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Home / Technical Articles / Using Core-Balance Current Transformer For Earth Fault Protection

Sensitive Earth Fault

The core-balance current transformer (or CBCT) is normally of the ring type, through the centre of which is passed cable that forms the primary winding. An earth fault relay, connected to the secondary winding, is energised only when there is residual current in the primary system.

Using Core-Balance Current Transformer For Earth Fault Protection
Using Core-Balance Current Transformer For Earth Fault Protection

The advantage in using this method of earth fault protection lies in the fact that only one CT core is used in place of three phase CT’s whose secondary windings are residually connected. In this way the CT magnetising current at relay operation is reduced by approximately three-to-one, an important consideration in sensitive earth fault relays where a low effective setting is required. The number of secondary turns does not need to be related to the cable rated current because no secondary current would flow under normal balanced conditions.

This allows the number of secondary turns to be chosen such as to optimise the effective primary pick-up current.

Core-balance transformers are normally mounted over a cable at a point close up to the cable gland of switchgear or other apparatus. Physically split cores (‘slip-over’ types) are normally available for applications in which the cables are already made up, as on existing switchgear.

SOURCE: Network Protection & Automation Guide -Current and Voltage Transformers

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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. Swapan Kumar Mukherjee
    Jun 26, 2022

    CBCT size will depend on the cable cross section/ bus bar size.

  2. karabi das
    Oct 22, 2019

    Please clarify me difference between 3P3L & 3P5L PT

    • moham
      May 07, 2021


      Using Core-Balance Current Transformer For Earth Fault Protection.

      Using Core-Balance Current Transformer For earth leakage protection.

  3. karabi das
    Oct 22, 2019

    can i connect earth leakage relay with nornal protection ct which is use for basically over current & earth fault protection??

  4. Syed Imran
    Sep 28, 2019


    Why do we connect CT & CBCT in distribution panel?

  5. Tarun
    Dec 10, 2018

    can we do the HI pot test on cable if CB CT connected in cable . please advsie

  6. smijo
    Jan 24, 2017

    Do CBCT have any direction for primary current ( Like Normal CT ) ???

  7. Y O U N E S
    Jan 05, 2017

    I want to design and calculate CBCT,which Formula i shoulad use?

  8. Rudolf
    Nov 10, 2016

    I have doubt about core balance CTs installation. Due to size of cables we have to use 2 CTs per cubicle = 2 single core cables per phase. This CTs are very close to each other (touching in some cases). Is it eventually problem (risk) that they will interfere each other? Problem with EMC?
    Thanks for advice.

  9. Rhea Johnson
    Apr 02, 2016

    Hi, can you tell me how to select a core balance current transformer? they have specified it to be PS Class? what calculations do I follow and where do i find them?

  10. Abhik
    Jan 12, 2016

    Observation : 3 separate armoured 1core cables for 3 phase passes through common cbct. During motor loading higher than rated current spurious earth fault is coming. Reason and remedy are reqd.

  11. Ramprasath
    Jul 10, 2015

    Can we use core balance CT in 11kv or 22kv?will it withstand those voltage

  12. maruti.suryavanshi
    Jul 18, 2014

    In 4 core cable (3p+N) do we pass N core through core balance?

    • Dipankar
      Apr 16, 2015

      In case of 4Core or 3.5 Core cable, if Neutral is passed through CBCT, then in case of any phase unbalance, the Earth fault CT connected to it will not trip since the differential current shall flow through the neutral. Thus the relay will not trip. But, if only the three phase cores of the cable is passed through the CBCT, then occurance of any phase unbalance as well as during earth fault the Relay connected to that CBCT will trip.

        Apr 05, 2018

        Whatever you said above, is TRUE only in 3 ph 4 wire system, for e.g. 415V (LV) distribution system. If it is 3 ph 3 wire system , like in 11kV or 6.6kV distribution systems, even if there is unbalanced loading, that is difference in R,Y, B phase currents, algebric sum (vectorial) current Ir+Iy+Ib =0 & hence there will/should be no current in CBCT secondary & relay will not operate. Only if there is Earth fault in one or two phases, there will be current in CBCT secondary & relay will operate.

  13. Thanh
    Jun 05, 2014

    Can anybody tell me what is the meanings of CBCT’s parameters below?
    Zero phase current ratio Pri./sec. 200mA/100mV (75~125mV at 2kohm); This zero phase sequence current transformer has the rated current of 80A. I used it for signaling a earth fault relay of a motor control unit.

  14. hadi218
    Oct 20, 2013

    In 4 core cable (3p+N) do we most pss N core through core balance?

  15. amira
    May 13, 2013

    Any advise about how to select/size the convenient CBCT? and who are the most common manufactures for this product.

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