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Instrument and protection CTs

Current transformers are used to supply information to the protective relays and/or current, power and energy metering “instruments”. For this purpose they must supply a secondary current proportional to the primary current flowing through them and must be adapted to network characteristics: voltage, frequency and current.

Learn How To Specify Current Transformers
Learn How To Specify Current Transformers (photo credit:

They are defined by their ratio, power and accuracy class. Their class (accuracy as a function of CT load and of overcurrent) is chosen according to the application.

A “protection” current transformer (CT) must saturate sufficiently high to allow a relatively accurate measurement of the fault current by the protection whose operating threshold can be very high. Current transformers are thus expected to have an Accuracy Limit Factor (ALF) that is usually fairly high. Note that the associated “relay” must be able to withstand high overcurrents.

An “instrument”current transformer (CT) requires good accuracy around the nominal current value. The metering instruments do not need to withstand currents as high as the protection relays. This is why the “instrument” CTs, unlike the “protection” CTs, have the lowest possible Safety Factor (SF) in order to protect these instruments through earlier saturation.

Some current transformers have secondary windings dedicated to protection and metering. These “instrument” and “protection” CTs are governed by standard IEC 60044-1 (in France NF C 42-502).

The matching of CTs with protection relays calls for a thorough knowledge of current transformers. The following section gives a few reminders of CTs corresponding to this use.

Characterisation of CTs

An example of a protection CT //

  • Rated primary current: 200 A,
  • Rated secondary current: 5 A.
An example of a protection CT
An example of a protection CT

Its accuracy load: Pn = 15 VA
Its accuracy limit factor is ALF = 10

For I = ALF. In, its accuracy is 5% (5P), (see figure 1)

To simplify, for the protection CT given in example, the ratio error is less than 5% at 10 In , if the real load consumes 15 VA at In. However these data are not sufficient. Also, it is useful to know the standard values.

Example of the nameplate of a current transformer with two secondaries
Figure 1 – Example of the nameplate of a current transformer with two secondaries

12 definitions related to current transformers //

≡ Rated (nominal) primary current I1

Defined by standards, it is chosen from the discrete values: 10 – 12.5 – 15 – 20 – 25 – 30 – 40 – 50 – 60 – 75 A and their decimal multiples.

≡ Rated (nominal) secondary current I2

Equals 1A or 5 A.

≡ Ratio (I1 / I2)

The primary and secondary currents are standard, thus these values are discrete. (Learn more about ratios of magnetic HV instrument current transformers – Here)

≡ Accuracy load

Load value on which the accuracy conditions are based.

≡ Rated (nominal) accuracy power Pn

Expressed in VA, it is the apparent power supplied to the secondary circuit for the nominal (rated) secondary current and the accuracy load. The standard values are: 1 – 2.5 – 5 – 10 – 15 – 30 VA.

≡ Real power Pr

In this technical article, it is the power corresponding to the real load consumption of the CT at In.

≡ Accuracy class

This class defines the error limits guaranteed on the ratio and on the phase shift in specified power and current conditions. For the nominal 5P and 10P classes, the table in figure 6 defines these limits.

Figure 2 // Errors on the module and the phase at nominal current
(according to standard IEC 60044-1)

Accuracy classCurrent error for
the nominal current as a %
Phase shift for the nominal currentComposite errors for the accuracy limit current as a %
5P± 1± 60± 1.85
10P± 310

≡ Special accuracy class

Class X is a class defined by British standard BS 3938. It must also be defined in the future standard IEC 60044-1 under the name of class PX. This class specifies the minimum value of the knee point voltage Vk of the CT.

It also imposes a maximum value of Rct (CT secondary winding resistance). Sometimes, it specifies the maximum value of the magnetising current Io at knee point voltage.

If we consider the magnetising curve V(Io) of the CT, the knee point voltage Vk is defined as the point on this curve from which a 10% increase in voltage causes a 50% increase in the magnetising current Io. Class X corresponds to a better metering accuracy than classes 5P and even more so 10P (see figure 3).

Voltages corresponding to different CT classes
Figure 3 – Voltages corresponding to different CT classes

It is always possible to find an equivalence between a CT defined in class X and a 5P CT or in some cases even a 10P CT.

≡ Real accuracy factor (Fp or Kr)

This is the ratio between the overcurrent corresponding to the nominal error and the rated current of the CT when the real load is different from the nominal load.

≡ Accuracy limit factor (ALF or Kn)

This is the ratio between the nominal overcurrent (e.g. 10 In) and the rated current (In).

≡ Short time withstand current

Expressed in kA, this is the maximum current Ith that can be withstood for one second (when the secondary is short-circuited). It represents the thermal withstand of the CT to overcurrents (the standard values are given by the standards mentioned in the appendix).

