Core-balance current transformers

Core-balance current transformers

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.

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

About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears. Professional in AutoCAD programming and web-design. Present on


  1. maruti.suryavanshi
    Jul 18, 2014

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

  2. 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.

  3. hadi218
    Oct 20, 2013

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

  4. [...] and earth is high. Also capacitive charging current is not large enough to compensate the same, so earth fault current is likely to be excessive.Hence resistance is connected between neutral to ground connection. [...]

  5. amira
    May 13, 2013

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

  6. [...] absence of an earth fault, the vectorial sum of the currents IΔ is equal to zero.In case of an earth fault, if the IΔ value exceeds the rated residual operating current IΔn, the circuit at the secondary [...]

  7. [...] screens surrounding the conductors, and/or armour and metallic wires surrounding the cables.During earth faults applied to directly earthed systems, these metallic paths are expected to carry a substantial [...]

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