Current Transformer: The Ultimate Guide (2025 Edition)

Current Transformers are the essential tools of measurement and protection in electrical systems. Whether we talk about power grids, industrial plants, or even commercial buildings, CTs enable current monitoring with high accuracy and safety. But the question arises: How do they operate, and why are they needed? Have a seat! This article will walk you through CTs, including the current transformer working mechanism, their types, differences, and uses. So, let’s go ahead and discover the world of CTs!

Figure no 1 Current Transformers

1) What is Current Transformer (CT)?

“A Current Transformer is a device that can control and reduce the currents in high-voltage systems (like power plants) and makes the current manageable for current measuring devices .”

It is broadly applied in power systems, industrial processes, and electric systems for protection. It electronically transfers a safe, accurately proportional value of current to measure instruments or protective relays.

Figure no 2 What is the current transformer

Usually, CT transformers come with a single-phase electric motor that incorporates a primary and secondary winding and magnetic core. The primary winding carries a high current, while the secondary winding provides a reduced current that is proportional to the primary current. This feature makes it possible for electrical engineers and technicians to connect to high-current and high-voltage electric circuits safely and measure the current.

• Current Transformer Operation Key Features

• Isolation: CTs allow for a clean separation of high-voltage circuits from the measuring devices, and thus, no direct contact is made with the circuit.

• Accuracy: Per IEEE standards, modern CTs are capable of achieving accuracies as high as 0.3% and still deliver the needed results.

• Burden Capacity: To prevent damage from occurring to current transformers, a burden resistor must be attached to the CTs.

2) Current Transformer Working Principle

To be straightforward, the basic Current Transformer operates based on electromagnetic induction. In case you didn’t know, electromagnetic induction can be best defined as;

“The process of converting electrical energy into magnetic energy and vice versa.”

To put it simply, this is how a Current Transformer works: in a CT, an alternating current that flows through the primary winding is used to make the core create a magnetic field. In the latter part of the transformer, there are secondary magnetic motors. These motes generate a magnetic field proportionate to the voltage induced in the secondary winding.

In the case of an autotransformer, we determine the voltage through the turns ratio of primary to secondary (the greater the windings in the primary coil, the less the voltage). To make it clear, a 10:1 ratio in windings means a voltage of 10,000V will result in only 1,000V on the secondary coil. Now, the secondary coil will transfer this reduced current to the measuring device.

Figure no 3 Working of Current transforer

The transformation ratio of a CT is given by::

Ip = Primary current

Is = Secondary current

Np = Number of turns in the primary winding

Ns = Number of turns in the secondary winding

One more thing to note is the assumption that a CT maintains that there is an accurate transformation of current from primary to secondary. This is achieved by connecting the primary and secondary windings magnetically through the core, which is around the wires. Since the induced current is in the secondary winding is a scaled-down model of the primary current, the monitoring and protection of electrical circuits can be done easily and efficiently.

To achieve the required precision, a violation unit (such as a measuring gadget or relay) should be connected to the secondary winding. If the secondary circuit is left unconnected, it can result in dangerously high voltage due to no opposing force to EMF (Electromotive Force), and that can potentially damage the CT or create safety hazards.

3) What is a Current Transformer Used For?

Current transformers are highly prevalent in the following fields of work:

a) Measurement

CTs are an integral part of all electrical measurement systems and allow for accurate high-current usage. CTs feed ammeters, wattmeters, and energy meters.

b) Protection

CTs are highly effective in protective relaying in power systems. They capture excessive currents (for instance, short circuits or overloads) and send command signals to protection relays, which actuate circuit breakers to avert damage.

Figure no 4 Uses of current transformer

c) Energy Management

CTs are used for power consumption monitoring and analysis in industrial establishments and commercial buildings. Energy monitoring through CTs improves load allocation and prevents unnecessary use of energy, thus decreasing operational costs.

d) Fault Identification

Current transformer CTs assist in identifying faults in electrical networks by monitoring the current of the region in real time. This action facilitates the mitigation of power outages and equipment harm. Moreover, fault detection is critical to system reliability since it minimizes the damage risks and system downtimes by allowing abnormalities to be addressed early.

e) Stability of The Power System

Power distribution and transmission rely on CTs to maintain stability with sufficient equal power allocation and load apportionment. CTs continuously monitor the current flow in transmission lines, detecting deviations and disorderly changes. Yes! Otherwise, it would lead to grid collapses and ensure service continuity of the power systems.

f) Automation of Industries

CTs have become increasingly prominent in industrial automation systems where current measurements need to be very accurate. These Transformers feed automation controllers as well as process control monitoring systems, allowing industries to be more efficient and safer.

g) Systems for Renewable Energy

CTs have grown to be very important in energy management systems to monitor and control the current in solar and wind power installations. They are also useful for monitoring energy production, controlling the load balance, and connecting the battery with the power grid, which increases the reliability of the renewable energy distribution systems.

