What are CT Coils?

Current Transformers (CTs) are essential components in modern electrical systems, providing accurate current measurement and system protection. This brief delves into the intricacies of CTs, covering types, specifications, practical applications, and manufacturing details.

Full Form and HSN Code of CT

CT stands for Current Transformer. The HSN Code for CTs is 8504, classifying them under electrical transformers, static converters, and inductors. These are typically winded on a toroidal shuttle winding machine.

Types of Current Transformers

  1. Bar-type: These CTs have a straight primary winding, suitable for installations where space is a constraint.
  2. Wound-type: CTs use multiple primary winding turns to achieve higher accuracy in precise current measurement applications.
  3. Toroidal: These CTs feature a donut-shaped core with no primary winding. The conductor carrying the primary current passes through the core, ensuring high accuracy and efficiency. Silicon steel or ferrite core materials in these CTs provide excellent magnetic properties and minimize energy loss during current transformation.

Primary Winding in CTs

The primary winding connects directly to the conductor that carries the current for measurement. This winding handles the full load current of the system, ensuring accurate transformation to the secondary circuit.

Determining the Ratio of CTs

A CT’s ratio is simply determined by dividing the Secondary Current (A) over Primary Current (A)​.

Interpreting CT Specifications like 100/5/5A & 50VA

  • 100/5: This ratio indicates a primary current of 100A and a secondary current of 5A, resulting in a 20:1 current transformation ratio.
  • 50VA: This rating specifies the maximum load the CT can handle on its secondary side without losing accuracy.


Polarity in CTs is important to maintain the correct phase relationship between the primary and secondary currents. Manufacturers mark CT terminals with polarity designations (H1, H2 for primary; X1, X2 for secondary) to ensure correct installation.

Difference Between CT and PT

  • CT (Current Transformer): Measures and reduces high currents to lower levels, enabling safe and accurate measurement by standard instruments. Technicians connect CTs in series.
  • PT (Potential Transformer): Steps down high voltages to lower levels for measurement and protection. Also known as voltage transformers, technicians connect PTs in parallel.

Consider checking out our in-depth article for technical details comparing CT and PT magnetics.

Potential Transformers: Types and Connection

  • Types: Potential transformers come in various types, including electromagnetic and capacitor voltage transformers. Each type suits specific applications and voltage levels.
  • Connection: Technicians typically connect PTs in parallel to the circuit requiring voltage measurement. They provide an accurate representation of the primary voltage on the secondary side at a lower, measurable level.

Is a CT an Inductor?

A CT is not a simple inductor. It is a transformer designed for current measurement and transformation, consisting of primary and secondary windings and operating based on electromagnetic induction principles.

Importance of CTs in Electrical Systems

CTs are vital in electrical systems for several reasons:

  • Accurate Measurement: They enable precise current measurements, crucial for metering and monitoring.
  • Safety: By reducing high current to safer levels, CTs protect measurement instruments and operators.
  • Protection: CTs help in detecting faults and triggering protective mechanisms to prevent equipment damage and ensure system stability.

Instrument Transformers – A CT Application

CTs reduce high currents to lower, manageable levels, allowing measuring instruments to take accurate readings. This step-down ensures standard measuring instruments can safely and accurately measure the current. CTs are integral to metering and protective relaying systems, providing necessary inputs for accurate readings and system protection.

Types and Classes of Current Transformers According to IEC 60441

The International Electrotechnical Commission (IEC) classifies CTs based on their performance characteristics:

  • Class P: Standard protection class for general applications
  • Class PR: High-accuracy protection with defined composite error
  • Class PX: Precision protection with defined limits for accuracy and linearity
  • Class TPS: Special protection with defined transient performance parameters
  • Class TPY: Protection with specific transient response characteristics
  • Class TPZ: Protection with stringent transient response requirements

Input and Output Ranges

At Shreejee Electronics, we manufacture CTs and PTs with various input and output ranges to meet diverse application requirements. Our products ensure high accuracy and reliability across different current and voltage levels.

Choosing the Right CT

When selecting a CT, consider the following factors:

  • Current Rating: Ensure the primary current rating aligns with your system’s current levels.
  • Accuracy Class: Select a CT with an accuracy class matching your application’s precision needs for metering or protection.
  • VA Rating: Ensure the VA rating is sufficient to drive the connected load without compromising accuracy.

Output Voltages and Flux Density

The output voltages of CTs vary based on their design and application requirements. The flux density of a CT coil is a crucial parameter that affects its performance. It determines the magnetic field strength within the core and impacts the accuracy of current measurement.

General Prices of CTs in Rupees

In India, the price range for CTs typically varies from ₹25 to ₹5000. The cost depends on the type, form factor, electrical specifications, insulation, and quality of the CT.

Where to Buy Current Transformers

You can purchase high-quality and reliable CTs from Shreejee Electronics. We design and offer customized CTs to meet various electrical application needs, ensuring safety and accuracy. We manufacture our products to the highest standards, ensuring dependable performance for all your current measurement and protection needs.

For more detailed information on CTs, their types, specifications, and applications, feel free to reach out to us. At Shreejee Electronics, we are committed to delivering top-notch solutions tailored to your specific requirements.

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