system to ensure safety and durability.

6. Core Type

CTs come in different core types, including:

Ring type CTs:

These are often used in protection circuits and can easily be fitted over the primary conductor.

Wound type CTs:

These have both primary and secondary windings and are typically used in low-current applications. • Your choice depends on the installation setup and the current levels involved. For example, ringtype CTs are ideal for retrofitting in existing systems.

7. Frequency

The frequency of the electrical system also impacts CT selection. Most systems operate at either
50 Hz or 60 Hz, but some industrial setups may have different frequencies. It’s important to
choose a CT designed to work at your system’s frequency for accurate performance.

CT Selection Process

Identify Application → Determine Primary Current → Choose Accuracy Class → Check Burden Rating → Select Core Type → Verify Frequency → Ensure Insulation

8. Environmental Conditions

Environmental factors such as temperature, humidity, and dust can affect the performance of a CT. In harsh environments, consider CTs that offer additional protection, such as weatherproofing or hermetically sealed enclosures.

Condition	CT Feature to Look For
High temperature	High-temperature rating
Humid or wet conditions	Waterproof or sealed CTs
Dusty environments	Enclosures with IP ratings

9. Physical Size and Mounting

The size of the CT and how it will be mounted is another practical consideration. For example, compact CTs are ideal for systems with limited space, while larger, heavy-duty CTs may be more suited for industrial setups with spacious control panels.

Choosing current transformer is essential for the accurate and efficient operation of
electrical systems. By considering the application type, primary current rating, accuracy class,
burden rating, core type, insulation level, and environmental factors, you can ensure that the CT
you select will meet your specific needs.
Paras Power provides a range of LT/HT current transformers designed to deliver reliable
performance in various applications. Whether you need a CT for metering, protection, or control,
our products are built to ensure accuracy and safety in any setting.

Factors to be considered when choosing current transformers