A core-balance current transformer (CBCT) is a type of current transformer used to detect ground fault currents in electrical systems. Also known as a zero-sequence current transformer, it is designed to measure residual or zero-sequence currents that result from an imbalance between the phase currents in a three-phase system.

The CBCT consists of a toroidal-shaped magnetic core, typically made of ferromagnetic material, with primary and secondary windings. Unlike traditional current transformers, which measure line currents, the CBCT’s primary winding typically does not pass through a conductor carrying the current to be measured. Instead, it encircles all three phase conductors together.

When the three-phase system is balanced and there is no ground fault, the currents in the three phases should cancel each other out, resulting in a net zero-sum current passing through the CBCT’s primary winding. However, if there is a ground fault in the system, causing an imbalance in the phase currents, a residual current will flow through the primary winding of the CBCT. This residual current induces a proportional current in the secondary winding, which can then be used for detection and protective purposes, such as tripping a circuit breaker or providing a signal for ground fault monitoring systems.

CBCTs are commonly used in electrical systems to provide ground fault protection and ensure safety by quickly detecting and isolating faults to prevent potential damage or hazards.