Protective relays—overcurrent, differential, directional and distance relays—depend on the CT secondary to represent the primary fault waveform with minimal distortion, correct polarity, and within specified ratio and phase error limits. Current transformers (CTs) are the primary sensing interfaces between high-current power circuits and the low-voltage protection and metering equipment used in substations and transmission networks. Please refer to the relevant re 10 Connection Examples: Current Tr CT star-point is towards the line or towards the busbar. They allow high currents to be safely measured, monitored, and controlled by stepping down the current to a standardized secondary value (commonly 1A or 5A).