Current Transformer

A current transformer, or CT, is a type of transformer that reduces or multiplies an alternating current so that its secondary winding carries a current proportional to, but much smaller than, the current flowing in its primary. Together with voltage transformers, current transformers are classed as instrument transformers, whose job is to scale down the large currents and voltages of a power system to small, standardized values that measuring instruments and protective relays can handle safely, while also isolating those measurement and protection circuits from the high voltage of the primary system itself. A current transformer presents only a negligible load to the primary circuit it monitors, and current transformers serve as the current-sensing units of the power system at generating stations, electrical substations, and in industrial and commercial power distribution generally.