Your image

This video was brought to you by, making power systems Intuitive, Open and Free for Everyone, Everywhere. Consider subscribing and supporting through This is a mechanism for you to support us financially so we can continue making high quality power system video tutorials. Our corporate sponsor for this topic is from Seattle, Washington. Contact them for industrial and commercial power system studies.

Brief Introduction to Current Transformers and Its Applications.

In part 2, we talked about the main characteristics of a Current Transformer with the help of an equivalent circuit model for an ideal CT. In this part, we shall briefly discuss CT accuracy class and its importance to metering and protection applications.

Let us first define the meaning of CT accuracy in simple terms. It is the extent, to which the current present in the secondary side of a current transformer is able to faithfully reproduce the current flowing in the primary side.

Under practical conditions, there will always be some error in the output current, when compared to the original current in the primary winding. The error is due to the magnetization current present in the secondary branch. This was discussed in more detail in previous parts.

With the basic definition understood, let us consider the importance of CT accuracy for “protection”. If we consider a fault on a low voltage bus in a 3 phase system, we would require the connected CT to be able to operate effectively at the highest current level present on that particular bus.

This is because we want the “CT” to accurately recognize the fault level, and deliver the signal to the relay connected to its secondary winding, which in turn would cause the “Circuit Breaker” to trip.

For protection, CTs are required to carry the fault currents which can be “10” times the rated full load current. For this reason, we require protection class CTs to saturate at a high value, as this would enable the CT to properly identify the high fault level signature for protective relays.

Examples of protection class CTs include the “5P20”, the “10C400”, as well as various others. We will be discussing these in more detail in later topics.

Let us consider the importance of CT accuracy class for “metering” purposes. We would want metering class CTs to accurately measure the current flowing on the primary side, to the full load current and beyond. These CTs are designed to have a lower saturation value.

This is because under fault conditions, we do not want our metering CT to become permanently damaged. The lower saturation limits the amount of maximum current which can flow through the core, thereby protecting it from damage. These type of CTs are normally used for energy metering and instrumentation purposes.

Since the accuracy class holds critical importance for metering applications, the percentage ratio error is only acceptable within a very small percentage… typically less than 1 percent.

Examples of metering class CTs include “0.15”, “0.3”, “0.3S” and various others. These values tell us about the percentage error, which will be discussed in more detail in following topics in the series.

To summarize the discussion, CT accuracy plays an important role for Protection and Metering applications. They are an important parameter to consider along with CT burden, sizing, polarity & saturation.

The examples used in this tutorial are derived from S.E.L’s paper, Current Transformer Accuracy Ratings. Rest assured, these are good and accurate examples. If mistakes are found, they will be described in detail on general

In part 4, we shall talk about the importance of CT sizing and how it can be used to minimize saturation.

We hope you have a continued interest in this topic and series as a student or professional. We also hope you find this content useful and enlightening. Please consider subscribing to… Making Power Systems Intuitive.

Greetings from the GeneralPAC Team!

We make high-quality Power Systems Video Tutorials on complex topics that are free and open to everyone!  Thank you so much for supporting us through Patreon so can continue doing good and valuable work.

What is Patreon and why do we use it?

Patreon is a fantastic portal that allows our fans and community to make monthly contribution (like Netflix subscription) so we can continue creating high-quality power systems video tutorials. In return, you get access to incredible perks like voting on future topics, getting your questions answered, access to VIP Q/A webinars with the creators of GeneralPAC, and much more! We THANK YOU for supporting us

Why do we need your support?

An incredible amount of time and effort is needed to develop high-quality video tutorials. Each video (Part 1 for example) takes approximately 10 hours to complete which includes learning the concept ourselves, brainstorming creative ways to teach and explain the concepts, writing the script, audio recording, video recording, and editing. It's no wonder why Hundreds-of-Thousands of people have watched, liked, subscribed, and left positive comments on Youtube channel. Your support truly makes all the difference.

Become a patron today!