The core balance current transformers are intended for providing earth leakage protection on 3 phase electrical system. These currents transformer are different from normal protective and metering current transformers due to their performance requirement.

In insulated or compensated neutral networks, earth leakage currents Are small and generally very much less than the nominal load current. Such earth leakage current should not be allowed to exist for long periods of time, since eventually it leads to insulation failure on the healthy phases and subsequent development in phase to phase or double phase to earth faults. To protect cable circuits and also overhead transmission lines with cable terminations, core balance current transformers are commonly employed.

Generally, it is sufficient to incorporate insulation monitoring for signaling appearance of earth leakage. The operating staff in such  cases will be able to take measures to switch the load over to other feeders and switch out the faulted circuits for repairs. Exception to this rule are circuits which supply power to peat pits; coal mines and similar loads where, in view of the safety considerations, the protection system is designed to switch out the circuit in the event of occurrence of earth leakage.

Core Balance Current Transformers are manufactured with one core and one secondary winding. The number of secondary turns does not need to be related to the cable rated current because no secondary current would flow under normal balnced conditions. This allows the number of secondary turns to be chosen such as to optimize the effetive primary pickup current. The choice of ratio should therefore be left to the manufacturer to obtain the best possible results. When mounted on insulated cables, irrespective of the voltage ratings of the cables, the insulation level of C.B.C.T. shall be 0.66/3kV.

Core Balance Current Transformers are used with suitable relays for the earth leakage protection purposes. C.B.C.T. encircles a 3 Phase, 3 core cable or 3 single core cables. During healthy conditions i.e. when there is no earth leakage current, the secondary of C.B.C.T. does not carry, any current as there is no magnetic flux in the core. In the event of occurrence of earth leakage an unbalance current sets up flux in the core of the C.B.C.T. and current flows through the secondary winding, causing the relay to operate.

If more than one 3 phase cables are used in parallel, it is recommended that separate C.B.C.Ts. for each cable should be provided. Secondary windings of all C.B.C.Ts. atre connected across one relay or separate relays may be provided for each C.B.C.T.

The purchaser should provide the following information while ordering a C.B.C.T.

1. Cable size or minimum I.D. required for C.B.C.T. (For guidance see Annexure 1 & 2).

2. Minimum earth leakage current to be detected.

3. Specifications of the relay to be used including relay burden and relay setting proposed to be used. (For guidance see Annexure 3)