They are used to measure the three-phase alternating current, when the current to be measured is too large to be applied directly to the measuring instrument.
The standard output current is / 5A. The accuracy is indicated by the class, the most used is class 0.5 (error 0.5%) and 1 (error 1%).
In three-phase busbar current transformers, current bars to be measured must pass through the center holes. They are the most used due to their good precision / price ratio for intensities greater than 100A.