≡ CT rated voltage

This is the rated voltage to which the CT primary is subjected. It is important to remember that the primary is at HV potential and that one of the terminals of the secondary (which must never be opened) is normally earthed.

Just as for any devices, a maximum withstand voltage for one minute at power frequency and a maximum impulse voltage withstand are also defined. Their values are defined by the standards.

For example: for a rated voltage of 24 kV, the CT must withstand 50 kV for 1 minute at 50 Hz and 125 kV at the impulse voltage.

CT with several secondaries

Some current transformers may have several secondaries dedicated to protection or to metering. The most typical cases are CTs with 2 secondaries, more rarely with 3 secondaries. Physically, these CTs group in the same mould the equivalent of 2 or 3 separate CTs that can have different classes and ratios (see figure 4 below).

Manufacturing principle of a CT with 3 secondaries (with 3 windings in the same mould)
Figure 4 – Manufacturing principle of a CT with 3 secondaries (with 3 windings in the same mould)

Current transformers – VIDEO sessions

What are CTs and why use them?

CT Polarity


Wye connected CTs

Delta connected CTs

Current transformer model

Reference // Cahier Technique Schneider Electric no. 194 – Current transformers: how to specify them by Schneider Electric

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Jan
    Aug 19, 2020

    Hi Edvard, Very usefull information! I have a client who wants to substitute the pre-defined Class X CT with an existing 5P20 CT due to space constraints. Are there any special considerations required by substituting a Class X CT (on diff protection) with a 5P20 CT specified for OC/EF protection?


  2. Salih Ahmad
    Jul 16, 2020

    You work great and bring us great benefit through your topics

  3. Hikmet Trnka
    Apr 29, 2019

    Info is very impressively.

    • Dipankar chatterjee
      Dec 14, 2019

      Where is Rct? How this is calculated? A detail must be given here.

  4. radojko micic
    Apr 28, 2019


  5. Yasir
    Apr 28, 2019

    Sir.i am very impressed your information

  6. Stephen Bwalya
    Apr 28, 2019

    Hi Edvard, Thank you for all this invaluable information and clarity you provide.

    Could you do an article for System Studies on a Distribution Network?

  7. Khouzem
    Dec 15, 2018

    What’s the main difference between CT of protection and CT of metering?

  8. Karthi- India
    Apr 12, 2018

    Hi.. I want to know how you are selecting the Magnetizing current limits for the knee point ( e.g) Vk=30mA , 45mA or .etc… and where the Vk , Vk/2 , Vk/4 are applicable / selected .? Very urgent .. please reply sir..

    • al-ibaan
      Nov 19, 2020

      Have you got an answer for that question ? Cuz I got the same inquiry from a client

  9. Saravanan
    Nov 29, 2017

    Sir I would like to know the difference between PS class, PX clear, TPX, TPY, TPZ. Please explain.

  10. Waseem sajjad
    Nov 15, 2017

    Info is very impressively

  11. MADHU M K
    Jul 24, 2017

    Dear EDVARD sir, The technical information which you are providing is very useful

  12. Amin Mustangin
    Apr 20, 2017

    How to get the PDF version of this article?

    • Edvard
      Apr 20, 2017

      See the blue button on your left ‘Get PDF’ and follow the simple instructions. This makes PDF for you.

      • Amin Mustangin
        Apr 20, 2017

        Thank you! I think should open it by using desktop/laptop.

        • Edvard
          Apr 20, 2017

          Yes, sorry I didn’t mention this information. On mobile it’s not available.

  13. Shantanu Deshpande
    Dec 08, 2015

    As always, the info is very impressively put together. Thanks man.

    Further could you take up following topics:-

    1. Metering CT: How to select a CT for an application. For eg. if we have a load of 400A on a source than can provide 800A, then should we go for a 400/1 CT, 600/1 CT or 1000/5 CT.

    2. Effects of Under sizing, Over sizing the CT

    3. CT callibration. Why and how ?

    I ask this because I am dealing with an environment where we have variable load. The load increases, reaches peak and decreases over a period of time.

    So would we be needed to change the CTs also with the load ?

    • D Verma
      Apr 21, 2020

      Select the CT based on the size of the highest load rating / current rating the respective bus bar rating.
      No adverse effect of over sizing.
      Standard CT shall be procured based on calibrated by authorized agency.

  14. Shreesh
    Dec 07, 2015

    Could you talk about US standards based current transformers . Please make distinction between US and IEC ratings — so it is easy for reader to understand the different electrical markets.

  15. sathish
    Dec 05, 2015

    Sir, I am very much impressed about ur site informations.

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