4) Types of Current Transformers

There are various types of current transformers available in the market; let’s discuss some prominent ones.

i) Bar-type current Transformer

This transformer consists of a solid bar that serves as the primary conductor, making it suitable for use in high-voltage ranges. This design allows for accuracy and high reliability. Bar-type CTs are typically used in power substations or transmission lines where precise measuring of high current levels is mandatory.

Figure no 5 Bar-type transformer

ii) Wound-Type Current Transformer

This type includes a primary coil that is placed around a magnetic core for low-current, precise applications. Such CTs are perfect for industrial tools and power distribution boards where direct current measurement is required.

iii) Toroidal Current Transformer

Unlike traditional models, this CT does not feature a physical primary winding; the primary conductor also functions as a core. Due to its small size, it is effective for residual current tracking. Apart from this, these CTs are commonplace in both residential and commercial circuit breakers for ground fault protection.

iv) Split-Core Current Transformer

These CTs are popular in energy audits, industrial control systems, and for temporary use where little or no modification is needed.

v) Protection Current Transformer 

This CT is specially manufactured for protective relays on power plants and assures accuracy under fault conditions. It is used in power transmission and distribution industries. Moreover, protection CTs are used to safeguard high-voltage devices and the electrical grids' operating range.

vi) Clamp-On Current Transformer

This is a type of CT that is commonly used for non-invasive measurements by electrical workers. It can be unclamped from a conductor instead of forcibly removing it from the circuit. It is also widely used during maintenance, fault diagnosis, and energy consumption audits because of its ease of use and wide range of usability.

Figure no 6: Clamp-on current transformer

vii) Multi-Ratio Current Transformer 

This one offers the versatility of multiple uses due to the selection of different transformation scopes, making it useful in most applications. These CTs are often used in substations and other industrial facilities that require flexible monitoring of current.

5) Current Transformer vs Potential Transformer

A Current Transformer (CT) is a large device meant for high currents that have to be lowered to a manageable level for easier measurement. It is used in series with the circuit being measured, and the output is connected to ammeters, energy meters, and protective relays.

A Potential Transformer (PT), or ( voltage transformer ), on the other hand, is built to lower voltage levels for measurement and protection purposes. It is connected in parallel with the circuit and used extensively in voltage metering and voltage monitoring circuits.

Feature	Current Transformer (CT)	Potential Transformer (PT)
Function	Measures current	Measures voltage
Primary Side	Connected in series	Connected in parallel
Secondary Side	Connected to ammeters, relays	Connected to voltmeters, protection devices
Output	Low current	Low voltage
Common Applications	Power monitoring, protection	Voltage regulation, metering

6) CT vs Normal Transformer: Key Differences

Even though Current Transformers (CTs) and typical transformers function under the principles of electromagnetic induction, each serves a different function in the electrical system. A CT’s purpose is to measure and monitor current, whereas a normal transformer is used for voltage transformation and power transmission.

Feature	Current Transformer (CT)	Normal Transformer
Purpose	Measures and lowers high currents for metering and protection.	Scale up or scale down voltage for power transmission and distribution.
Primary Winding	It has very few turns or consists of a single conductor passing through the core.	Has multiple turns to step up or step down voltage efficiently.
Secondary Winding	Provides a proportional current output to measuring instruments or relays.	Delivers transformed voltage or power to the connected load.
Load Connection	Current transformer connection is done to measuring instruments like ammeters and protective relays.	Directly connected to electrical loads for power supply.
Accuracy	Designed for precise current measurement with minimal errors.	Designed to efficiently transfer power with minimal loss.
Saturation	It can saturate under excessive current, affecting measurement accuracy.	Usually designed to avoid saturation for efficient energy transfer.
Insulation Level	Requires high insulation due to direct connection with high-current conductors.	Insulated based on voltage levels but not necessarily for extreme currents.
Application	Used in electrical metering, protection systems, and energy monitoring.	Used in power distribution, voltage regulation, and electrical transmission.

Normal transformers are used to stabilize energy and regulate voltage, while CTs are primarily used for protection and monitoring. Knowing these distinctions provides insight for making informed selections of transformers for specific purposes.

If you are looking for quality current transformers, Delixi Electric can be the top choice. Delixi Electric is committed to delivering high-quality Current Transformers that require accurate measures and protection for power systems. Our transformer's accuracy, safety, and durability make them suitable for industrial and grid utilization.

They also have advanced insulation and reliable performance because of our stringent quality control. Check out our full range here.

7) FAQ

1. What Should You Never Do with a Current Transformer?

A CT should never be left open when energized, as the secondary will produce dangerously high voltage. Such high voltage can destroy insulation, cause electric shock, or damage other connected devices.

2. Why is CT called a step-up transformer?

A CT steps down the current while increasing voltage proportional to the current. It behaves like a step-up transformer while in open secondary voltage because it generates very high secondary voltage.

3. What is P1 and P2 on a CT?

P1 and P2 are the primary terminals of a CT, and the direction of the current is correct in relation to current measurement. Current enters through P1 and exits through P2, which is also critical for the current protection system to work.