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Current transformers

A current transformer is, in many respects, different from other transformers. The primary is connected in series with the network, which means that the primary and secondary currents are stiff and completely unaffected by the secondary burden.

Instrument Transformers Application Guide
Instrument Transformers Application Guide

The currents are the prime quantities and the voltage drops are only of interest regarding excitation current and measuring cores.

How to specify them?

Important main factors when selecting current transformers are:

Standard (IEC, IEEE international) //

  • Rated insulation level (service voltage)
  • Altitude above sea level (if >1000 m)
  • Ambient temperature (daily temperature or average over 24 hours) − Rated primary current
  • Rating factor (maximum continuous current)
  • Rated secondary current
  • Short-time current
  • Dynamic current
  • Number of cores
  • Burdens (outputs) and accuracies for each core − Pollution level (creepage distance)

Rated insulation level

The current transformer must withstand the operational voltage and overvoltages in the network. Test voltages are specified in the standards in relation to the system voltage. These tests shall show the ability of a current transformer to withstand the overvoltages that can occur in the network.

The lightning impulse test is performed with a wave shape of 1.2/50 μs and simulates a lightning overvoltage.

For current transformers with a system voltage of 300 kV and more the switching impulse test is performed with a wave shape of 250/2500 μs simulating switching overvoltages. It is performed as a wet test. For voltages below 300 kV a wet power frequency test is performed instead.

The dielectric strength of air decreases as altitude increases. Consequently, for installation at an altitude higher than 1000 m above sea level, the external insulation (arcing distance) of the transformer has to be adapted to the actual site altitude.

Note that as far as the internal insulation is concerned, the dielectric strength is not affected by altitude.

Typical designs of CTs
Typical designs of CTs

According to IEC 61869-1 the arcing distance under the standardized atmospheric conditions is determined by multiplying the withstand voltages required at the service location by a factor k.

k = em · ( H – 1000 ) / 8150


  • H = Altitude above sea level in meters
  • m = 1 for power frequency and lightning impulse voltage
  • m = 0.75 for switching impulse voltage

According to IEEE dielectric strength that depends on air should be multiplied by an altitude correction factor to obtain the dielectric strength at the required altitude.

Title:Instrument Transformers Application Guide – ABB
Size:9.9 MB
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Instrument Transformers Application Guide
Instrument Transformers Application Guide by ABB

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More Information


  1. Dragan Tabakovic
    Mar 13, 2020

    Arteche used to have it on their web

  2. Dragan Tabakovic
    Mar 13, 2020

    Very good publication offered by Knut Sjoval

  3. Nhat Anh
    Jan 09, 2017

    The link was dead. Could you reup ?

    • Edvard
      Jan 09, 2017

      Should be ok now. Thanks for noticing.

  4. Gopalkrishnan G pillai
    Apr 29, 2015

    The highly reliable source of back up source is said to be batteries of the Ups. Most of the thwthe batteries (Alcad) declare the shelf life for 20ys of Nicd bateries.
    But pratically there failure starts from 9 yrs on wards even though they are maintained at +/- 1 at 25.c . In order to curtail the losses can you recommend best practice to overcome the failure , also charge discharge frequency cycle , Does and donts for ni-CD batteries.

  5. prakash chand yadav
    Oct 19, 2014

    Most useful for revise of knowledge.

  6. José Jamal
    Oct 19, 2014

    É um documento que resume de uma clara e objectiva esta matéria que estudei ao longo de alguma cadeiras do curso. Obrigado por partilhar um tão precioso conhecimento!

  7. Murali Kavitarkika
    Oct 19, 2014

    I am looking for a Comparison Chart ( or something similar/Equivalent) to the Classes of Accuracy of CTs and VTs in IEC as well as IEEE ( European/American Standards) For Example what will be the equivalent of 5P20 accuracy in IEEE Standards. Is there any ready made chart or formula to do this


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