What Is a Current Transformer (CT)? Working, Uses & Applications

Introduction

A Current Transformer (CT) is a vital component used in electrical systems for measuring and monitoring current flow safely. CTs are commonly found in electrical panels of industries, commercial buildings, and power distribution networks. They help convert high current levels into small, manageable values that can be read by meters and protective relays.

What Is a Current Transformer (CT)?

A Current Transformer is an instrument transformer designed to reduce high AC current to a lower, proportional current.
For example, a CT with a ratio of 100/5A means:

If 100A flows in the main cable,
The CT outputs 5A on the secondary side.

This allows measuring instruments and protection devices to analyze high current without directly handling dangerous levels.

How Does a CT Work?

A CT works on the principle of electromagnetic induction:

The main power cable passes through the CT core.
When current flows, it creates a magnetic field.
The CT converts this into a smaller current in its secondary winding.
This reduced current is sent to meters, relays, or monitoring systems.

Where Are CTs Used?

1. Electrical Panels

CTs monitor phase currents in:

Main distribution boards (MDB)
Sub-distribution boards (SDB)
Energy metering panels

2. Energy Meters

To measure:

Power consumption
Load distribution
Phase imbalance

3. Protection Systems

CTs feed data to:

Overcurrent relays
Earth-fault relays
Differential protection systems

4. Industrial & Commercial Buildings

For:

Load monitoring
Power factor correction systems
Harmonic analysis

5. Power Plants & Substations

Used in:

High-voltage protection
Grid monitoring
SCADA systems

CTs in Real Electrical Panels (Image Explanation)

In the provided image, you can see:

Three CTs (red-core devices) installed on R, Y, and B phase cables.
Each CT has a clearly marked ratio label.
Wires from the CT secondary side are connected to measuring and monitoring devices.
Cable ties secure the CTs in place to prevent misalignment.

This setup is typical in commercial and industrial electrical panels.