Zero-sequence current transformers (CTs) are used to measure and protect against ground faults in electrical power systems. They are designed to detect the zero-sequence current that flows in the neutral conductor during an unbalanced condition. The test turns on a zero-sequence CT are additional turns that are wound on the core of the CT and are used to test the accuracy of the CT's performance. The test turns allow a known current to be injected into the CT to verify its functionality.