Introduction to Current Transformers 

By on February 8th, 2013


High Voltage SF6 Current Transformers
Image Source: Courtesy Siemens
Current transformers (CTs) are used to convert high level currents to a smaller more reasonable level for use as inputs to protection relays and metering equipment.  Within electrical systems, current transformers are essential to ensure the correct functioning and control of equipment and for providing operational data and information.

This introductory note looks at the construction of current transformers and their specification.

There are two broad categories of current transformer:

Measuring CTs -  provide signals to meters and instruments

Protection CTs - provide signals to protective relays to enable correct operation under steady state and transient conditions.

Current transformers work on a similar principal to normal voltage transformers.  Two (or more) winding are wound round a magnetic core.  Current flowing in one winding [the primary] creates a magnetic field which drives current in the other winding [the secondary].  The ratio of the primary turns to the secondary turns provides the current scaling.

Example: a 600:5 ratio CT, for every turn on the primary would have 600 turns on the secondary.  A primary current of 600 A would cause 5 A to flow in the secondary. 

The physical construction of a current transformer can be as simple as one primary winding and one secondary winding on a core.  Quite often the construction is more complex with several secondary windings providing different protection and instrumentation needs. 

Specification of current transformers, typically considers the following:

  1. turns ratio - of the primary to secondary current (i.e. 1200/1)
  2. burden - the normal load in VA that the CT can supply
  3. accuracy factors -  the accuracy limits of  (both steady state and transient) 
  4. physical configuration - number of primary or secondary windings, size, shape, etc.

Safety: if a CT secondary is not connected to any load, then it should be short circuited.  If the secondary of the CT was left open during operation, then you would effectively have a transformer with one turn on the primary and many turns on the secondary.  Large and potentially dangerous voltages would be induced at the secondary terminals. 

Specification of Current Transformers

Current Transformer Accuracy

Accuracy of a current transformer is measured by the composite error.  This is defined as the difference between the ideal secondary RMS current and that of the actual secondary current.  It takes into account current errors, phase error and harmonic errors.

Current transformer intended for protection applications need to cover a wide range of current.  Then current value up to to which they will maintain accuracy is the 'accuracy limit current'.   The ratio of the accuracy limit current to the rated current is the 'accuracy limit factor'.

Measuring CT Accuracy Class

Accuracy for measuring current transformers is achieved by allocating an accuracy class to the CT.  For each class the standards define a maximum allowable current and phase displacement error for different load conditions.

± percentage
current/ratio error
± phase displacement error minutes
Current 5% 20% 50% 100% 120% 5% 20% 100% 120%                                                      
0.1 0.4 0.2   0.1 0.1 15 8 5 5 precision measurements
0.2 0.75 0.35   0.2 0.2 10 15 10 10 precision measurements
0.5 1.5 0.75   0.5 0.5 30 45 30 30 high grade kWhr meters
1 3 1.5   1.0 1.0 60 90 60 60 general measurements
3     3   3         general measurements
5     5   5         approximate measurements

Protection CT Accuracy Class

Protection current transformers are defined as either 5P or 10P.  For each of these the current error, phase displacement error and accuracy limit factor are defined

Class Current
Limit Factor
5P ± 1% ± 60 minutes 5
10P ± 3% - 10

Class 'P' current transformers are generally used for overcurrent protection applications.  For more demanding applications, additional specification is required.  In this instance the maximum useful emf is often used - specified as the 'knee-point' of the excitation curve (point at which a further 10% rise in emf, requires a 50% increase in excitation current).

In addition to the above, other current transformer specifications are also in widespread use:

  • P - general purpose with accuracy defined by composite error and steady state primary current
  • TPS - low leakage with performance defined by secondary excitation and turns ratio error
  • TPX - defined by peak instantaneous error during specified transient duty
  • TPY - as per TPX, but remanent flux limited to 10%
  • TPZ - breaker failure application CT with large air gap

Name Plate Ratings

Typical Current
Transformer Nameplate
mage: Courtesy Schneider Electric
All current transformers should have a name plate attached.  The image shows and example of a typical nameplate for a current transformer with one primary and two secondary windings (click for a larger version of the image).

