Short Answer:

A core-balance current transformer (CBCT) is used to detect earth faults (leakage currents) in electrical systems. Its purpose is to sense any imbalance between phase conductors and generate a signal if a leakage or fault current flows to the ground, which is not part of the normal return path.

In normal conditions, the sum of currents in all phase conductors is zero, so the CBCT outputs nothing. But during an earth fault, the imbalance is detected, and the CBCT sends a signal to the protective relay to trip the circuit quickly and safely.

Detailed Explanation:

Purpose of a core-balance CT

A core-balance current transformer (CBCT), also known as a zero-sequence CT, is a special type of current transformer used in ground fault protection systems. It plays a crucial role in detecting leakage currents or faults between a phase conductor and the earth, especially in low-voltage and medium-voltage systems where personnel safety and equipment protection are vital.

The CBCT provides sensitive and accurate detection of even small leakage currents, making it especially useful in residual current protection, earth fault relays, and differential protection schemes.

Working Principle:

• A CBCT has a toroidal magnetic core through which all phase conductors (L1, L2, L3) and sometimes the neutral conductor pass.
• Under normal conditions, the vector sum of the currents in all conductors is zero (since outgoing and returning currents cancel out), resulting in no magnetic flux in the core, and hence, no output.
• If an earth fault occurs, part of the current flows through the ground, causing an imbalance among the phase currents.
• This imbalance induces a magnetic flux in the core of the CBCT, generating a secondary current, which is sensed by the connected earth fault relay.
• The relay then trips the breaker to disconnect the faulty circuit.

Applications:

1. Earth Fault Protection:
   • Widely used in industrial panels, distribution boards, and motor control centers to detect ground leakage.
2. Neutral Grounding Protection:
   • Protects equipment from insulation failures and shock hazards by isolating the faulty section.
3. Generator and Transformer Protection:
   • Used for sensitive earth fault detection in the neutral side of generators or transformers.
4. Cable Protection:
   • Detects leakage in underground cables or long feeders where traditional overcurrent protection may not respond to low-level faults.
5. Residual Current Detection:
   • Ensures safety in residential and commercial electrical systems by disconnecting circuits during small leakages (as in RCCBs/ELCBs).

Advantages:

• Provides quick and sensitive detection of earth faults.
• Simple and effective for protection of people and equipment.
• Can detect very low fault currents, unlike traditional overcurrent protection.
• Helps maintain system stability and safety.

Conclusion:

The core-balance CT is an essential protection device that monitors the balance of current flowing through all conductors in a system. Its main purpose is to detect earth faults by identifying any current imbalance and sending a signal to disconnect the faulty circuit. This helps prevent electrical hazards, equipment damage, and ensures safe operation of the power system.