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Electrical characteristics – primary circuits

The primary circuits of the current transformer must withstand the constraints related to the medium voltage network to which it is connected.

Current Transformers Selection Guide
Current Transformers Selection Guide

Rated frequency

This is the frequency of the installation. A CT defined for 50 Hz can be installed on a 60 Hz network with the same level of accuracy. However, the opposite is not true.

For a non-referenced unit, it is vital to indicate the rated frequency on the order from.

Rated voltage of the primary circuit (Upn)

General case:

Insulation level continuity for the whole installation will be ensured if the rated voltage of the CT used is the rated voltage of the installation. The rated voltage determines the insulation level of the equipment. Generally we choose the rated voltage based on the duty voltage, Us, according to the following table:

Rated voltage of the primary circuit - General case
Rated voltage of the primary circuit – General case

Specific case:
If the CT is installed on a bushing or a cable providing insulation, the CT can be LV ring type.

Primary service current (Ips)

Knowledge about the primary service current will enable us to determine the rated primary current for the CT taking into account any eventual derating.

The service current depends on the power traversing the primary windings of the CT.

S = apparent power in VA
Ups = primary service voltage in V
P = active power of the motor in W
Q = reactive power of the capacitors in VAR
Ips = primary service current in Amp

We will have :

Incoming cubicle:

Incoming cubicle - Primary service current

Generator incomer:

Generator incomer - Primary service current

Transformer feeder:

Transformer feeder - Primary service current

Motor feeder:

Motor feeder - Primary service current

η = efficiency of the motor

If you do not know exact values for j and h as a first approximation, you can assume that: cos j = 0.8 ; h = 0.8

Capacitor feeder:

1.3 is a de-rating factor of 30% which compensates for heat-up due to harmonics in the capacitors.

Capacitor feeder - Primary service current

Bus tie:

The Ips current in the CT is the highest permanent current that can circulate in the connection.

Title:Current Transformers Selection Guide – Schneider Electric
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Current Transformers Selection Guide
Current Transformers Selection Guide

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More Information


  1. Maxwell
    Jul 10, 2021


    What are the down sides of using a 1200:5 CT on a 1600 A bus.


  2. Madhu
    Oct 08, 2019

    How to select correct rate of ct

    Jun 03, 2019

    i am dgm(e&m) in NCL Coal India

    May 13, 2019

    hi , i have 630A ,440v breaker ,i want to install CT in each phase what ratio CT should i install in bus bar of that breaker ,busbar size is 20*35 in each phase

  5. Amir
    Nov 30, 2016

    are there any effects for different starting motor when we want to selecting the CT?
    Sample :
    I have motor cap : 2000 kW.
    When we using Soft starter, basic design choose 600/5,
    What if we using DOL? we need to change the CT ratio or not?

  6. Rama
    Jun 24, 2015

    Useful article

  7. Ahmed
    Jun 20, 2015

    Hello brothers ,
    Any one has the reference showing how much can the transformer loading % for permanent


  8. Mohammad Alshammari
    Jan 18, 2014

    can you give me some information about important test for CT
    and what is the best device we can use ?

    • nauman
      Jan 19, 2014

      Ct megnatisation test,ct winding resistance,polarity test and ct ratio test s are important….

  9. Nisar Bahleem
    Oct 24, 2012

    Current Transformers please any one elaborates the CTs made by ENERGOINVEST in 1970s according to their construction/design and selection.?

    Nisar Bahleem

  10. mohammed ashwaq
    Sep 22, 2012

    hey bro am the continous follower of your technical aritcles…am very much passionate about my electrical engg…..and wanna always enhance my self with more and more of this kind of articles…but…as am MEP field working as electrical engg here in dubai….i would love if you can suggest me some books or some site which can help me … my work

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