Sizing of Current Transformers

The correct sizing and specification of transformers is essential to ensure trouble free operation of protection and instrumentation systems.  There is a full electrical note dedicated to this at:

How to Size Current Transformers

Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

comments powered by Disqus

Latest Questions:

  1. Electrcial Distribution Cabinets B15
  2. Breakers
  3. Cable AC Resistance calculation
  4. Induction Machine Types
  5. Zone Delimiters in distribution network
  6. Voltage level
  7. Lightning Protection calculation Free Software
  8. How many motors, or of what kind, do I need to get the output to be 5 Volts under load?

most popular notesMost Popular Notes:

newsletter logo

Our Newsletter

Receive updates on new posts by email
down arrow

Electric Power Distribution Engineering, Third Edition
Electric Power Distribution Engineering, ...
Turan Gonen
Hardcover - 1061 pages
Solar Electricity Handbook - 2015 Edition: A simple, practical guide to solar energy - designing and installing solar PV systems.
Solar Electricity Handbook - 2015 Edition: ...
Michael Boxwell
Paperback - 204 pages
Photovoltaic Design and Installation For Dummies
Photovoltaic Design and Installation For ...
Ryan Mayfield
Paperback - 384 pages
How to Solar Power Your Home: Everything You Need to Know Explained Simply (Back to Basics Conserving)
How to Solar Power Your Home: Everything ...
Martha Maeda
Paperback - 336 pages
Independent Energy Guide: Electrical Power for Home, Boat, & RV
Independent Energy Guide: Electrical Power ...
Kevin Jeffrey
Paperback - 280 pages
Power System Relaying
Power System Relaying
Stanley H. Horowitz, ...
Hardcover - 398 pages
Energy Systems Engineering: Evaluation and Implementation, Second Edition
Energy Systems Engineering: Evaluation and ...
Francis Vanek, Louis ...
Hardcover - 672 pages
Smart Power Grids 2011 (Power Systems)
Smart Power Grids 2011 (Power Systems)
Hardcover - 696 pages
Submarine Power Cables: Design, Installation, Repair, Environmental Aspects (Power Systems)
Submarine Power Cables: Design, ...
Thomas Worzyk
Hardcover - 296 pages
Electric Power Substations Engineering, Third Edition (Electrical Engineering Handbook)
Electric Power Substations Engineering, ...
Hardcover - 536 pages
Renewable and Efficient Electric Power Systems
Renewable and Efficient Electric Power ...
Gilbert M. Masters
Hardcover - 712 pages
Electrical Power System Essentials
Electrical Power System Essentials
Pieter Schavemaker, ...
Hardcover - 340 pages
Do It Yourself 12 Volt Solar Power, 2nd Edition (Simple Living)
Do It Yourself 12 Volt Solar Power, 2nd ...
Michael Daniek
Paperback - 128 pages
Daniel Johnson
Kindle Edition - 36 pages
Power Electronics and Renewable Energy Systems: Proceedings of ICPERES 2014 (Lecture Notes in Electrical Engineering)
Power Electronics and Renewable Energy ...
Hardcover - 1607 pages
Grid Converters for Photovoltaic and Wind Power Systems
Grid Converters for Photovoltaic and Wind ...
Remus Teodorescu, ...
Hardcover - 416 pages
Photovoltaics: Design and Installation Manual
Photovoltaics: Design and Installation ...
Solar Energy ...
Paperback - 336 pages
Solar Photovoltaic Basics: A Study Guide for the NABCEP Entry Level Exam
Solar Photovoltaic Basics: A Study Guide ...
Sean White
Paperback - 168 pages
Build Your Own Small Solar Power System
Build Your Own Small Solar Power System
Gavin Webber
Kindle Edition - 59 pages
Renewable Fuel Standard:: Potential Economic and Environmental Effects of U.S. Biofuel Policy
Renewable Fuel Standard:: Potential ...
Committee on Economic ...
Paperback - 250 pages
Photovoltaic Systems Engineering, Third Edition
Photovoltaic Systems Engineering, Third ...
Roger A. Messenger, ...
Hardcover - 528 pages
Power Electronics for Modern Wind Turbines (Synthesis Lectures on Power Electronics)
Power Electronics for Modern Wind Turbines ...
Frede Blaabjerg, Zhe ...
Paperback - 120 pages
Power System Monitoring and Control
Power System Monitoring and Control
Hassan Bevrani, ...
Hardcover - 288 pages
Wind Energy Engineering
Wind Energy Engineering
Pramod Jain
Hardcover - 352 pages
Convex Optimization of Power Systems
Convex Optimization of Power Systems
Joshua Adam Taylor
Hardcover - 209 pages
Power Quality: Problems and Mitigation Techniques
Power Quality: Problems and Mitigation ...
Bhim Singh, Ambrish ...
Hardcover - 596 pages
Multi-Stage Flash Desalination: Modeling, Simulation, and Adaptive Control (Engineering Systems and Sustainability)
Multi-Stage Flash Desalination: Modeling, ...
Abraha Woldai
Paperback - 352 pages
Large-Scale Solar Power Systems: Construction and Economics (Sustainability Science and Engineering)
Large-Scale Solar Power Systems: ...
Dr Peter Gevorkian
Hardcover - 395 pages
Power from Pellets: Technology and Applications
Power from Pellets: Technology and ...
Stefan Döring
Hardcover - 223 pages

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note