A Core Balance Current Transformer (CBCT), also known as a Zero Sequence CT or Residual Current Transformer, is a special type of current transformer used primarily for earth fault protection in electrical power systems. Unlike conventional split core current transformer that measure the current of individual conductors, a CBCT encloses all three phase conductors (and sometimes the neutral) within a single magnetic core.

Working Principle

In normal balanced operation, the vector sum of the three phase currents (Ia + Ib + Ic) equals zero. This means no net magnetic flux is induced in the CBCT core, and hence no secondary current flows.

During an earth fault (ground fault), the sum of the phase currents is not zero (because fault current flows through the ground instead of returning via phases). This creates a net residual flux in the core, which induces a proportional current in the CBCT secondary winding.

The secondary output is connected to a protection relay (e.g., Earth Fault Relay or Residual Current Device), which trips the circuit breaker if the fault exceeds a set threshold.

Equation:
Residual Current,

Ires=Ia+Ib+IcI_{res} = I_a + I_b + I_cIres=Ia+Ib+Ic

If Ires≠0I_{res} \neq 0Ires=0, the CBCT detects it and signals a fault.

Applications of Core Balance Current Transformer

1. Earth Fault Protection – Detects leakage or ground faults in distribution systems, transformers, and switchgear.

2. Motor & Generator Protection – Provides sensitive protection against winding-to-ground faults.

3. Industrial Safety Systems – Installed in switchboards to improve personnel and equipment protection.

4. Neutral Current Detection – Useful in detecting unbalanced loads or leakage current.

Comparison: Core Balance Current Transformer vs. Split Core Current Transformer

Illustrative Diagrams

1. Core Balance Current Transformer

All three phase conductors pass through a common core. In normal operation, flux cancels out; under fault, residual flux induces secondary current.

2. Split Core Current Transformer Example

 Opens and clamps around a single conductor, commonly used for retrofitting in metering systems.